J - (PDF, 101 mb) - USAID

The University of Michigan
Gambia River Basin Studies
Aquatic Ecology
and
Gambia River Basin Development
Prepared for
U.S. Agency for International Development (USAID)
and Gambia River Basin Development Organisation (OMVG)
Contract No. 685·0012·C·OO·2158·00
September 1985

PREFACE
This document summarizes results of a two-year study of the aquatic
resources of the Gambia River. The material is presented in concise form
that should be readily comprehensible to the informed layman. Technical
terms are either avoided or defined on first usage. Chapters 3 through 7
stand alone in the sense that each one presents a self-contained concept
or a small set of concepts. The reader may chose to read selected
chapters should he or she have interest in only one or two topics. A
considerable body of supporting documents was used to prepare this
document. Readers are referred to these documents for more detail on any
one subject.
While each of Chapters 3 through 7 stands alone, a common thread runs
among them. That thread begins with the environmental studies and ends
with the proposed monitoring/surveillance program. The links between the
chapters are as follows: a detailed and extensive ecological study of the
Gambia River was conducted because of the lack of any holistic study of
the river system. Certain segments of the aquatic flora and fauna were
never described before this project. Furthermore, an integrated sampling
approach was used to test hypotheses about the river environment that
include all segments of the river ecosystems. The results of that
ecological survey are presented in Chapter 3. The Gambia River was also
studied as a regional resource. The value of the industrial and
artisanal fisheries was addressed as a source of domestic food and
employment within the basin. These two studies, which served to further
characterize the Gambia River, are presented at the end of Chapter 3.
Once the extant Gambia River system was reasonably understood, evaluation
of impacts was conducted. Using case histories from other river basin
development programs, impacts to the Gambia River from the proposed
development program were identified. The spatial and temporal extent of
those impacts were determined, as well as the intensity of the changes.
The impacts are presented in Chapter 4. Mitigative measures for these
impacts were suggested when possible, and impacts which defied or do not
require mitigation were indicated. A management program was offered as
well as a development scenario to best implement the mitigating
measures. The mitigating measures, development scenario, and management
iii

program are presented in Chapter 5. The economics of the proposed Gambia
River Basin development programs are presented in Chapter 1. Following a
similar format to the presentation of the impacts, the economics of each
of five development scenarios are considered. Finally, suggestions were
,
made as how to continue the field studies in order to monitor the changes
to the river as development proceeds. A monitoring program is
recommended in Chapter 7.
ACKNOWLEDGEMENTS
. An enormous amount of effort went into the successful completion of
this study. A great many people contributed well beyond their normal
occupational requirements. Our extreme gratification goes to the
following:
The River Basin Development Office (RBDO) of the United States Agency
for International Development in Dakar for having the vision to undertake
and finance such a study. In particular, we would like to thank Lew
Lucke and David Hunsberger of that office for their assistance and
administrative flexibility. Dr. Karl Lag1er, Chief of Party, University
of Michigan Gambia River Basin Studies (GRBS), for invaluable advice and
unending patience. The River Resource Team - Thomas Berry, Loren Flath,
Marc Healey, Gerald Krausse, Donna Page, Phil Schneeberger, Heang Tin,
and Marion van Maren -- gave their best efforts to make the project
succeed. The crew of the R/V Laurentian, in particular captain Ed
Dunster and crew member Glen Tompkins, provided the life support and
scientific systems that made the sampling a success, including
carrying-out some relatively unusual sampling plans. The families of the
Rive Resource Team for their support and patience during the field
studies; the administrative staff in Michigan, especially Dr. A.M.
Beeton, Norah Daugherty, Nelson Navarre, and Mark Weishan. The African
scientists from The Gambia, Guinea, and Senegal who worked side-by-side
with the River Resource Team to make this project succeed; OMVG for their
support, and especially Andre deGeorge who was considered the eleventh
member of the team. And last but not least, to our wives, Pat and Marcia,
who endured many lonely months so that we could come to Africa.
Ann Arbor, Michigan
September 1985
iv
Russell A. Moll
John A. Dorr III

PREFACE AND ACKNOWLEDGEMENTS • • .
UST OF ACRONYMS AND ABBREVIATIONS •
1. EXECUTIVE SUMMARY. • • • • • • •
TABLE OF CONTENTS
2. BASIN DEVELOPMENT OBJECTIVES AND PURPOSE OF STUDY. •
2.1.
2.2.
Development Objectives ••
Purpose of Study • • •
3. PRESENT STATUS OF AQUATIC RESOURCES IN THE GAMBIA RIVER.
3.1. Hydrological Synopsis of the Gambia River Basin••
3.1.1.
3.1.2.
3.1.3.
3.1.4.
3.1.5.
3.1.6.
General Description of
Streamflows. • • • •
Tides. • • •
Salinity ••
Sediments. •
Groundwater.
. . iii
xv
1
9
9
11
15
15
the Climate • 15
18
21
23
24
28
3.2. Current Ecological Conditions in the Gambia River. 29
3.2.1.
3.2.2.
3.2.3.
3.2.4.
3.2.5.
Lower Estuary Zone • •
Upper Estuary Zone • •
Lower Freshwater River
Upper Freshwater River
Headwaters Zone. • ••
3.3. Current Status of Fisheries••
4. IMPACTS.. •
4.1.
4.2.
3.3.1. Artisanal Fisheries•••
3.3.1.1.
3.3.1.2.
3.3.1.3.
3.3.1.4.
Zone.
Zone.
Gambian Atlantic Coast
Gambian estuarine and river fisheries.
Senegal.
Guinea • • •
3.3.2. Industrial Fisheries. 85
3.3.2.1.
3.3.2.2.
Domestic companies •
Foreign companies••
Summary. • • • • •
Hydrology of Reservoir Operation .
v
32
39
57
66
76
78
79
79
81
84
84
85
88
91
95
101

4.2.1­
4.2.2.
4.2.3.
4.2.4.
4.2.5.
Balingho Salinity Barrage••••••••
Kekreti Storage Dam. • • • • • • • • • • • • ••
Tandem Operation of Kekreti and Balingho Dams.
Guinean Dams • • • • • • • • • •
Irrigation Networks. • • • • •
4.3. Lower Estuary. • • • • 108
101
103
104
106
108
4.3.1- No Development or Steady State · · · . · · · · 109
4.3.2. Kekreti Storage Dam. · · 110
4.3.3. Kekreti Storage Dam and Guinean Dams · 111
4.3.4. Kekreti Storage Dam and Balingho
Salinity Barrage · · · · · · · · ·
4.3.5. Kekreti Storage Dam, Guinean Dams,
and Balingho Salinity Barrage. ·
4.4. Upper Estuary. . . · · · · · · · 115
4.4.1- No Development or Steady State · · · · . · . · · · · 116
4.4.2. Kekreti Storage Dam. · · · · · · 116
4.4.3. Kekreti Storage Dam and Guinean Dams · 118
4.4.4. Kekreti Storage Dam and Balingho Salinity
Barrage. · · · · · · · · · · · · · · · · 119
4.4.5. Kekreti Storage Dam, Guinean Dams, and
Balingho Salinity Barrage.
4.5. Lower Freshwater River Zone. · · · · · · 125
4.5.1- No Development or Steady State 127
· · · · · 4.5.2. Kekreti Storage Dam. · · · · · · · · · . 128 ·
4.5.3. Kekreti Storage Dam and Guinean Dams
4.5.4. Kekreti Storage Dam and Balingho Salinity ·
Barrage. · · · · · · · · · · · ·
4.5.5. Kekreti Storage Dam, Guinean Dams, and
Balingho Salinity Barrage. ·
4.6. Upper Freshwater River Zone. · · · · · · · · ·
4.6.1­
4.6.2.
4.6.3.
4.6.4.
4.6.5.
No Development or Steady State ••••
Kekreti Storage Dam. • •
Kekreti Storage Dam and Guinean Dams • • • • •
Kekreti Storage Dam and Balingho Salinity
Barrage. . . . . . . . . . ...
Kekreti Storage Dam, Guinean Dams, and
Balingho Salinity Barrage••
4.7. Headwaters Zone•• 138
4.7.1­
4.7.2.
4.7.3.
No Development or Steady State •
Kekreti Storage Dam. • •
Kekreti Storage Dam and Guinean Dams •
vi
112
115
125
130
130
132
132
133
134
138
138
138
140
141
141

4.7.4.
4.7.5.
Kekreti Storage Dam and Ba1ingho
Salinity Barrage • • • • • • • •
Kekreti Storage Dam, Guinean Dams,
and Ba1ingho Salinity Barrage.
• • • 144
• • 145
5. IMPACT MITIGATION AND MANAGEMENT STRATEGIES •• • • 147
5.1. Impact Mitigation. • • • • • • 147
5.1.1. Primary (Physical-Chemical) Impacts•• • • • 147
5.1.1.1. Regulated annual streamf10ws · · · . 151
5.1.1.2. Altered streamf10ws. · · · · · · · · · · . 151
5.1.1.3. Altered thermal regimes. · · 151
5.1.1.4. Anoxic bottom waters in reservoirs · · 153
5.1.1.5.
5.1.1.6.
5.1.1. 7.
5.1.1.8.
Altered nutrient regimes in
reservoirs · · · · · · · · · ·
Altered suspended solids load.
Increased underwater light ·
transparency •
Increased suspended · · solids · · · due · · to dam
153
154
154
and irrigation network construction. 154
5.1.1.9. Modification to river banks. 155
5.1.1.10. Loss of seasonal floodplains · · 155 · · · · ·
5.1.1.11. Development of drawn-down zones. 155 · · · 5.1.1.12. Increased evaporation from reservoirs'
surfaces 156
· · · · · · · · · · 5.1.1.13. Lack of tidal mixing above salinity
barrage. 156
· · · · · · · · · · · · · ·
5.1.1.14. Increased tidal amplitude downstream
from the barrage 156
· · · · · 5.1.1.15. Removal of saltwater from above · ·
Balingho 156
· · · · · · · · · · 5.1.1.16. Absence of a salinity gradient in ·
the river. 156
5.1.1.17. Formation of acid-sulfate · · · · · · · soils. · · ·
157 ·
5.1.1.18. Sediment accumulation in bo10ns. 157 ·
5.1.1.19. Formation of hypersa1inity downstream
of the barrage · · ·
157
5.1.2. Secondary (Biological) Impacts · · · · · · 158
5.1.2.1. Species shifts in reservoirs · · · · · 158
5.1.2.2. Increased algae growth in reservoirs · 158
5.1.2.3. Benthic species shifts · · · · · · · · 158
5.1.2.4. Increased fish stocks in reservoirs. · 159
5.1.2.5. Aquatic weed growth in reservoirs
and irrigation canals. · · · ·
5.1.2.6. Evapotranspiration • · · · · · ·
5.1.2.7. Elimination or reorganization
of mangrove forests. · · · · ·
vii
159
159
160

6.2.2. Artisana1 Sector • • • • • • 179
6.2.2.l.
6.2.2.2.
Finfish fisheries.
Shellfish fisheries••
• 180
• 185
6.3. Development Implications and Tradeoffs • • 188
6.3.l.
6.3.2.
6.3.3.
6.3.4.
6.3.5.
6.3.6.
Overview • • • ••
No Development •
6.3.2.l.
6.3.2.2.
Finfish fisheries••••••
Shellfish fisheries. • • • •
Kekreti Storage Dam. • • • • • •
Kekreti Storage Dam and Guinean Dams •
Kekreti Storage Dam and Ba1ingho
Salinity Barrage • • • • • • • •
Kekreti Storage Dam, Guinean Dams,
and Ba1ingho Salinity Barrage.
188
189
• • 190
• • 191
• • 193
• • 196
• • 197
7. MONITORING AND INSTITUTIONALIZATION. 207
7.l.
7.2.
7.3.
7.4.
REFERENCES •
Monitoring and Future Studies. • • • • •
Parallel Studies • • • • • • ••
Institutionalization • • • • •
Future Fishery Monitoring and Management
APPENDIX I. PRESENTATION OF IMPACTS BY TYPE •
ix
. . . . . . . .
205
207
210
211
212
215
219

3.1.
3.2. Average Monthly Streamflow in 1983 and Percent of Long
Term Average. • • • • • • • • • • • ••
3.3. Summary of Tide Characteristics.
3.4. Physical Characteristics and Upstream Boundaries
of Five Ecological Zones of the Gambia River. •
3.5.
3.6.
3.7.
3.8.
3.9.
3.10.
Average Monthly Streamflow. •
LIST OF TABLES
Mean Values of Physical-Chemical Variables Collected
at Kedougou During the Rising Waters Field Trips
(June 1983) . . . . . . . . . . . . . . . . . . . .
Mean Values of Physical-Chemical Variables Collected
at Kedougou During the Flood Waters Field Trip
(September 1983)••••••••••••••••••
Recorded Marine Artisana1 Catches by Species
for the Gambia Atlantic Coast, June 1982 to July 1983
Catches of the Gambian Estuarine and River Artisana1
Fishery from June 1982 to July 1983 • • • • • • • •
Seasonal Catch of Fish by Artisanal Fishermen
in the Lower River Segment of the Gambia River.
Schedule of Fees for Registered Vessels and
Export Duties in The Gambia • • • • •
Page
· · · · 19
20
22
. · · · · 31
74
75
· . . . 80
3.11. Economic Value of Fish Products (July 1982 to June 1983). • 87
4.1. Anticipated Impacts to the Aquatic Environment,
Flora and Fauna of the Gambia River from the
Proposed River Basin Development Program. • • • • • • • • •• 93
4.2. Distribution of Anticipated Impacts by Zones
in the Gambia River . . . . . . . . . . . . . . · · · · 96
4.3. Degree of Risk or Extent of Impact Associated with
Each Development Option . . . . . . . . . . · · · ·
4.4. Maximum Irrigation Area and Limit of Saline Penetration
From the Autonomous Operation of the Kekreti Dam. • • • 105
5.1. Suggested Mitigation Measures for Anticipated Impacts to the
Gambia River from Proposed Development Program Impact
Mitigation Measure. • • • • • • • • • • • • • • • • • • • • • 148
xi
81
82
86
98

Figures
LIST OF FIGURES
2.1. The Gambia River Basin •••••••
3.1. Dam Sites in the Gambla River Basin.
3.2. Longitudinal Movement of Salinity Throughout the Year in
the Gambia River Estuary • • • • • • • •• ••••
3.3. Salinity Cross-section in the Gambia River During Late
July 1973. . . . . . . . . . . . . . . . . . ..
3.4. Salinity Cross-section in the Gambia River During Early
October 1973 •• •• • • • • •
3.5. Seasonal Ranges of Physical-chemical Variables from
the Lower Estuary Zone • • • • • •
3.6. Seasonal Ranges of Chemical Variables from the Lower
Estuary Zone • • • • • • • • • • •
3.7. Seasonal Ranges of Physical-chemical Variables from the
Upper Estuary Zone • • • • • • • • • • • • •
3.8.
3.9.
Seasonal Ranges of Chemical Variables from the
Upper Estuary Zone • • • • • . • • • • •
Seasonal Ranges of Chemical Variables from the Upper
Estuary Zone • • • . • • • • • • • •
3.10. Changes in Physical-chemical Variables with Distance
and Time in a Mangrove Bolon near Balingho • • . •
3.11. Changes in Chemical Variables with Distance and Time
in a Mangrove Bolon near Balingho•••.••
3.12. Seasonal Ranges of Physical-chemical Variables from the
Lower River Zone. • . • • • • . • • ••••
3.13. Seasonal Ranges of Chemical Variables from the
Lower River Zone ••.••••
3.14. Distribution of Ch1orophyll-A in the Lower 300 km of the
Gambia River • • • • • • • • •
3.15. Seasonal Ranges of Physical-chemical Variables from
the Upper River Zone • • • • • • • • • • •• 68
3.16. Seasonal Ranges of Chemical Variables from the
Upper River Zone • • . • • • • . • • •• • • • • • • •• 71
xiii
10
16
25
26
27
35
37
41
44
46
51
54
60
63
65

Figures
5.1. Average streamf10ws at Gou1oumbou from 1970-1980 and
Projected Flows after the Construction of the Kekreti Dam
and a 70,000 ha Irrigation Network. • • • • • • • • • • • • 152
xiv

LIST OF ACRONYMS AND ABBREVIATIONS
CRED Center for Research on Economic Development
USAID United States Agency for International Development
GRBS Gambia River Basin Studies
ART Agrar-und Hydrotechnik
HHL Howard Humphrey Limited
OMVG Gambia River Basin Development Organization
ORSTOM Office de la Recherche Scientifique et Technique Outre-Mer
NPE National Partnership Enterprise, Ltd.
FMC Fish Marketing Company, renamed National Fish Marketing Board
(The Gambia)
xv

1. EXECUTIVE SUMMARY
This chapter presents the highlights of a large and detailed study of
the aquatic resources of the Gambia River. The guiding principal in this
study. was that only by understanding the extant system could informed
judgements be made in regard to the possible effects of river basin
development and the magnitude of those effects. The body of information
developed as a result of this study includes economic as well as
ecological data. The reader is encouraged to review the source documents
from which this document was composed for details on any aspect of the
river system. These source documents include: physical-chemical studies
(Berry et al. , 1985) , plankton studies (Healey et al. , 1985) ,
macroinvertebrate studies (van Maren, 1985), fish studies (Dorr et a1.,
1985), acid-sulfate soil problems (Colley, 1985), mangrove studies
(Twilley, 1985), fishery economics (Josserand, 1985), and hydrological
studies (Harza, 1985).
The University of Michigan Gambia River Basin Studies (GRBS) was
divided into four studies dealing with four topics: aquatic resources,
public health, wildlife/vegetation, and socioeconomics. The objectives
of the four studies were approximately the same, although somewhat
modified to accommodate each individual discipline. The first objective
of all four studies included a review of existing literature concerning
the Gambia River. This base of existing information was then used to
develop a plan of study of the river basin with emphasis on those items
which were either never or incompletely evaluated in the past. The next
objective of the GRBS was to produce a good understanding of the extant
system. This understanding was then used to address the next objective,
prediction of impacts to the river from the different development
programs that have been offered for the Gambia River. Those development
programs include up to five dams (four freshwater impoundments and one
salinity barrage) to provide water for as much as 85,000 ha of irrigated
agriculture in the Gambia River Basin, and support hydroelectric
generation. The final major objective was to suggest measures to
mitigate the identified impacts.
The study of the aquatic resources of the Gambia River was conducted
on a zonal basis. Prior research revealed that a great deal of diversity
1

2
in the aquatic environment existed along the river from the estuarine
mouth to the streams of the headwaters. Therefore, the river was divided
into five zones and detailed studies were conducted at one or two
locations in each of the five zones. These zones are: lower estuary,
upper estuary, lower freshwater river, upper freshwater river, and
he.adwaters. The salient ecological characteristics of each of the five
zones are listed below.
• Lower Estuary. This is a wide and relatively shallow segment of the
river which is dominated by the influx of water from the coastal marine
environment. The lower estuary assumes most of the chemical, physical,
and biological characteristics of the adjoining ocean waters. The
semi-diurnal tides constantly mix the waters of the lower estuary and
carry a typical marine flora and fauna. The banks of the lower estuary
zone are lined with mangrove forests which extend up to 10 km inland from
the edge of the river. The net primary productivity from these mangrove
forests provides the nutrient base for many of the marine and estuarine
organisms, including several rich finfish and shellfish fisheries. These
fisheries are the most productive of the entire river.
• Upper Estuary. The upper estuary zone has highly seasonal
characteristics because of the annual flood. During the dry season
(October to June), this segment of the river is estuarine and supports
primarily a marine flora and fauna. But, during the rainy season,
floodwaters change much of this zone to freshwater and cause an exodus of
the marine species. Tidal currents are a major factor which mix the
river waters from top to bottom as well as keep water flowing through the
numerous small mangrove-lined bolons (creeks) which branch from the main
river channel. Similar to the lower estuary zone, the main source of
organic matter to the upper estuary zone comes from the mangrove forests
which extend up to 3 km inland from the river bank. Much of the mangrove
detritus is carried downstream from the upper to lower estuary zone. The
mos t luxuriant forests along the river are found in this zone. The
mangrove detritus supports productive fisheries, although they are
seasonal in nature.
• Mangrove ecosystems. These systems are an integral and fundamental
part of the estuarine zones of the Gambia River. It is a consequence of
their high net productivity that the estuary has a rich and diverse

4
• Upper Freshwater River. This zone has many characteristics in common
with the lower river zone including a highly seasonal suite of
characteristics and relatively unproductive fisheries. The major
differences between the two zones are that the upper river zone is not
tidally mixed and has no net streamflow during much of the year (the
river dries to pools).
• Headwaters. This section of the river consists of the dendritic
small streams and rivers running through the Fouta Dja110n Mountains of
Guinea. The aquatic environment is dominated by the annual rains; the
river has no net streamflow during the late dry season and then becomes a
torrent during the rainy season. Relative to its lower reaches, this
segment of the river is unproductive and supports a very meager fishery.
The Gambia River provides a large amount of food to local populations
through the estuarine and coastal fisheries. Considering that this
portion of Africa is attempting to increase domestic food production, the
Gambia River is a valuable regional resource. The artisana1 fisheries of
the river and adjacent coast employ up to 16,000 people and produce over
8100 metric tons of fish annually. The commercial fisheries in Gambian
coastal waters provide about 7300 metric tons of fish annually, which
generate revenues to the local economy via fishing fees, export taxes,
and prOVide a source of foreign exchange. The annual harvest of both the
artisana1 and commercial fisheries could be increased, primarily by
exploitation of certain coastal and estuarine stocks. In contrast,
freshwater stocks may be fully exploited at the present. Major
impediments to expanding the coastal and estuarine fisheries are wasteful
handling and preservation techniques and poor marketing networks. Lack
of stock and catch assessment data also prohibit establishment of maximum
sustained yield and economic benefit from the fishery resource. Finally,
a regional fisheries development and management program is needed for the
Gambia River Basin and adjoining coastal waters.
Forty-one impacts to the aquatic resources of the Gambia River were
identified and presented as a consequence of the proposed development
programs. These impacts were considered in two manners, by zone
(Chapter 4) and by type (Appendix I). The presentation of impacts by
zone reveals the effects of each development scenario in each of the five

5
zones. Five different development scenarios were considered in each zone
for a total of twenty-five.
The identified impacts were divided into three categories: primary,
secondary, and tertiary. Primary impacts were considered those which
affect the physical and/or chemical environment of the river. Examples
of the nineteen primary impacts include altered patterns of streamflows,
changing of portions of the river from riverine (riverlike) to lacustrine
(lakelike), elimination of tidal mixing upstream from the salinity
barrage, and removing the salinity gradient from the estuary. These
primary impacts in turn create changes to the biota of the river. The
changes to the biota are designated as the secondary impacts. Examples
of the eleven secondary impacts are: shifts in species from riverine to
lacustrine, extensive growth of aquatic weeds, destruction of mangrove
forests upstream from the salinity barrage, and increased fish production
in the newly created reservoirs. The human populations of the Gambia
River basin will interact with the new aquatic environment and in turn
generate a set of impacts by their behavior. These eleven impacts are
considered tertiary impacts and examples include: addition of nutrients
and pesticides to the river from irrigated farming, expanded fishing
activities, and migration of people toward the new freshwater resources.
Mitigation of the identified impacts stems from the concept that
primary impacts generate all others because of their fundamental nature.
Thus the key to reducing or eliminating each impact is to identify the
basic change to the physical and/or chemical environment and to mitigate
that change. For example, mitigation of impacts due to mining activities
is achieved by preventation of mine wastes and waste waters from entering
the river. Viewed from this perspective, it becomes apparent that over
half (21) of the impacts cannot or do not require mitigation. Nine of
the forty-one impacts are favorable and thus do not require mitigation.
Twelve impacts cannot be mitigated and thus should be accepted as
inevitable consequences of river development.
The consideration of impacts by zones demonstrated that one
particular project, the Balingho Salinity Barrage would generate a very
large set of undesirable impacts which cannot be readily mitigated.
Furthermore, these impacts will occur in the most productive segment of
the river. As a result, the salinity barrage appears a poor choice for

6
one of the development options with respect to the aquatic resources.
This concept was supported by economic considerations as well. Under
these conditions, the logical development program is to first develop the
Kekreti Dam, and evaluate the consequences of that project. Construction
of the salinity barrage should be delayed until a definite need is
established and the associated impacts more carefully studied.
Twenty impacts should respond to mitigation, primarily by careful
management of water resources. These mitigation efforts fit into a
coherent nine-point management program of the following:
i)
ii)
iii)
iv)
v)
vi)
vii)
viii)
ix)
produce a controlled annual flood;
artificially mix the reservoirs;
limit sediment release during construction;
control aquatic weed growth by mechanical methods;
carefully manage irrigated cropland;
provide a physical separation or barrier between irrigated
cropland and the river;
impose strict environmental mining safeguards;
place new roads and settlements at a distance from the
river;
create a I km-wide greenbelt on both sides of the river.
This management scheme will provide considerable protection to the
quality of the environment and aquatic resources. But implementation
must take place only when all facets of the basin development program
have been considered; some of the management steps may not be
economically feasible (e.g. artificially mixing of reservoirs) or
socially acceptable (e.g. keeping settlements away from rivers).
The economics of the proposed development program for the Gambia
River Basin were considered in relation to the aquatic resources. The
basis from which the economic analysis was drawn was that of the
fisheries. The reason for this approach arises from the recognition that
the only portion of the aquatic resources that can readily be addressed
economically is the fisheries. The bulk of the current fisheries of the
Gambia River are centered in the estuary and adjacent coastal waters.
The finfish fisheries are an important component of the domestic Gambian
economy while the shrimp fishery generates a substantial amount of
foreign exchange for The Gambia.
The proposed development programs will affect the fisheries economics
in two ways: increases in fish yields in the freshwater segments of the

2. BASIN DEVELOPMENT OBJECTIVES AND PURPOSE OF STUDY
2.1. DEVELOPMENT OBJECTIVES
The Gambia River flows over 1100 km from the Guinean highlands to the
Atlantic Ocean on the west coast of Africa (see Figure 2.1.). Much of
the terrain through which the river flows is arid and supports only
limited agriculture. Agricultural output has not increased over the past
decade despite vigorous efforts toward developing food self-sufficiency
throughout West Africa. This stagnant productivity has been greatly
fostered by a decade-long trend of diminished annual rainfall.
The more developed nations have supported a variety of programs to
enhance agricultural output of West Africa. Most of these programs have
focused on the more efficient use of freshwater. The emphasis on use of
freshwater originates from two sources. First, development programs in
other parts of Africa have also sought to enhance agricultural output by
management of water resources. Second, water has limited production far
in excess of any other factor.
The African nations of The Gambia, Guinea, Guinea-Bissau, and Senegal
have recognized that the problem of managing the Gambia River will
require a multinational effort. In response to that problem, the Gambia
River Basin Organisation (OMVG) was created with the overall charge to
further water resource-based development of the Gambia River. OMVG has
in turn created a development program based on the construction of up to
five dams along the Gambia River. These dams would make a system of
reservoirs to store as much as possible of the annual discharge of the
river. The water could then be released throughout the course of the
year for irrigation, hydropower generation, industrial applications, and
mining. Because the basin is characterized by two seasons (a nine-month
dry season and a three-month rainy season), much of the annual discharge
of the river occurs as annual flood which is unused as it passes out to
sea. The dams and their reservoirs will regulate this discharge for
human use throughout the year.
An additional benefit of the reservoirs is the potential for
development of extensive fisheries in each of the newly formed lakes.
Currently the Gambia River supports relatively rich coastal and estuarine
9

11
fisheries, but a rather poor inland fishery. This inland fishery is
apparently overfished and yields have been declining over the past
decade. The reduced fishery may also result of the reduced streamf10ws
in the river during the ten-year drought. The projections for the Gambia
River are that fish yields from the reservoirs should greatly increase
the annual catch from the freshwater portion of the river.
The expectations from development of the Gambia River are an overall
improvement in the quality of life in the basin. This improvement in the
quality of life would result from a combination of factors which include:
increased domestic agricultural output, increased local employment,
improved regional transportation, and increased foreign exchange for new
products. With these improvements to the quality of life, the countries
in the Gambia River Basin should then be able to develop their economies
by diversification of the base and enhance output of goods and services.
All of these benefits will not arise without some costs both to the
local society and the natural environment. Precedent has shown that
expectations have rarely been met for many of the river basin development
. projects undertaken in Africa (Freeman, 1974). In some cases the people
have ended up worse-off after the projects have been completed than
before. Economic loss of natural resources can often exceed the gains
made from poorly managed irrigation projects. Thus, the potential
benefits of the Gambia River Basin development program will most
certainly not be met simply by constructing the dams. Careful management
will be required to use the new resources to best advantage. In some
cases the development program should be modified to protect existing
natural resources from replacement by less valuable new resources.
2.2. PURPOSE OF STUDY
The failures of previous river basin development programs provide
more than ample justification for an extensive review of the proposed
program for the Gambia River Basin. The United States Agency for
International Development (USAID) working with OMVG requested a major
study of the Gambia River Basin in an effort to avoid some of the
mistakes encountered with other development programs. The University of
Michigan Gambia River Basin Studies (GRBS) was a fout-part effort

12
covering the different aspects of the basin which will be affected by the
development program: public health, wildlife, aquatic resources, and
social and economic resources. This report deals with the findings of
the study of the water and living aquatic resources (or river
resources). The overall structure of the study dictated that many common
objectives were shared among the four disciplines. Examples of these
common objectives include a detailed understanding of the current
conditions within the Gambia River Basin, predictions of the changes in
the basin from the development program, and suggestions as to how
mitigate adverse impacts from development. But the objectives of each of
the fouI.' disciplines were tailored to needs of each specific program.
The objectives of the river resources study are presented below.
The first objective addressed as part of the river resource study was
an evaluation of existing knowledge concerning the aquatic resources of
the Gambia River. Before field sampling was initiated, an analysis of
previous studies of the Gambia River was conducted. This analysis served
to reduce the probability that repetitive studies would be conducted on
the Gambia River. As the field study of the Gambia River was only one
year in duration, the program could not afford to repeat past research.
The primary form of the analysis was a literature review culminating in
an extensive bibliography of research either conducted on the Gambia
River or relevant to it.
After the literature review was complete, the second and largest
objective was begun: to gain an understanding of the current ecological
conditions of the Gambia River. The literature review revealed that many
aspects of aquatic ecology had either never been investigated, or were
not understood. Consequently, this study strived to fill in those
portions of the general ecology of the river which were missing in order
to yield a comprehensive understanding of the aquatic system. A
completely detailed study of the aquatic ecology of the river was not
possible. Rather J the intent was to reveal the major processes which
drive biological production within the river ecosystems. This objective
was to include all segments of the river from the highly saline estuary
at the river's mouth to the small streams of the river headwaters in the
Guinean Highlands.

13
The ultimate goal of river basin development is to improve living
conditions. As discussed above, this goal is primarily met in economic
terms such as enhanced agricultural output, improved foreign exchange,
and augmented fish yields. The analysis of the potential increase of
fish yields associated with development of the GambiaRiver was a major
aspect of the river resource study. Thus the third objective was to
determine the current value of the fisheries associated with the Gambia
River. This value was estimated in several ways including: foreign
exchange earnings for fishery products, food stuffs for local
consumption, and local employment. The intersection of the fishery
economic and ecological objectives was made by focusing the ecological
understanding of riverine and estuarine fish on economically important
species. In most instances, the economically important fishes were also
ecologically important species.
The understanding of the ecology of the Gambia River as well as the
economic value of the fisheries provided the base of information for
completion of the next objective, prediction of the major impacts
associated with basin development. The development program will in many
places dramatically alter the physical regime of the Gambia River. For
example, the estuarine segment of the river upstream from Balingho will
be eliminated after the salinity barrage is constructed. Along with the
altered physical regime will be a suite of biological changes to the
river ecosystem. Prediction of these changes will allow an evaluation of
the consequences of river basin development. Given this information,
managers can decide if the benefits of development outweigh the costs.
The impacts are presented in ecological or economic terms.
The final objective of this study was to determine the extent of the
major impacts from river basin development and suggest mitigation of
those impacts if possible or necessary. The operational policies of dams
are most often determined by engineering and/or hydrologic factors. Thus
impacts to the aquatic organisms must be accepted as part of the costs of
development. Those impacts which cannot be mitigated have been
identified. Other impacts that do not require mitigation emerge as
beneficial aspects of basin development. Finally a suite of impacts
which can be reduced by changes in operational policy were identified, as
well as a management policy to effect those reductions. Programs to
monitor impacts and their mitigation have also been suggested.

3. PRESENT STATUS OF AQUATIC RESOURCES IN THE GAMBIA RIVER
3.1. Hydrological Synopsis of the Gambia River Basin
The planning and design of dams for the Gambia River has required an
extensive understanding of the hydrology of the basin. Studies conducted
by a variety of European consultants began as early as 1975 and will
continue well into the future as planning proceeds. As a result of the
numerous hydrologic reports, the Gambia River Basin Studies placed little
effort into collection of original hydrologic data. Rather, a project
hydrologist summarized the major findings and presented them in a report
(Harza, 1985). Those results were combined with the biological field
results to develop inferences about the ecology of the Gambia River
(section 3.2.). A synopsis of the hydrologic data is given below which
provides the basic facts concerning the water characteristics of the
basin.
3.1.1. General Description of the Climate
The Gambia River Basin is a semitropical region located between
11°30' and 15°00' north latitude and 11°00' and 16°30' west longitude.
2
The basin covers 77,380 km, of which 13%, 72%, and 15% lie in The
Gambia, Senegal and Guinea, respectively (Figure 3.1). The basin has
three distinct geographic regions: a hilly upper watershed in Guinea, a
rolling continental basin in Senegal and the eastern half of The Gambia,
and a very flat coastal plain in the western half of The Gambia. The
principal source of water to the Gambia River is rainfall in the upper
watershed and southeastern continental basin.
Climatological data for the basin are collected at a variety of
locations, but are primarily derived from eight stations in the basin.
The historical record is quite variable but begins as early as 1919 for
rainfall and as late as 1975 for evaporation data (Harza, 1985). The
climate of the basin is characterized as semitropical with distinct wet
and dry seasons. The wet season begins in March or April in the upper
watershed and moves slowly north to begin in June or July in the northern
continental basin. The wet season ceases in September in the north and
October or November in the south. Winds during the rainy season are
15

17
prevailing southwesterlies and carry high moisture content from the
Atlantic Ocean. The dry season occupies the remaining portion of the
year and is characterized by a virtual absence of rain. Dry season winds
are easterly, northeasterly, or northerly and carry large quantities of
dust and silt from the North African deserts; these are called Harmattan
Winds. The annual cycle of wet and dry seasons depends on the formation
and migration of a low pressure zone called the Intertropical Convergence
Zone.
Air temperatures in the basin follow a seasonal pattern with annual
minimums in December and January, and maximums in April or May. The
daily mean minimums in the basin are about 15 to l7°C, while the mean
maximum approaches 40°C. Temperatures in the highlands of the upper
watershed are slightly (3-5°C) cooler. Daily sunshine averages from 5 to
6 hours during the rainy season to over 9 hours in March. Mean relative
humidity on the continental basin ranges from 30% in January to over 80%
in June. Humidity along the coast rarely falls below 50%. Winds are
generally light and variable during the early dry season (0.8 m/ s in
November) to moderate just prior to the annual rains (2.4 m/s in May);
winds are calmer inland compared to coastal areas. Evaporation rates in
the basin are high (approx. 2500 mm/year), but vary considerably between
December (about 4.5 mm/day) and May (about 9.5 mm/day).
Evapotranspiration, estimated by Coode and Partners et ale (1979) and
Agrar-und Hydrotechnik and Howard Humphrey Ltd. (AHT/HHL) (1983), was
also high and typically the same or slightly more than evaporation.
As mentioned above, rainfall is distinctly seasonal throughout the
basin as well as occurring in a south to north gradient. For the
1928-1981 average, rainfall ranged from 1600 mm/yr in the southern
reaches of the watershed to less than 600 mm/yr in northeastern areas
(Harza, 1985). February or March has always been the driest month of the
year and August the wettest. Monthly rainfall averages range from 0.0 mm
in February or March to over 500 mm in August. 1983, the year of the
Gambia River Basin Study field investigations, was one of the driest in
history. Rainfall ranged between 20 and 70% of the long-term average,
constituting a one in a one-hundred year drought. Central Gambia near
Kuntaur was the driest portion of the basin in 1983.

3.1.2. Streamflows
18
Streamflow data for the Gambia River Basin are extremely sparse. In
the Gambia, all water level gauges are affected by the tides; because
discharge measurements are not taken, streamflows cannot be estimated.
The station at Gouloumbou provides the farthest downstream streamflow
data, approximately 525 km upstream, or half the total length of the
river. Streamflow records from Gouloumbou extend back to 1952. Eight
water level gauges and 16 staff gauges were installed in Senegal along
the Gambia River and its tributaries between 1972 and 1978. Seven gauges
are in place in Guinea, two automatic recorders and five staff gauges,
all installed during 1975-76. Tile streamflow database has considerable
gaps despite the installation of new equipment. For example, there are
still no high flow rating curves. Flow duration curves have been
calculated for the river at Kedougou and Gouloumbou from the gauge data,
and simulated for the Sandougou and Koulountou locations (AHT/HHL 1983).
Streamflows in the Gambia River and its tributaries respond directly
to rainfall, but have marked regional differences. In the upper
watershed annual runoff is about 25% of annua+ precipitation, which drops
to 10% for the continental basin and 1-2% for the coastal plain.
Despite the rather sparse historical streamflow database, 11
locations on 9 different tributaries have provided enough information to
compute average flows for the period 1970-1982. These are presented in
Table 3.1 by monthly averages. The results show that The Gambia,
Koulountou, and Sandougou Rivers have year-round flow. The recent
drought has caused all rivers except the Gambia to cease to flow during
the late dry season. Table 3.2 shows the 1983 streamflows and the
percent of the longterm average. The extent of the 1983 drought is
evident from these data; verbal histories indicate that the 1970-82
period was dry compared to the first half of the 20th century, thus
making the 1983 drought loom even more severe over the long-term. Tables
3.1 and 3.2 also show the seasonal nature of streamflows, with the annual
flood always cresting in September.
The annual flood of the Gambia River provides a considerable amount
of water which inundates a large area. Between 1953 and 1983 flood water
levels at Gouloumbou rose an average 9.1 meters above dry season levels,

TABLE 3.2.
AVERAGE MONTHLY STREAMFLOW IN 1983
AND PERCENT OF LONG TERM AVERAGE
Drainage
Station Area
Name River (km 2) M J J A S 0 N D J F H A Annual
Kedougou Gambia 7550 n.a. 4.7 34.3 118.0 183.7 81.7 20.0 10.7 4.7 1.9 n.a. n.a. 39.0 a
Percent of Long Term Average - 38 46 37 57 59 47 55 45 36 - - 49
Wassadou J Gambia 21200 0.2 5.7 55.3 120.0 255.0 99.2 24.4 9.0 3.4 1.3 0.4 n.a. 47.9b Percent of ong Term Average 40 66 62 24 41 39 36 34 31 27 24 - 37
Gouloumbou I Gambia 42000 0.3 14.8 64.2 126.0 295.0 134.0 46.1 20.3 13.4 5.0 1.3 0.4 59.1c
Percent of Long Term Average 10 95 56 25 41 33 39 5.0 89 63 29 11 36
I
SOURCE: From Harza, 1985.
NOTES: a) Source: Direction des Etudes Hydraul1ques, Tambacounda •
Unpublished valuea.
b) Some dry season flows near 0.0 m3/s missing.
c) Some dry season flows affected by tidal influence estimated.
N
o

22
TABLE 3.3.
SUMMARY OF TIDE CHARACTERISTICS
Mean Time of
Distance Propagation Average
From Banjul in Range
Station in km h min in m
Banjul - 0 00 1.68
Tendaba 103 4 32 1.52
Ba1ingho 130 5 36 1.33
Brumen Bridge 132 5 40 1.70
Paka1iba Bac 184 7 54 0.90
Kaur 199 8 24 1.30
Chamen Bac 228 9 39 0.93
Kuntaur 254 10 36 1.44
Jaha11y 270 11 30 1.33
Patchar 282 11 54 1.25
Georgetown 295 f 12 30 1.19
Bansang 312 13 24 1.08
Sam! Tenda 362 15 24 0.89
Basse 404 17 10 0.82
Fatoto 478 20 18 0.65
Gou1oumbou 525 22 20 0.10
SOURCE: From Harza, 1985.
Univerlity of Michigan, Gambia River Baain Studlel, 1985.

23
Tidal harmonics (period of tidal oscillations) produce strong
reversing currents in the Gambia River. These currents are asymmetrical
in their strength, with ebb tide currents exceeding flood currents. The
Danish Hydraulic Institute (1982) estimated maximum ebb tide currents at
up to 0.9 mls and maximum flood currents at 0.7 m/s; these estimates were
supported by observations during the Gambia River Basin Study. The
harmonics of the tides cause water in the river to oscillate upstream and
downstream. But, the stronger ebb currents lead to net downstream flow
which matches streamflows at Gouloumbou (HHL, 1974). An additional
aspect of tidal harmonics is the presence of an unusual two-week
downstream tidal- surge in the vicinity of Bansang. Field studies by HRS
(1977) show that net downstream flow almost stops during neap tides, but
then surges once every two weeks with the spring tides. Tidal waves also
assist in seasonally flooding low-lying lands adjacent to the Gambia
River.
3.1.4. Salinity
The propagation of tidal waves over 500 km up the Gambia River and
the low dry season streamflows allows saltwater to penetrate the river as
far as 250 km upstream. Saltwater intrusion is a fundamental aspect of
the ecology of the river. The extent and duration of the intrusion
controls the nature of the aquatic flora and fauna as well as the amount
of crops grown along the river.
Given the importance of salinity in the estuary, HHL conducted an
extensive field study of saline water movements in 1972 to 1974. Their
results (HHL, 1974) were further verified by models (HHL, 1984) and field
data (Berry et al., 1985). The details vary among years because of the
magnitude of the annual flood, but a basic pattern has emerged. On a
longitudinal basis, salt water migrates upstream each year during the dry
season to a recent maximum penetration of about 250 km. The salt
frontier (boundary between fresh and salt water) reaches its maximum
penetration in early June and remains more or less stationary until
mid-August. The annual flood rapidly pushes the salt frontier downstream
to its minimum penetration in September. That location depends greatly
upon the size of the flood, but recently has been between 70 and 130 km
above the river mouth (1973 and 1984, respectively). Verbal historical

24
records indicate the salt frontier moved downstream as far as the river
mouth during the larger floods of the first half of the century. After
passage of the flood, salt water begins to move back upstream in late
October. Between October and June, the salt frontier moves upstream at
an average rate of 15 km/month, increasing toward 20 km/month at the end
of the dry season. The flood pushes the frontier over 125 km downstream
in about five weeks. Longitudinal salinity gradients range from 0.40
ppt/km (parts per thousand/km) to 0.17 ppt/km in September and May
respectively (HHL, 1974). Figure 3.2 shows the migration of salinity
during the HHL study.
Many estuaries develop a typical salt wedge where fresh water
overlays denser salt water (McLusky, 1971). The Gambia River with its
low dry season streamflows does not exhibit this phenomenon between
October and August. But, during the peak of the annual flood, a distinct
salt wedge was observed by the HHL study. Both vertical and longitudinal
salt wedges were observed between 50 and 90 km upstream. Figures 3.3 and
3.4 show dry and flood season salinity cross-sections, with the salt
wedge evident in Figure 3.4. Further, Coriolis Forces cause more saline
water to accumulate along the left or south bank. Observations during
the Gambia River Basin Studies indicated that flood tides tended to move
upstream along the left bank.
Throughout the estuary, tidal waves create oscillations of water in
the river. As a result, observations at a fixed location on the river
bank show a cycle in the salinity. Salinities appear to increase during
flood tides and decrease during ebb tides.
3.1.5. Sediments
The Gambia River carries a relatively low sediment load due to its
armored bottom and low streamflows during the dry season (Harza, 1985).
The best longterm sediment database is available from observations by
ORSTOM (1978) beginning in 1974. The best spatial database was assembled
by our Gambia River Basin Studies, which includes over 20 locations in
the river. The results of these two databases agree very well.
Suspended sediment loads in the Gambia River are extremely low during
the dry season, never exceeding 50 mg/L in the freshwater portion of the
river. The annual flood brings an increase with runoff and suspended

28
sediment loads averages about 100 mg/L. AHT/HHL (1984) calculated the
total sediment load reaching the Kekreti Reservoir as 265,000 tons/yr.
They estimate that no more than 0.3% of the live storage of the reservoir
would be lost to sediment over a 50 year lifespan and no more than 0.5%
over a 100 year period. Suspended sediment concentrations in Guinea were
extremely low, usually less than 20 mg/1. The loss of reservoir volume
for the Kouya, Kankakoure, and Kogou Foulbe Reservoirs for 100 year
lifespans was estimated at 0.2%, 2.1%, and a.8% respectively (Harza,
1985) .
Suspended sediment loads in the estuary are higher, typically
exceeding 100 mg/L (Berry et al., 1985). These higher sediment loads
were attributed to tidal currents scouring soft bottom sediments.
Suspended sediment concentrations were particularly high during spring
tides. Although suspended sediment loads were high in the estuary, net
downstream movement was moderate. River and bolon bottoms were covered
wi th a thick layer of soft material, which was at least 25 m thick at
Balingho. RRI (1984) estimates that the Balingho REservoir would fill
only to -9 m Gambian Datum (GD) after 100 years. This is about 10 m
below lowest water levels of the live storage volume of the reservoir.
3.1.6. Groundwater
Although an extensive groundwater survey has not been conducted in
the Gambia River Basin, the basic structure of the aquifers is known
(Harza, 1985). The four major aquifers are: Shallow, Eocene Sand,
Maestrichtian Sand, and Hardrock. The Shallow Aquifer underlines all of
The Gambia and part of Senegal oriental. Water depths range from a to 50
m. The Eocene Sand Aquifer begins along the southern boundary of the
basin and extends into the Casamance region of Senegal. Water depths
begin between 50 to 100 m. The Maestrichtian Sand Aquifer is found under
the entire basin, with water depths from a m in eastern Senegal to over
500 m at the coast. Hardrock Aquifers are found throughout the basin,
but their areal extent is variable and not totally known. The
Maestrichtian Sand Aquifer is the most important source of ground water.
-4 -2 2
Its transmissivity ranges from 2 x 10 to 4 x 10 m Is (Harza,
1985) •

29
3.2. Current Ecological Conditions in the Gambia River
The prediction of impacts to the Gambia River from development of the
basin must originate in a good understanding of the extant system. While
several studies of fisheries (Johnels, 1954; Scheffers and Conand, 1976),
mangroves (Giglioli and Thornton, 1965), and dissolved materials (Leesack
et al., 1984) have been completed on the river, detailed ecological
studies have not been conducted. Entire segments of the flora and fauna
were never described before June 1983 when this project began its field
program. Those segments included plankton, fish larvae, and
invertebrates. A major objective of the Gambia River Basin Studies
(GRBS) was to complete the lacking ecological description in order to
identify potential impacts. This section summarizes those ecological
studies.
The ecological studies were organized to gather a large amount of
original field data. This approach was adopted for two reasons:
• The lack of information from previous investigations
required that at a minimum some new data be gathered to
fill in missing information gaps;
• The sampling strategy used was that of coordinated
multidisciplinary sampling; Le., many different types of
samples were collected at once to enhance the understanding
of river ecology.
The overall objectives of the field program included the ability to make
statements concerning the Gambia River from inferences rather than
suppositions. The distinction being that inferences arise from data
collected at the location under investigation. Supposition, in contrast,
is extrapolating results from studies conducted outside of the area of
interest to the location under study. Those results made by inferences
require a great deal more original data than those made by supposition,
but the former yield much stronger and more relevant results that does
the latter.
The ecological descriptions given below are brief and simplified.
The object of this presentation is to give a synopsis of the findings
without delving into excessive detail (for example, detailed species
lists are generally avoided). The detail can be found in a number of
project's technical documents. These documents include:

30
• fish studies (Dorr et a1., 1985);
• invertebrate studies (van Maren, 1985);
• plankton studies (Healey, et a1., 1985);
• physical-chemical studies (Berry et a1., 1985);
• mangrove studies (Twilley, 1985);
• fishery-economic studies (Josserand, 1985);
• acid-sulfate soils (Colley, 1985);
• hydrologic studies (Harza, 1985).
Because the Gambia River includes a diverse array of environments
from coastal marine to small freshwater streams,
approach was employed for .the ecological studies.
work, (see Montei11et and P1aziat, 1979), the river
ecological zones. These zones were:
• lower estuary;
• upper estuary;
• lower freshwater river;
• upper freshwater river;
a stratified sampling
Based on prior field
was divided into five
• headwaters.
Eventually an additional investigation of the mangroves bo10ns was added
to the sampling program in recognition of the extremely important role of
mangroves in the overall function of the Gambia River. Physical and
chemical characteristics were used to define each zone and then
approximate geographic boundaries were set among zones (Figure 3.1).
Those characteristics are given in Table 3.4. The presentation of the
ecological results follows this same zone-by-zone approach.
Within each zone, additional stratification was used for the sampling
program. A primary sampling site was chosen in each zone and served as
the location where most samples were collected. Four field trips were
conducted in all but the headwaters zone where only three trips were
conducted. These trips covered the four hydrologic seasons of: rising
waters (July); floodwaters (October); declining waters (December); and
low flow (March). The floodwater trip was omitted in the headwaters
zone. The four field trips also matched four of the five traditional
seasons recognized by the Mandinka residents of the basin:
August); Kountchamaro (September to October); Sanyano
December); and Ti1ikando (December to May).
Sama (July to
(October to

31
TABLE 3.4.
PHYSICAL CHARACTERISTICS AND UPSTREAM BOUNDARIES OF FIVE
ECOLOGICAL ZONES OF THE GAMBIA RIVER
Zone
Lower Estuary
Upper Estuary
Lower Freshwater River
Upper Freshwater River
Headwaters
Upstream Boundry
Mootah Point
Kuntaur
Gouloumbou
Senegal-Guinea Border
Source near Labe
Univerl1ty of Michigan, G_bia River Badn Studi.., 1985.
Physical Characteristic
High salinities (above
30 ppt.), extensive
tidal flushing
Presence of salinity
(1-30 ppt.), tidal
mixing
Freshwater all year,
tidal mixing
Freshwater river, no
tidal mixing
Small streams and
rivers flowing through
mountainous terrain

32
Within each zone during each field trip, sampling was stratified
further to investigate the effects of short-term temporal and small-scale
spatial variation. Short-term temporal variation was sampled as a split
factor with time of day (day or night) and phase of tide (ebb or flood)
in the two estuarine and lower river zones. In the upper river and
headwaters zones short-term temporal variation was investigated by
sampling at four different times during the day: day, dusk, night, and
dawn. Small-scale spatial variation was addressed as longitudinal (along
the river), latitudinal (across the river), and depth. The final
experimental design used was a Latin Square sampling program (Winer,
1976; Netter and Wasserman, 1974). More details of this sampling program
can be found in Berry et ale (1985).
3.2.1. Lower Estuary Zone
The lower estuary is primarily an extension of the coastal marine
waters into wide, slow-flowing river. The wide mouth of the Gambia River
allows tidal waves readily to move into the lower reaches of the river.
The twice-daily flushing of the river brings a constant renewal of
coastal marine water into the lower estuary zone. Thus, even during the
rainy season when freshwater discharge was high, the chemical
characteristics of the lower estuary were very similar to those of the
coastal environment.
The physical characteristics of the lower estuary zone are typical of
tidal rivers in West Africa. The lower Gambia River is wide (up to
15 km) and a relatively shallow area with a main channel running down the
middle of the river. The primary sampling site for this zone was near
Dog Island Point (about 15 km upstream from Banjul). In this location,
the river was slightly over 6 kin wide. A large mud bank extended 2 km
from the south shore of the river and a smaller mud bank about 500 m from
the north shore. The south shore mud bank was covered with only 0.5 m of
water at low tide. The main channel was about 3.5 km wide and up to 12 m
deep. The bottom of most of the lower estuary was soft, silty mud.
The banks of the river are cut by numerous meandering creeks or
"bolons" as they are called locally. Extensive tidal flats covered with
Rhizophora and Avicenia mangrove forests were found on both sides of the
river. In some places these forests extended 10 km from the river.

Tidal flushing was extensive with the tidal
Point about 40 minutes after passing Banjul.
as Banjul, or between 1.5 and 2 meters.
33
waves reaching Dog Island
Tidal ranges were the same
The waters of the lower estuary were highly saline with salinities
never falling below 28.5 parts per thousand (ppt), and rarely below 31
ppt (Figure 3.5). The effects of freshwater dilution in this zone were
minimal and primarily confined to the edges of the river during the rainy
season. Other chemical characteristics were also dominated by the
coastal ocean waters. Soluble reactive silica levels were low when
compared to the rest of the river, whereas soluble reactive phosphorus
concentrations were high (Figure 3.6). Soluble nitrogen concentrations
were extremely variable because of the influence of the mangrove
ecosystems which tended to strip nitrogen from the water.
The highly buffered condition of seawater also prevailed in the lower
estuary. Alkalinities and pH were extremely stable throughout the year
(Figure 3.5). The pH rarely varied more than 0.1 standard units during
anyone field trip, and the range of the means for each of the four field
trips was only 0.09 units.
There was some degree of seasonality in that two distinct thermal
regimes existed. A warm season with water temperatures near 30° C was
observed from June through early November (Figure 3.5). A cool season
was found from early November through mid-May with water temperatures
about 23° C. The thermal regime of the lower estuary, similar to the
coastal ocean, was controlled primarily by the influence of the North
Equatorial Current (Sverdrup et a1., 1942). As a result, the
temperatures in the lower estuary were out of phase with the rest of the
river and were usually about 2° C colder.
Analysis of the data from the Latin Square experimental design by
Analysis of Variance (ANOVA) showed that short-term temporal and
latitudinal variation were important factors affecting the distribution
and concentrations of dissolved and suspended materials. The tide/time
of day factor had an effect on many variables by virtue of the tidal
mixing processes. Each flooding tide brought coastal ocean water into
the lower portions of the river to mix with the water already in the
estuary. This coastal ocean water had several properties which were
slightly different from the river water. For example, flooding tides may

36
LEGEND FOR FIGURES 3.5. and 3.6.
Units on the X-axis indicate the four hydrologic seasons of the
Gambia River: R-rising waters; F-annual flood; D-declining water and
L-Iow flow. The top line of each of each graph is the maximum, the
middle line is the mean, and the lower line is the minimum.

38
have cooler waters and higher salinities than ebbing tides. As a result,
the mixing process from tidal waves was considered a vital element in
maintaining the marine characteristics of the lower estuary.
Latitudinal variation was also considered an important factor in the
dynamics of the lower estuary. Large differences were observed between
samples collected at four different locations across the river.
Nitrate-nitrogen concentrations occasionally dropped to almost zero along
the edges of the river. These nitrate-depleted waters were attributed to
the activity of the mangrove ecosystems. On the ebbing tide, the water
drained from the mangrove bolons and flowed along the edges of the
river. Thus comparison of the samples between the middle of the river
and the edges of the river indicated they were usually significantly
different. These factors combined to give the lower estuary region a
fair degree of spatial heterogeneity.
The marine characteristics and heterogeneity of the lower estuary
zone provided a very suitable habitat for coastal marine species. In
many regards the lower estuary zone could be considered a tongue of the
ocean extending inland about 50 km. However, the lower estuary zone is
substantially more productive than many coastal areas due to the influx
of organic detritus from the mangrove forests. The fauna of this region
was typically marine throughout the year. The plankton, invertebrates,
and fish species were those commonly found in the coastal ocean. Marine
phytoplankton and zooplankton were present and abundant at all times of
the year. Invertebrates such as sea urchins, cuttlefish, shrimp, crabs,
and starfish were commonly found in the lower estuary zone (van Maren,
1985). The juvenile and adult fish community was also composed
principally of species typical of the coastal ocean. The number of
species and overall biomass of fish captured in the lower estuary zone
were greater than for any other zone in the river. Species from diverse
habitats and trophic levels were prominent. Pelagic planktivores were
represented primarily by sardines (Sardinella maderensis) ; bonga
(Ethmalosa fimbriata); and lefflefo (Ilisha africana). Mid-water
piscivores/omnivores included: drums (Fonticulus elongatus,
Pseudotolithus senegalensis, and P. brachygnathus); borama (Pentanemus
guinquarius); and kujalo (Polydactylus quadrifilis). Among fish
considered demersal detritivores/omnivores, several catfish species

39
(Arius latiscutatus, A. houdeloti, A. mercatoris, and Galeichthys
feliceps), and shine nose (Galeoides decadactylus) were dominant.
The fish catches in the lower estuary were by far the largest of any
of the five zones of the the Gambia River (see sections 3.2 and 3.3).
This was also true for the penaeid shrimp catches. Levels of
phytoplankton biomass and rates of primary production were not especially
high, indicating a detritus-based food web (Twilley, 1985). The large
number of detritivore and scavenging species (such as crabs and
bottom-feeding fish) support the detritus-based food web concept. A rich
and constant source of organic matter required to support this food web
enters the river from the semi-diurnal flushing of the mangrove bolons
and banks.
3.2.2. Upper Estuary Zone
The physical, chemical, and biological characteristics of the upper
estuary zone stand in contrast to the lower estuary zone. While both
sections of the river contained brackish water during the year and were
surrounded by mangrove forests, similarities between the zones ended with
these two points. The major distinctions between them included a rather
different river morphometry, highly seasonal dynamics of the upper
estuary, and a different flora and fauna.
The primary sampling site for the upper estuary zone was located just
downstream from Elephant Island, about 155 km upstream from Banjul (see
Figure 3.1). The river at this location occupied a relatively deep
channel which was surrounded by extensive, luxuriant mangroves. The
sides of the river channel were steep, dropping to 8 to 10 m deep, less
than 10 m from the mangroves. The depth of the channel gradually
increased to a maximum of 18 m. The main river channel was approximately
500 m wide just below Elephant Island. The morphometry at the sampling
site appeared typical of much of the entire upper estuary zone. In a few
locations mud banks were visible along the edges of the river on the
"outside" of bends in the river.
The river had a very large number of meandering mangrove bolons.
These bolons were large, ranging from 2 to 15 km long. The extremely
level terrain along the river allows the mangroves to extend up to 3 k.m
back from the river. The exchange of materials, especially detritus,

40
between the bolons· and the river was extremely large (Twilley, 1985).
Behind the mangroves a large series of floodplains extended up to another
1 km from the river. During the rainy season, these plains were flooded
at high tide with between 0.25 and 0.5 m of water.
The dominant physical force in the upper estuary zone was tidal
mixing. The river had the same semi-diurnal pattern of tides as Banjul,
but was over 7 hours later at Elephant Island compared to Banjul. Tidal
amplitudes were approximately 1.2 m at Bai Tenda, about 0.5 m less than
Banjul. Flushing due to tides was extensive, and surges we re observed
running the full length of the bolons and out onto the floodplains behind
the bolons. Tidal and river currents in the main channel produced
sufficient mixing to keep the concentrations of dissolved substances and
temperature uniform throughout the water column. During spring tides,
currents often exceeded 3.0 km/hr. These currents served to mix a great
deal of soft sediments into the water column which was observed as high
suspended solids concentrations. Currents also scoured the sides and the
soft, silty bottoms of the bolons to stir up the mud.
The seasonal aspects of the upper estuary zone were composed of two
factors, the annual thermal cycle and the streamflow pattern. The
thermal cycle was tied primarily to air temperatures. The upper estuary
zone was evidently far enough from the ocean that there was no thermal
effect from the North Equatorial Current. Temperatures declined almost
monotonically from the rising water field trip (July), to the low water
field trip (March). The thermal range was from a mean of 30.2° C to
24.6° C, respectively.
Whereas the thermal cycle in the upper estuary zone influenced
various processes such as respiration rates, the most important annual
cycle was streamflow. The increased streamflows of the Gambia River
during the annual flood reduced the salinities at the upper estuary site
from 13.7 ppt in July to 0.2 ppt in October (Figure 3.7). Salinities
then increased to 2.2 ppt by December and to 11.2 ppt by March. These
changing salinity levels placed an enormous amount of stress on the
aquatic flora and fauna of the upper estuary zone. Low but measurable
salinities between 0.5 and 12 ppt are very difficult for many aquatic
species to colonize; these waters are often characterized by low species
diversity (McLusky, 1971). Along with the annual cycle of salinity

42
LEGEND FOR FIGURE 3.7.
Units on the X-axis indicate the four hydrologic seasons of the
Gambia River: R-rising waters; F-annual flood; D-declining water and
L-low flow. The top line of each of each graph is the maximum, the
middle line is the mean, and the lower line is the minimum.

46
LEGEND FOR FIGURES 3.8. and 3.9.
Units on the X-axis indicate the four hydrologic seasons of the
Gambia River: R-rising waters; F-annual flood; D-declining water and
L-low flow. The top line of each of each graph is the maximum, the
middle line is the mean, and the lower line is the minimum.

47
large gradients in small distances (Berry et a1., 1985), movement of
water along the river produced significant changes in physical and
chemical conditions. Furthermore, the mangrove ecosystems had a dramatic
effect on the chemical conditions of the water. During the ebbing tides,
water from the mangrove bolons entered the river channels and mixed with
river water to produce new chemical conditions.
In the upper estuarine reaches, an area of large salinity
fluctuations, only a restricted number of marine animals persist. Those
animals either tolerate a wide range of salinities or use strategies to
escape from low salinity. Examples of each class of these animals are
juvenile penaeid shrimp and polychaet worms, respectively. Freshwater
invertebrates, mainly represented by lakefly larvae (Nematocera and
Chaoborida), were found only during the annual flood. The bulk of
invertebrate benthos of the upper estuary zone was composed of true
brackish water species that find their physiological optimum somewhere in
the range between freshwater and seawater. Examples of these species
include the periwinkle Tympanotonus fuscata, the shrimps Crangon and
Palaemonetes and the marsh crabs Sesarma spp.
The plankton and nekton (small floating plants, animals and swimming
animals) moved into and out of the upper estuary zone with the changes in
salinity. During the high salinity months (February through July), a
diverse and rich estuarine community was found. This community was
composed of:
• marine phytoplankton and zooplankton;
• shrimps;
• crabs;
• jellyfish;
• coastal fish species.
Artisanal fishermen also moved into this region following the shrimp
migrations.
Planktivorous bonga (Ethmalosa fimbriata) and lefflefo (Ilisha
africana) were prominent but not nearly as abundant in the upper estuary
as in the lower estuary zone. A drum (Fonticulus elongatus) was
relatively abundant throughout the water column. Ninebone (Elops
senegalensis) and kujalo (Polydactylus quadrifilis) were other
wide-ranging omnivores. In the demersal habitat, catfish of four

different genera
present and a
(Arius,
solefish
numerous. Overall, species
than in the lower estuary.
48
Chrysichthys,
(Cynoglossus
diversity in
Synodontis, and Schilbe) were
senegalensis) was relatively
the upper estuary zone was less
Many of the coastal marine species appeared to use the upper estuary
as a spawning and/or nursery ground. Larval and juvenile forms were
abundant in many of the mangrove bolons (Dorr et a1., 1985; van Maren,
1985). The floodplains are a probable site of active fish spawning
(Welcomme, 1979). But, the meager rains of the 1983 rainy season
prevented any sustained inundation of the floodplains and subsequent use
as a nursery area.
The upper estuary zone was similar to the lower estuary in that the
food chain was apparently detritus-based. Rates of primary production
were low and the euphotic zone (portion of the water column with
sufficient light intensity to support algal photosynthesis) was
relatively small, about 1.0 to 1.5 m below the surface. Bacterial
metabolism rates were extremely high, especially near the river bottom
(Healey et a1., 1985). The high inputs of mangrove detritus was a rich
and constant source of organic material for the bacteria (Twilley,
1985). Many of the invertebrates and fish found in the upper estuary
were scavengers or detritivores.
3.2.2.1. Mangrove ecosystems. The importance of the mangrove
ecosystems to the ecology of the upper estuary zone became evident early
in the study. As a consequence of this perceived importance, a separate
investigation of mangrove ecosystems was carried out. The main purpose
of the mangrove study was to determine the exchange of materials between
the river and mangrove bolons in the upper estuary zone, as well as
partially to characterize the dynamics within the bolons.
The mangrove ecosystems of the Gambia River extend from the river's
mouth to the extreme extent of saltwater penetration, about 250 km
upstream, near Kuntaur. The mangrove forests along the Gambia River are
composed of up to seven species of trees in three different genera. Each
mangrove genus has its own salt tolerance, thus the species composition
of the forests vary with distance upstream. Those near the ocean are
somewhat stunted and sparse because of hypersalinity on the mud flats
(hypersalinity is a condition where the salt content of the water

49
significantly exceeds that of seawater, due to high rates of
evaporation). The mangroves near the upstream limit of saltwater
penetration grew only in small isolated clumps at the river's edge. The
mangroves near Bai Tenda were by far the most luxuriant growing in dense
stands with streamside specimens over 30 m tall; the primary sampling
location for the mangrove study was among these luxuriant stands. There
were two major reasons the Bai Tenda location was chosen as the study
site. First was the ability to combine the mangrove study with the
ecological study of the upper estuary zone. Second was that this section
of the mangroves along the Gambia River will be eliminated by the
construction of the salinity barrage.
Along the Gambia River near Bai Tenda, mangroves typically border the
bo10ns and main river channel. The bo10ns are extremely convoluted and
up to 15 kIn long. Tidal scouring keeps the bo10ns relatively open with
channel depths 1 to 5 m and 5 to 30 m in width. The narrowest bo10ns are
often totally overgrown by arching branches of Rhizophora racemosa.
The Bai Tenda area bo10ns were flushed twice per day by tidal waves
that move so rapidly up them that a distinct surge can be observed. When
water levels reach three-fourths of high tide level or higher, the mud
flats where the mangroves grow became inundated. The mangroves stands
often extend several hundred meters back from the bo10n bank. The bo10ns
near Bai Tenda were so numerous as to produce a continuous forest that in
some places extend up to 3 kIn away from the river. Farther downstream
the forests extended up to 10 kIn from the river. The mangrove forests
were not continuous in the lower estuary zone because the salt pans or
mud flats covered with hypersaline water caused numerous openings among
them.
The mangrove ecosystems were very active biologically and as a
result, had a significant effect on most water quality parameters. These
effects were investigated by the following experimental design:
• at different phases of the tide (high water, ebb, low
water, flood);
• different times of the day (day, dusk, night, dawn).
samples were collected in the middle of the main Gambia River channel and
at up to five different locations within a mangrove bo10n. Samples were

50
then compared at one location over time, or at one time among several
locations. Net changes in the value of each water quality parameter
emerged from these comparisons.
In the main river channel and at the bolon mouth, changes in most
variables over one tide cycle were small (Figures 3.10 and 3.11). In
contrast, changes over the same period several kilometers up the bolon
were large. For example, alkalinity increased over 70% between a daytime
high water and the following nighttime low water. Changes in
alkalinities at the bolon mouth were trivial. A similar pattern was
observed for conductivity and pH (Figure 3.10). Conductivities in the
upper reaches of the bolon increased about 30% between the daytime high
water and nighttime low water, while pH shifted about 0.2 units. Both of
these variables did not change significantly either in the middle of the
Gambia River or at the bolon mouth adjacent to the main river.
Dissolved nitrate-nitrogen underwent an enormous change between high
and low water in the bolon system. In the upper reaches, nitrate
concentrations dropped from 200 ug/t at high tide to 20 ug/t at low tide
(Figure 3.11). Nitrate levels were unchanged, both in the middle of the
river as well as at the bolon mouth. Silica concentrations in the upper
reaches of the bolon showed a large increase between high and low tide
(Figure 3.11). These large, rapid changes in nutrient concentrations in
the upper reaches of the mangrove bolons indicated an extremely active
biological community. The uptake and release of essential elements for
growth are attributed to the mangrove trees themselves and/or the
associated flora and fauna of the mangrove ecosystem.
Biological samples indicated a large and active community growing in
the mangrove ecosystem. The algal population was extremely rich; the
highest chlorophyll levels of the entire one-year study were from the
mangrove bolons. The highest rates of primary production by
phytoplankton were measured in association with these high chlorophyll
concentrations. A large and metabolically active bacterial population
was observed in several bolons. Palaemonetes shrimp, marsh crabs, and
mollusks are among the most abundant permanent inhabitants of the
mangroves. Animals that spend part of their life cycle in this
environment are the crabs (belonging to the genus Callinectes) and the
pink shrimp (Panaeus duorarum) •

Units
channel.
51
LEGEND FOR FIGURES 3.10. and j.11.
on the X-axis are meters up the bolon from the main river
All samples were collected in one 24-hour period.

56
Mangrove bo10ns play a very important role during the development of
the shrimp. While low salinity seems not to be a requirement of the
young pink shrimp, the food and protection from predators offered by the
mangroves are key factors for their growth and survival. The juvenile
shrimp feed on detritus (fallen mangrove leaves) enriched by bacteria and
fungi growing on them. Shelter is provided by the intricate roots of the
mangrove trees standing in the water. The coating of decomposing
microorganisms on the leaf particles from the mangroves provides these
particles with a high protein/carbohydrate ratio, so that they have a
high nutritive quality. The mangrove leaves form the base of a detrital
food web that includes, at various trophic levels, the majority of fish
and invertebrates in the estuary. Therefore, shrimp yields and the local
area in mangrove forests are positively correlated (Snedaker, 1978).
The abundance of the mangrove oyster Crassostrea gasar living on the
proproots of the mangrove trees is directly related to the amount of
mangrove root area available. There is no reason to expect that the
permanently high salinities of the lower estuary mangrove bo10ns impair
oyster growth. But at high salinities, oysters are subjected to more
predation and greater parasitism by animals that are normally eliminated
by reduced salinities. Gunter (1955) found heavy mortality of oysters
caused by the oyster borer Thais and the stone crab Menippe in an
estuarine area where salinities had increased due to a prolonged
drought. Both the oyster borer and stone crabs are found in the lower
reaches of the Gambia River estuary. Furthermore, the crab Ca11inectes
is known to prey heavily on oysters (Lunz, 1947).
Fish standing stocks were abundant. A few species of fish appeared
to reside in the mangrove bo10ns rarely emerging into the main river
channel. A diverse fish community was supported in the mangrove bo10ns.
The most numerous of the species captured was the catfish Chrysichthys
nigrodigitatus, a voracious omnivore which probably invaded bo10ns
primarily in search of food. A mud skipper (Porogobius sch1ege1i) and a
c1upeod (Pe110nu1a vorax) were also extremely abundant. A cyprinodont
(Ap10chei1ichthys normani) was numerous, and a mullet (Liza fa1cipinnis)
was common.
Results of field sampling combined with hydrologic data provide a
generalized sequence of events concerning the mangroves in the Gambia

57
River estuary. Areal net primaru productivity from the mangroves,
especially streamside Rhizophora spp., esceeds in situ algal productivity
by almost a factor of 10. This productivity is flushed by tides from the
floor of the mangrove forests into small mangrove bolons (creeks) and
from there into the main river channel. Once immersed in warm water, the
mangrove detritus begins to degrade rapidly, enriching estuarine waters
with both organic and inorganic nutrients. This material is covered with
layer of microorganisms which are constantly remineralizing the nutrients
in the detritus. While tidal action keeps the detritus suspended in the
water column and surging back and forth along the river channel,
streamflow in the river provides a net downstream drift. As the detritus
and nutrients slowly move downstream, inputs from other mangrove forests
continue to enrich the estuarine waters.
The constant and rich source of mangrove detritus serves as the
primary source of energy for the estuarine fauna. The influx of material
stimulates a food web which feed on those organisms. It is this rich
fauna that is the primary distinction between nearshore oceanic and
estuarine waters. Furthermore, researchers in West Africa believe that
the flux of organic material out of mangrove-lined estuaries serves to
stimulate coastal fisheries as well. As a result, mangrove ecosystems
have been characterized as one of the most productive aquatic habitats in
existence (Snedaker 1978). That productivity appears to extend beyond
the geographic boundaries of the mangrove estuaries themselves.
3.2.3. Lower Freshwater River Zone
The lower freshwater portion of the Gambia River is characterized as
a large, slow-flowing river that has distinct tidal mixing. This zone
was almost as long as the two estuarine zones combined extending from
Kuntaur at 250 km upstream to Gouloumbou at 510 km upstream. The primary
sampling site for this zone was about 3 km downstream from Bansang, which
was 310 km upstream from Banjul (see Figure 3.1).
At the primary sampling site, the river was between 100 to 150 m
wide. The banks were relatively steep, beginning about 2 m above the
water's surface at high tide and dropping to 3.5 or 4 m below the river's
surface. The river bottom was relatively flat with water depths varying
from 3.5 to 5 meters. The bottom was composed of a mixture of soft sand
and firm mud.

58
Tidal forces were very much evident at the Bansang sampling site with
the same semi-diurnal pattern of tides as observed at Banjul. Tidal
waves took approximately 14 hours to proceed upriver from Banjul to
Bansang. Tidal amplitude was reduced to approximately one meter, or
about half that at Banjul. Tidal currents were much weaker than in the
estuarine zones. The maximum current observed was not much greater than
l.0 km/h. Asymmetry existed in tidal currents during the flood season
field trips. The ebbing tide produced a current which was about twice
the speed of the flooding current; the flood season streamflow added to
the ebb tide current.
A distinct seasonality was observed in the lower river which was
controlled almost entirely by the annual flood. The only seasonal cycle
that was not under the direct influence of the flood was the thermal
cycle. Water temperatures followed air temperatures and decreased
monotonically from an annual maximum close to 31° C in July to a minimum
of 26° C in March (Figure 3.12).
All the other variables displayed a seasonal trend that was tied to
the annual flood. Because seawater intrusion did not occur in the lower
river zone, the seasonality associated with the annual flood was either
due to different chemical conditions in runoff during the rainy season,
or to concentration of dissolved materials by evaporation during the dry
season. Conductivity and alkalinity were especially tied to the
hydrologic cycle in the river. The flood waters of the Gambia River had
very low conductivities and alkalinities. A large drop was observed in
the amount of alkalinity in the lower river zone between the July and
October field trips (Figure 3.12). After the peak of the flood had
passed, alkalinities slowly increased toward their mid-summer annual
maxima. Conductivity followed the same seasonal trend (see Figure 3.12).
The trend in pH was somewhat the opposite, with values increasing
steadily from the observed mean low of 7.0 in July to the highest mean
level of 7.7 in March. Figure 3.12 shows that pH and alkalinity were not
totally coupled because the curves are not mirror images of one another.
This indicated a contribution to alkalinity other than dissolved carbon
dixoide. It is possible that the effect of evaporation and very low
streamflows during the dry months concentrated certain dissolved
materials which in turn affected pH. Percentages of dissolved oxygen

59
also increased monotonically between July and March; this could also have
a very minor effect on pH values.
Soluble nutrient concentrations, in particular soluble reactive
silica and nitrate-nitrogen, had seasonal cycles which were closely tied
to the annual flood. Runoff during the rainy season was enriched with
nitrogen and a pulse of high nitrate runoff was observed moving down the
Gambia River beginning at Kedougou in June (Berry et al., 1985). By the
time of the first field trip at the lower river sampling site, nitrate
concentrations were somewhat elevated over dry season levels (Figure
3.13). Concentrations continued to increase until they reached a maximum
which coincided with maximum streamflows. After the flood passed,
nitrate levels fell downward to the limit of analytical detection and
remained at these low levels throughout the dry season. Soluble reactive
silica concentrations showed a somewhat opposite pattern beginning at low
concentrations in July, increasing by the October field trip, and
remaining high throughout the dry season (Figure 3.13). Based on the
silica results, one would conclude that the flood waters had not arrived
in the lower river by July because silica levels remained low until
October. In contrast, the nitrate results indicated that flood waters
which were enriched with nitrogen had already arrived at Bansang by
July. Of course, it is possible that the nitrate enrichment observed in
July was due to local runoff, and not from the major annual flood.
Soluble reactive phosphorus concentrations declined between July and
March but were highly variable within anyone field trip (Figure 3.13).
The lower river zone had one major factor that was unique compared to
the other zones. A large and relatively productive phytoplankton
population was found dUring the July field trip. Average chlorophyll
concentrations were 12.7 ug/L, which was four to £ive times higher than
any other average values. The chlorophyll concentrations dropped to an
average 2.3 ug/L by the October field trip, a level commonly found
throughout the rest of the river. This large plankton population was
also very productive yielding the highest rates of photosynthesis found
throughout the river over the one-year study. Overall, even with the
presence of this large phytoplankton population, the river was considered
dominated by respiration and not photosynthesis. Plankton cell counts

62
LEGEND FOR FIGURES 3.12. and 3.13.
Units on the X-axis indicate the four hydrologic seasons of the
Gambia River: R-rising waters; F-annua1 flood; D-declining water and
L-low flow. The top line of each of each graph is the maximum, the
middle line is the mean, and the lower line is the minimum.

64
showed this phytoplankton population was primarily composed of the diatom
of the genus Melosira.
Enrichment of the river water by nitrate may have promoted an algal
bloom during the early rainy season. The area of high chlorophyll
concentrations extended 50 km downstream from Bansang. A second area of
high algal biomass was observed in the upstream section of the lower
estuary (Figure 3.14). A similar but smaller area of high algal biomass
was found during the March field trip near the Kuntaur.
Inferences from the Latin Squares ANOVAs indicated the continued
importance of tidal mixing, even though the sampling site was more than
300 km from the ocean. Analysis of the July field trip results showed
that the tide/time of day factor significantly affected all of the
physical and chemical variables (Berry et a1., 1985). In other words,
the mean value of each of the fourteen variables was different, depending
upon stage of the tide (ebb or flood) and/or time of day (day or night)
when the samples were collected. The importance of this factor
diminished throughout the four field trips until March when only eight of
the fourteen variables were significantly affected by tide/time of day.
The only other factor which had any important effect on the physical
and chemical variables was depth of sample. During the last two field
trips, when streamflow was low, vertical stratification developed at the
lower river sampling site. The March results showed a moderate degree of
thermal stratification (about 0.5 0 C) from top to bottom, and some minor
accompanying vertical stratification of conductivity, suspended solids
and soluble reactive silica.
The benthic invertebrate fauna of the lower river zone resembles
that of lakes (van Maren, 1985). This fauna included burrowing mayfly
and odonate nymphs, lakefly larvae and some mollusks commonly found in
African lakes. But the presence of polychaet marine worms in the bottom
mud indicated that there was still some saltwater influence at least in
the interstitial water of the lower river.
The other portions of the biological community of the lower river
zone were relatively sparse. The plankton community was a freshwater
flora and fauna with relatively low biomass except for the first field
trip. Standing stocks of fish were relatively low. Bottom feeding
catfish (primarily Schilbe mystus, Chrysichthys nigrodigitatus,

67
Water temperatures declined from a mean annual maximum of 32.2° C in July
to a minimum of 23.5° C in December (see Figure 3.15), the largest annual
range of all five zones.
Seasonality in all other variables was tied to the hydrologic cycle.
Conduc tivity, alkalinity and soluble reactive silica displayed the same
pattern as was found in the lower river zone. Conductivity and
alkalinity declined from annual maxima in June to annual minima in
October (Figure 3.15). Concentrations then gradually increased as the
flood receded. The separation of alkalinity and pH was evident because
the two curves did not resemble one another. Diel variability in pH was
substantial and thus masked seasonal trends.
Concentrations of particulate materials were especially tied to the
flood and dry cycle. Suspended solid concentrations were high and
variable during the flood, but declined to almost zero in December
(Figure 3.15). Chlorophyll concentrations were the opposite of suspended
solids, increasing as the particulate material disappeared from the water
column.
Dissolved nitrate-nitrogen and soluble reactive silica displayed the
same pattern of flood season enrichment as in the lower river zone.
Nitrate concentrations were highest in the flood, giving credibility to
the hypothesis that nitrate enrichment originated from the initial runoff
of the first rains. Silica, on the other hand, had low concentrations
during the early rainy season but increased steadily throughout the year
(Figure 3.16).
Time of day had a large effect in determining the observed
concentrations of most variables as indicated by the Latin Square
ANOVAs. The lack of tidal fluctuations would appear to relegate the time
of day effect to diel variability alone. But, the time of day factor
actually had two sources of short-term temporal variation: diel and
periods of several days. This latter source was dominant during the two
rainy season field trips. A large rain storm occurred at Kedougou during
the first field trip between the day sampling on June 25 and the dawn
sampling on June 26. Water levels in the river increased rapidly about
0.2 m after the rain. During the second field trip in October, a rain
event had occurred just before sampling began, after which water levels

70
LEGEND FOR FIGURE 3. 15.
Units on the X-axis indicate the four hydrologic seasons of the
Gambia River: R-rising waters; F-annual flood; D-declining water and
L-low flow. The top line of each of each graph is the maximum, the
middle line is the mean, and the lower line is the minimum.

72
LEGEND FOR FIGURE 3.16.
Units on the X-axis indicate the four hydrologic seasons of the
Gambia River: R-rising waters; F-annual flood; D-declining water and
L-low flow. The top line of each of each graph is the maximum, the
middle line is the mean, and the lower line is the minimum.

73
fell more than 1.0 m during the two and one-half days of sampling that
followed.
Many variables showed large changes after a rain event or immediately
after the rain, compared to several days after a rain (Tables 3.5 and
3.6). During the first field trip, pH decreased 0.8 units after the rain
while the dissolved oxygen saturation decreased more than 30%. Soluble
reactive silica concentrations increased about 15%. Dissolved nitrate
dramatically increased from below the limit of detection to almost 80
ug/L.
Similar trends were observed during the second field trip.
Streamflow rapidly declined between the first (day) and last (dusk)
sampling periods (Table 3.6). Both soluble and total nitrate, and
soluble and total phosphorus decreased by about 50% over the two and
one-half day sampling period. Soluble reactive silica increased about
15% during the same period. Suspended solids decreased more than 75%
during the two and one-half day span. These results emphasize the
extreme importance of rain events on the chemical characteristics of the
upper river. Not only do concentrations of many substances increase
immediately following a rain event, but because streamflows are elevated,
total loadings (amount of material added to the river) are vastly
increased.
Time of day variation still persisted
trips, but was primarily associated with
Diel trends were observed for water
insolation.
probably as
respiration.
during the two dry season field
photosynthesis and respiration.
temperature which was due to
Patterns were also observed for pH and dissolved oxygen,
a consequence of daily photosynthesis and nocturnal
The biological community of the upper river zone was typical of
streams and small rivers. The Gambia River near Kedougou offers a
variety of microenvironments which are reflected in the presence of a
diversified benthic fauna. The predominant invertebrate groups are
characteristic of moderately to fast-running waters. The stonefly
(Neoperla spio) was particularly abundant in the rapids. Blackfly larvae
were found in this same type of habitat, mainly on the dead leaves
trapped under stones. The bottom fauna of the stagnant and slowly

74
TABLE 3.5.
MEAN VALUES OF PHYSICAL-CHEMICAL VARIABLES
COLLECTED AT KEDOUGOU DURING THE RISING WATERS FIELD TRIP
(June 1983)a
June 25 June 26 June 26
Variables (day) (dawn) (dusk)
Temperature (C) 32.5 31.6 31.9
Conductivity 89.1 89.3 101.0
Dissolved Oxygen (mg/L) 8.73 6.68 8.10
Dissolved Oxygen (% sat) 120.7 90.0 111.3
pH 7.75 6.50 6.95
Alkalinity (mg/L) 40.0 39.3 42.8
Silica (mg/L) 8.2 8.4 9.6
Nitrate (ugN/L)

75
TABLE 3.6.
MEAN VALUES OF PHYSICAL-CHEMICAL VARIABLES COLLECTED AT
KEDOUGOU DURING THE FLOOD WATERS FIELD TRIP
(September 1983)a
Variables
Temperature (C)
Conductivity
Dissolved Oxygen (mg/L)
Dissolved Oxygen (% sat)
Ch1orophy11-a (ug/L)
Phaeo-pigments (ug/L)
Phaeo-fraction (%)
pH
Alkalinity (mg/L)
Suspended solids (mg/L)
Silica (mg/L)
Phosphate (ugN/L)
Nitrate (ugN/L)
Total phosphorous (ugP/L)
Oganic phosphorous (ugP/L)
Total nitrogen (ugN/L)
Organic nitrogen (ugN/L)
Sept. 29
(day)
26.60
35.5
7.62
94.8
0.54
0.97
64.
6.91
18.1
143.9
11.1
1.2
12.0
66.4
65.2
534.
522.
Sept. 29
(night)
26.50
48.0
7.41
92.3
0.30
0.76
72.
6.96
17.0
171.8
11.5
2.0
9.0
72.1
70.1
551.
542.
Sept. 30
(dawn)
26."25
40.3
7.44
93.3
0.27
0.51
65.
7.33
19.9
52.2
13.2
1.2
1.6
35.3
34.1
262.
260.
Sept. 30
(dusk)
27.00
36.0
7.12
88.9
0.21
0.49
70.
6.94
19.8
38.5
12.6
1.1
7.9
30.8
29.7
267.
259.
NOTE: a) Gambia River streamf10ws declined significantly between the
day and dusk sampling periods.
University of Michigen, G-.ble River Be.iD Studie., 1985.

76
running parts of the upper river was primarily composed of the nymphs of
different mayflies.
In the upper river zone, fish species diversity was high relative to
other zones, but standing stock in terms of biomass was low. The most
abundant species in the upper river zone were small generalists. In the
riffle portions of the river, two alestids (Hemigrammopetersius
septentrionalis and Brycinus longipinnis) were very abundant. In slower
flowing waters, Pellonula vorax and Brycinus nurse were numerous. In
deep pools and backwaters, several species of catfishes were captured
including Synodontis gambiensis, Chrysichthys nigrodigitatus, and Schilbe
mystus. Cichlids (predominantly Tilapia occidentalis) were also an
important ecological component in the upper river zone.
3.2.5. Headwaters Zone
The headwaters zone is the dendritic network of small rivers and
streams that compose the source of the Gambia River. This zone is
contained entirely in the Fouta Djallon Mountains of northern Guinea (see
Figure 3.1). The headwaters (of the Gambia River) end at an escarpment
that is located close to the Guinea-Senegal border, approximately 970 km
upstream from Banjul.
The two distinguishing features of the upper portion of the Gambia
River are the relatively small size of the streams, and the sharp
elevation drop through the mountains. The primary sampling site for the
headwaters zone was about 15 km from Balaki. At this site the river was
about 30 m wide and between 0.5 and 3.0 m deep. The main river was
composed of a series of rapids and pools, with the pools from 20 to
3,000 m long. During the late dry season (April through early June),
water only occassionally flows over the rapids and among the pools. At
the primary sampling site, the river runs through a distinct gorge with
banks between 5 and 10 m high. The bottom of the river and lowest meter
of the gorge walls are scoured to bed rock, which is slate. Above the
slate, the walls are mostly loose rock and lateritic gravel. The slate
bottom of the river is highly slabbed.
In the headwaters zone, the Gambia River falls rather rapidly in
elevation with an average drop of 4.2 m/km (Humphreys, 1974). At
Kedougou in the upstream end of the upper river zone the elevation drop

77
is 1.1 m/km, whereas in the vicinity of Gouloumbou the river falls only
0.02 m/km.
The Gambia River was characterized by the same hydrologic cycle as in
the upper and lower river zones. Similar to the other two freshwater
zones, the thermal cycle was not totally dominated by the annual pattern
of wet and dry seasons. The thermal cycle was comparable to the upper
river zone with average water temperatures ranging from 30° C in June to
22° C in December.
A seasonal cycle in water chemistry was evident, but the annual range
was much smaller than the other zones. The primary characteristics of
the river in the headwaters zone was a dilute and relatively stable
aquatic medium. Alkalinity and pH varied very little over the year. The
pH was usually just slightly basic at between 7.20 and 7.84. Diurnal
trends in pH were minimal, with a shift of only 0.1 to 0.2 units between
early morning and evening. Conductivities showed seasonality by
increasing from 30 to over 100 umho/cm from December to June. During the
June field trip, a rain event upstream of the sampling site caused
streamflows to increase from 0 (no flow) to about 40 cubic meters per
second in less than four hours. Conductivities dropped from 108 to 72
umho/cm overnight between the nonflowing and flowing waters conditions,
respectively.
Dissolved nutrient concentrations were relatively consistent
throughout the year, although a small rainy season to dry season cycle
was evident. Dissolved nitrate-nitrogen concentrations were very low,
often at or below the limit of analytical detection. During December,
nitrate concentrations were low but measurable at about 0.05 mg/L. By
June the concentrations were below 0.01 mg/L but increased to 0.05 mg/L
after the river began to flow.
Soluble reactive phosphorus and silica displayed the same pattern.
Concentrations were highest in December (about 15 ug/L for phosphorus and
10 mg/L for silica). Concentrations for both of these variables dropped
to their annual lows before the rain event in June (about 4 ug/L for
phosphorus and 7 mg/L for silica).
The river waters of the headwaters zone were clear with Secchi disc
depths of over 2.0 m. Despite this clarity, the river appeared
unproductive. Algal photosynthesis was extremely low as measured both by

79
sustainable yields were not made because of insufficient long-term catch
data. The true economic value of the artisanal fisheries was difficult
to assess because the product either was used as barter or moved to the
marketplace without inclusion in government surveys. Nonetheless,
estimates of economic values were made from available data and presented
in Chapter 6. The value of the artisanal fisheries was also couched in
terms of employment provided within the basin.
Much of the material presented below was summarized from six reports
which were produced as part of the Gambia River Basin Studies. The
primary source of material was from Josserand (1985), which included a
three-week survey of artisanal fisheries in The Gambia and Senegal.
Supporting material came from reports by Josserand et ale (1984);
Saidykhan, (1984); Dorr et ale (1985); Moll et ale (1984); and van Maren
(1985). The authors of these documents drew heavily upon the statistics
of fish catches provided by the Fisheries Department, Gambian Ministry of
Water Resources and the Livestock Service, Senegal.
3.3.1. Artisanal Fisheries
3.3.1.1. Gambian Atlantic coast. A very active artisanal fishing
community exists along the Gambian coast. The entire coast has numerous
small fishing fleets composed of motorized canoes which are generally
between 7 and 10 meters long. The largest and most active community is
centered near Brufut. Upwards of 600 canoes operate along the coast
employing as many as 3,000 fishermen and their helpers (Josserand,
1985). The artisanal fishermen caught slightly more fish between July
1982 and June 1983 than did the commercial fishing companies; 8,116
metric tons were taken by artisanal fishermen versus 7,275 metric tons
for commercial harvest (Josserand, 1985). The fish catch was
diversified (Table 3.7) and included pelagic fish, demersal fish and
crustaceans. The dominant catch by far was bonga (Ethmalosa fimbriata),
accounting for 5,271 of the 8,116 metric-ton catch (Josserand, 1985).
Approximately two-thirds of the coastal artisanal fishermen in The
Gambia were foreigners. These were primarily long-term residents, which
in many ways could be considered Gambians. Some of the fishing was
seasonal with the Senegalese fishing along the coast during the dry
season after the crops had been harvested. The predominant method of
fishing was with gill nets and surround nets.

80
TABLE 3.7.
RECORDED MARINE ARTISANAL CATCHES BY
SPECIES FOR THE GAMBIAN ATLANTIC COAST,
JUNE 1982 TO JULY 1983
(Catch in metric tons)
Bonga
Catfish
Shark/skates
Lady fish
Cassava fish
Sompat
Barracuda
Jotor
Kujeli
Mullet
5,270.5
717.8
390.0
338.5
317.7
153.7
147.0
116.6
112.6
109.8
I
I
Sole
Mackerel
Sacca
Sardinella
Banda
Tapandarr
Shine nose
Grouper
Tilapia
Fotta
Snapper
65.3
49.6
46.7
38.5
35.8
34.4
31. 7
23.7
19.5
16.2
5.3
Lobster
Other
0.3
74.7
Total 8,115.9
SOURCE: Data from Ministry of Water Resources and
Environment, Fisheries Department. Table
taken from Josserand (1985).
Univers1ty of M1ch1gan. G_b1a R1var 1I..1n Stud1... 198L
The largest impediment to maintaining a successful fishery for most
of the coastal artisanal fishermen has been marketing. Once the
perishable fish have been landed on the beach and cleaned, the lack of an
efficient mechanism to transport the product to market was evident.
Processing of fish product was poor, often utilizing the wasteful
sun-drying technique. Until recently, the road network leading to the
fishing beaches was poor, but now has been vastly improved. But

81
coordinated transportation of fish products to market was still lacking
by early 1984. Experience in Senegal has shown that demand for fish
products in interior villages was very high, as long as the product can
be brought to market.
3.3.1.2. Gambian estuarine and river fisheries. This fishery was
considerably smaller than the coastal artisana1 fishery. The official
listed catch between July 1982 and June 1983 was 1,242 metric tons

83
The Gambian Fishery Department estimated the number of artisana1
fishermen working the estuarine and river fisheries at 1800: 300 in the
upper river and 1,500 in the lower river. Josserand (1985) has set this
total closer to 3,000 as a result of his field survey of the artisana1
fishing community. Both estimates included the 300 shrimp fishermen
associated with National Partnership Enterprises, Ltd. (NPE). The
discrepancy between the two estimates probably lies in the seasonal
nature of the occupation. Many artisana1 fishermen turn to farming
during the rainy season and back to fishing after the crops are harvested.
In the lower river, most of the fishermen were Gambians (90%), while
in the upper river they were predominantly foreign (70%) (see Josserand,
1985). These foreign fishermen were primarily Senegalese and Malian,
with representation from Guinea-Bissau. All of the artisana1 fishermen
tend to use similar techniques, irrespective of national origin. These
techniques include shifting from fishing in the main channel to the
floodplains and bo1ons, and back to the main channel, depending upon the
season. One segment of the artisana1 fishery that is totally untabu1ated
is the shellfish harvest. A significant bivalve population has been
observed growing on the proproots of many Rhizophora mangroves. This
fishery provides a substantial amount of local food to the lower river
communities. But it is not included in the fishery surveys. This lack
of inclusion may originate from the fact that oyster and other shellfish
harvests are conducted by women who work alone. These women often just
wade into the estuary, collect their harvest, and trade or sell it
locally.
The same problems of lack of marketing and wasteful processing that
plague coastal fishery plague the estuarine and river artisana1 fishery.
Road networks in The Gambia are rapidly improving, but this is primarily
in response to the groundnut industry. Roads from the main highway to
the river are usually in poor shape and deteriorate quickly during the
rainy season. Roads in the lower river section through the mangroves are
few and far between. While the fishermen know that fish catches have
declined over the past ten to fifteen years, they feel that marketing
problems remain the biggest impediment to expanding the artisana1 fishery
(Josserand, 1985).

85
Gambia or Senegal. Second, the fisheries were seasonal in nature and
thus contributed food to the local diet for only a portion of the year.
The Guinean artisanal fisheries were viewed as a minor resource which
would only be enhanced by the river basin development program.
3.3.2. Industrial Fisheries
The catch and economics of the industrial sector sector can be
separated into two categories, finfish and shellfish. Data on catch of
finfish in Gambian waters have been compiled by' the Fisheries Department
in The Gambia. But little information beyond that compiled by the Gambia
River Basin Studies is available on finfish catch in Senegalese or
Guinean portions of the basin, or on any aspect of shellfish fisheries.
The primary economic benefit received by the local economy from
industrial fishing is derived through foreign exchange and domestic
employment. The latter is mostly facilitated by artisanal fishermen
contracted to industrial fish processing and export companies. Economic
revenues from industrial fishing come from three general sources:
foreign vessel fishing fees, export taxes on fish products, and return of
some of the export value of the catch to the local economy.
Recent revenues from these sources, summarized in Tables 3.10 and
3.11, illustrate the relatively small portion of the FOB catch value that
is reflected back into the local economy. Additionally, most companies
tend to undervalue their catches and thus reduce export taxes. The
values presented in these tables are probably minimums and could be
considerably higher than those cited here.
While The Gambia exports a proportion of the fishery products taken
from its waters, it also imports a considerable amount of high-value fish
products. Approximately 600-700 metric tons or about 10% of the total
export volume is imported annually. Unfortunately, these imports are of
high-priced products and are equal to almost one-half of the value of the
exports. These imports include mainly canned, salted, smoked, or dried
high-value fish.
3.3.2.1. Domestic companies. The largest Gambian-owned fishing
company is National Partnership Enterprises, Ltd. (NPE) based in Banjul.
This company uses a variety of fishing techniques including contracting
of artisanal fishermen to exploit the pink shrimp (Penaeus duorarum) and

87
TABLE 3.11.
ECONOMIC VALUE OF FISH PRODUCTS (JULY 1982 TO JUNE 1983)
Company Vessel Fees Export Tax FOB Value
Seagull 500,000a 60,000b 600,000c,e
NPE-Shrimp n.d. 500,000d 2,500,000e
NPE-Demersa1 n.d. 150,000f 1,000,000e
Other Foreign
250,000a 150,000f 1,000,000e,g I
Total 750,000 860,000 5,100,000
SOURCE: Adapted from Josserand (1985) •
NOTES: a) Based on fee schedule in Table 3.6.
b) Fee schedule of 10% of FOB value.
d Based on an FOB value of 0.1 Dalasis/kg and an annual
catch of 6,000 metric tons.
d) Fee schedule of 20% of FOB value.
e) Data from Josserand (1984).
f) Fee schedule of 15% of FOB value.
g) Estimated as 10% of regional catch.
Unlver.lty of Mlchlgan, G_bla R1ver saal11 Studl.., 1985.

88
high-value demersal fish stocks (e.g., solefish). NPE processed about 15
metric tons of fishery products from July 1982 to June 1983, of which
about 227 metric tons were shrimp (van Maren, 1985). Between July 1983
and June 1984, the NPE-processed shrimp catch increased to 412 metric
tons (van Maren, 1985). Over 300 fishing canoes use stake nets and
ancillary fishing gear in the harvest of NPE shrimp. NPE-sponsored
shrimp and demersal finfishing employed upwards of 1,000 people in The
Gambia during peak fishing periods. An additional unquantified number of
people were employed in the fishing supply, service, and processing
sectors. The NPE catches are high-value products with an annual export
value of 2-4 million Dalasis. The fishing activities of NPE provide an
excellent source of foreign currency for The Gambia because about 95% of
its product is exported.
The other major industrial fishing company that is Gambian-owned and
also located in Banjul is the Fish Marketing Company (FMC). This is a
nationalized company established in 1977 to take over the premises of a
bankrupt Japanese firm (Josserand, 1985). FMC has the charge of issuing
export permits for fish products taken from Gambian waters. The company
also handles a small amount of fish products which are purchased from
artisanal fishermen for export. The volume of this product has ranged
from 400 metric tons to 102 metric tons per year. Most of the FMC
purchases have been from the marine coastal artisanal fishing sector. If
current plans for FMC are realized, it will become the dominant company
in the industrial fishing sector. Through a $20 million (U. S. dollars)
financing agreement with the African Development Bank and European
technical assistance, FMC will obtain an extensive fleet of trawlers, a
jetty, and processing facilities. FMC, to be renamed the National Fish
Marketing Board, plans to fish for sardines (Sardinella), bonga
(Ethmalosa fimbriata) and other marine coastal species. In addition, FMC
expects to purchase 30% of its total product from artisanal fishermen 'for
processing in its facilities which would represent a major increase in
economic benefit to the economy and people of The Gambia.
3.3.2.2. Foreign companies. The dominant foreign-owned company
fishing Gambian waters and operating out of Banjul is the Ghanaian-owned
Seagull Cold Stores, Ltd. This company paid approximately 78% of all
fees and taxes collected by the Gambian government from foreign fishing

89
concerns. Seagull catches also comprised more than 80% of the total
industrial fish harvest in Gambian waters during 1983 (Josserand, 1984).
Between July 1982-June 1983, Seagull processed 5,944 metric tons of the
total 7,275 metric tons of industrial fish product. Nearly all of this
product was sardines which were exported to Ghana because there is a
limited market for these fish. Seagull is a totally foreign-owned and
foreign-operated company employing considerable foreign labor. As a
result most of the benefit of this company's activities to The Gambia
comes from foreign exchange associated with fishing fees and taxes.
Up to eight smaller foreign fishing companies have fished Gambian
waters since the early 1970s. During July 1982-June 1983, the combined
fleet totaled eleven trawlers which took about 800 metric tons of fish.
This total represents about 11% of the total industrial catch from
Gambian waters. Josserand (1985) estimated the annual value of the total
catch in the trinational coastal waters of Senegal, The Gambia, and
Guinea to exceed 10 million Dalasis. But the actual value of the portion
of fish caught in Gambian waters by these companies was estimated at less
than one-tenth of their total catch in these West African waters.

4. IMPACTS
The ecological study of the Gambia River was combined with results
from river development programs in other African rivers to generate a set
of probable impacts for the Gambia River. These impacts cover a large
range of effects from strictly ecological impacts to those that arise
from human. activity along the banks of the newly formed reservoirs. The
presentation of impacts from the development of the Gambia River is given
below on a zone-by-zone basis. This approach was chosen because it was
consistent with the other portions of the study which also used a zonal
approach. Impacts from five zones are presented. These zones are:
• lower estuary
• upper estuary
• lower freshwater river
• upper freshwater river
• headwaters
Discussion of impacts by zones was used not only because it was a method
consistent with the other parts of the study, but also because the five
proposed dams affect each zone differently.
One of the major conclusions of the ecological study was that the
physical and chemical environment of the Gambia River is the major factor
which determines the nature of the aquatic flora and fauna. This
somewhat obvious conclusion has a major influence on the nature of the
impacts which should arise from river basin development. Alterations of
the physical and chemical environment will ultimately affect the entire
composition of the biota in the river and surrounding marshes. Given
this premise, the anticipated impacts to the Gambia River have been
categorized as: primary, secondary, and tertiary. primary impacts are
those which affect the physical and/or chemical environment, e.g. a
change in the salinity regime in the estuary from the salinity barrage.
Primary impacts generate secondary impacts which are changes to the
riverine biota, e.g., elimination of estuarine shrimp upstream from the
salinity barrage. Tertiary impacts are those which can be considered
anthropogenic, e.g., the resettlement of people along the banks of the
new freshwater lake created by the salinity barrage. The discussion of
91

92
impacts within each zone will begin with the primary impacts and work
through the tertiary impacts.
The consideration of impacts on the river resources of the Gambia
River has an additional level of hierarchy in that five different
development scenarios were considered. The proposed development program
for the Gambia River Basin calls for up to five dams. But the particular
combination of these five dams has never been exactly specified • Given
this uncertainty, it is impossible to consider all possible 32 (including
the no-dam possibility) combinations of dams. Therefore, the five mos t
probable development scenarios were considered:
• no development
• Kekreti only
• Kekreti plus three Guinean dams
• Kekreti plus Balingho
• all five dams
In addition to the five combinations of dams discussed below,
different irrigation schemes were also addressed as part of the
development program. Again, the irrigation schemes have not been
precisely defined, causing ambiguity as to how the irrigation program
will affect the aquatic environment. The discussion below made several
assumptions about the irrigation program. First, the incorporation of
land into the irrigation network would be a slow process, not exceeding
2500 ha per year. Second, a maximum of 85,000 ha would come under
irrigation. Third, all available freshwater would be used for either
irrigation, or salinity control, and hydropower generation. Implicit
with the implementation of the irrigation network will be the use of
intense agricultural practices.
The approach stressed in the discussion of the impacts has been one
of the high degree of interrelationship among the various processes which
drive the aquatic ecosystems of the Gambia River. This. approach was
adopted as part of the overall sampling strategy and carried through the
consideration of impacts given below.
The discussion of impacts given below, as in the rest of this report,
was written at a level for many people. However, the field of aquatic
ecology has a moderate amount of technical material which may be

unfamiliar to the informed layman. In particular, certain terms as
epilimnion, anoxia, thermal stratification, etc. are not commonly used
outside of the technical field of aquatic sciences. These terms actually
embody concepts rather than just simple definitions. In order that the
more ecologically informed reader may proceed through the text without
the problem of excessive explanation of terms, the impacts are presented
in two forms. The material given below is the main discussion of impacts
and is presented on a zone-by-zone basis. Some technical terms are
invoked without definition. Appendix 1. presents the impacts on a type
basis, e-.g. primary, secondary, and tertiary. In this discussion, some
of the concepts concerning aquatic ecology which are invoked below are
explained in more detail.
93
4.1. Summary
The impacts projected for the Gambia River as a result of the various
river basin development programs are presented below on a zone-by-zone
basis. However, all but one of the projected impacts are expected to
occur in at least two or more zones under the different development
options. A total of forty-one impacts are reported among the five
zones. These impacts are listed in Table 4.1 by type: primary,
secondary, and tertiary. These impacts are then cross-listed in Table
4.2 by zone, i.e. the manner in which they were presented in the text.
An explanation of each impact by subject matter (analogous to Table 4.1)
is given in Appendix I. The presentation in Appendix I. also serves to
provide detail on several technical terms for readers who are unfamiliar
with the principles of aquatic ecology.
The presentation below as well as in Appendix I does not specify the
degree of risk associated with each impact in each zone. That assessment
is attempted in Table 4.3, where the risk of each impact is ranked as
high, medium, low, or nonexistent. This coarse ranking was conducted on
a totally subjective basis. The degree of risk is often biased from the
perspective of the concerned party. Thus, Table 4.3 should primarily
serve as a guide for others to aid in compiling their own risk
assessments. After reading Chapter 4, readers should be able to compose
their own versions of Table 4.3, using Tables 4.1 and 4.2.

94
TABLE 4.1.
ANTICIPATED IMPACTS TO THE AQUATIC ENVIRONMENT, FLORA AND FAUNA
OF THE GAMBIA RIVER FROM THE PROPOSED RIVER BASIN DEVELOPMENT PROGRAM
Impact
Physical-Chemical Impacts
Alteration of seasonal flow patterns in the Gambia River
Altered streamf10ws in the river
Altered thermal regime within reservoirs
Anoxic conditions in the bottom of reservoirs
Altered nutrient concentrations in the Gambia River
Altered suspended sediment loads
Increased underwater light penetration
Elevated suspended sediment loads during construction
Modification to river banks by soil erosion and lack of
seasonal inundation
Permanent loss of seasonally inundated floodplains
Development of a draw-down zone in each reservoir
Increased evaporation from the reservoirs and floodplains
Lack of tidal mixing upstream of the barrage
Increased tidal amplitude downstream from the barrage
Exclusion of saltwater upstream of the barrage
Lack of salinity gradient in the Gambia River estuary
Formation of acid-sulfate soils
Sediment accumulation in the mangrove bo10ns
Formation of hypersaline water
Biological Impacts
Shifts in aquatic species toward 1imnetic organisms 20
Enhanced algal production in reservoirs 21
Changes in the benthic invertebrate species composition
in the reservoirs 22
Increased fish production and harvest in reservoirs 23
Explosive growth of aquatic weeds 24
Elevated rates of evapotranspiration 25
Elimination of mangroves upstream from the barrage and alteration
of mangrove erosystem structure downstream of barrage 26
Disruption of estuarine migration routes 27
Elimination of marine plankton upstrem of barrage 28
Enhancement of fish production in the lower portion
of the Gambia River 29
Elimination of invertebrate communities upstream of barrage 30
No.a
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19

TABLE 4.1. (can't.)
95
Impact No.a
Anthropogenic Impacts
Increased use of herbicides, pesticides and fertilizers 31
Changes in traditional fisheries toward lacustrine
(lake) species 32
Change in diet toward fish 33
Increase in irrigated cropland 34
Mining pollution 35
Human resettlement adjacent to river and reservoirs 36
Changes in disease vectors 37
Deforestation 38
Changes in occupations 39
Altered routes of commerce and transportation 40
Changes in the distribution of wildlife 41
NOTE: a) No. stands for number of impact in Tables 4.2. and 4.3.
Un1vln1ty of "1ch1,aa, G.b1a R1ver Bal1n Stud1.. , 1985.

96
TABLE 4.2.
DISTRIBUTION OF ANTICIPATED IMPACTS BY ZONES IN THE GAMBIA RIVERa
ND no impacts
Lower Estuary Zone
K 1, 2, 5 (minor), 6 (minor), 8
KG same as K
KB 1, 2, 5, 6, 7, 8, 14, 16, 19, 23, 26, 27, 29, 31, 40
KGB - same as KB
ND no impacts
Upper Estuary Zone
K 1, 2, 5, 6, 8 (minor), 10 (minor)
KG same as K
KB 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 15, 16, 17, 18, 20,
21, 22, 23, 24, 25, 26, 27, 28, 30, 31, 32, 33, 34, 36, 38, 39,
40, 41
KGB - same as KB
Lower Freshwater River Zone
ND 5, 10, 24, 25, 31, 34, 36 (minor), 38, 41
K 1, 2, 3 (minor), 5, 6, 8, 9, 10, 12, 24, 25, 31, 34, 36, 37, 38,
39, 40, 41
KG same as K
KB 1, 2, 3, 5, 6, 8, 9, 10, 12, 13, 18, 21, 24, 25, 27, 30, 31, 34,
36,37,38,39,40,41
KGB - same as KB
Upper Freshwater River Zone
ND - 5, 10, 24, 25, 31, 36, 38, 40, 41
K 1, 2, 3, 5, 6, 7, 8, 9, 10, 11, 12, 20, 21, 22, 23, 24, 25, 27
(minor), 31,32,33,34,35,36,37,38,39,40,41
KG same as K
KB same as K
KGB - same as K

TABLE 4.2. (con't.)
ND -
K -
KG -
KB -
KGB -
KEY:
97
Headwaters Zone
5,24,25,31,35 (minor), 36, 38, 39, 40, 41
same as ND
1, 2, 3, 4, 5, 6, 7, 8, 9, 10, ll, 12, 20, 21, 22, 23, 24, 25,
31,32,33,34,35,36,37,38,39,40, 41
same as K
same as KG
For development scenarios - ND = No Development other than
normal growth without dams;
K = Kekreti Dam only;
KG = Kekreti Dam and three Guinean Dams;
KB = Kekreti Dam and Balingho Salinity Barrage;
KGB = Kekreti Dam, three Guinean Dams, and Balingho Salinity
Barrage.
NOTE: a) Impacts are listed for five different development scenarios
within each of the five ecological zones. Impact numbers refer
to type of impact listed in Table 4.1.
University of P'I1cl\l.an, G..bia River Buln Studl... 1985.

Impact No.
Primary
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Secondary
20
21
22
23
24
25
26
27
28
29
30
Tertiary
31
32
33
34
35
36
37
38
39
40
41
98
TABLE 4.3.
DEGREE OF RISK OR EXTENT OF IMPACT ASSOCIATED
WITH EACH DEVELOPMENT OPTION
NO K
M
M
L
L
L
NOTE: Degree of risk is assigned an arbitrary rank of
high (H), medium (M), low (L), or Iloll-existent
(blank space).
KG
M
M
L
L
L
KB
H
H
H
M
M
H
H
H
H
H
H
H
M
M
M
KGB
H
H
H
M
M
H
H
H
H
H
H
H
M
M
M

TABLE 4.3. (con't.)
99
Upper Estuary Lower River
Impact No. NO K KG KB KGB NO K KG KB KGB
Primary
1 H H H H H H H H
2 H H H H H H H H
3 M M L L L L
4 M M
5 M M H H L H H H H
6 M M H H L L M M
7 M M
8 L L M M M M H H
9 H H H H H H
10 L L M M L H H H H
11 M M
12 M M M M M M
13 H H
14
15 H H
16 H H
17 H H
18 H H
19 H H
Secondary
20 H H
21 M M L L
22 M M
23 H H
24 M M L M M M I M
25 M M L M M M M
26 H H
27 L L
28 H H
29
30 H H L L
Tertiary
31 H H M H H H H
32 H H
33 M M
34 H H M M M M M
35
36 H H L H H H H
37 L L L L L L
38 M M L M M M M
39 M M M M M M
40 M M L L L L
41 M M L L L L L

102
high volumes, thus producing a small live volume. Despite this small
vertical range, the surface area of the reservoir will expand from 294
km 2
to 716 km 2 . Thus, the Balingho Salinity Barrage Reservoir will
be characterized by extremely shallow depths, especially when full. The
average full depth is only 2.1 m.
The reservoir will fill quickly during the annual flood and water may
be spilled through gates for a few days to two months per year. Analysis
of recent streamflow records show the minimum period of spilling if the
barrage has been in place from 0 days in 1978 to a maximum of 70 days in
1974. The average for the 14 year period between 1970 and 1983 was 25
days of spill per year. During the remaining portion of the year, the
gates will remain closed.
The maintenance of the water level in the Balingho Reservoir within
strict limits is for a variety of reasons, not the least of which is to
prevent formation of acid-sulfate soils (see more detail on this impact
below). Simulations have shown that dry season evaporation alone will
cause the surface of the reservoir to fall below +1.3 GD. Upstream
withdrawals for irrigation will deplete water levels further. (Because
of these limitations, RRI estimated that a maximum of 5,000 ha rather
than the original estimate of 30,000 ha of land could be developed for
double- cropped irrigation with water from Balingho Reservoir.) For this
reason, two operational schemes have been proposed to hold surface water
levels at a minimum of +1.3 GD. The first scheme is to allow salt water
to intrude into the bottom of the reservoir and thus ··float" a lens of
fresh water on the denser salt wedge. However, simulations by RRl (1984)
showed that density currents and wind mixing would probably disrupt the
vertical stratification and the reservoir would become uniformly salty at
about 3 parts per thousand (ppt) salinity. The only other alternative is
to build a water storage dam to hold the Balingho Reservoir level equal
to or above +1.3 GD.
For the purposes of the impact discussion below, the operational
policy of the Balingho Salinity Barrage was considered only in tandem
with an upstream dam (Kekreti Storage Dam). The gates of the barrage
were considered closed for 11 months of the year. Because the upstream
reservoir fills during the rainy season, its storage will consume about
60% of the fresh water that might be spilled over the barrage during the

103
annual flood. Once the upstream reservoir is full, a brief period of
spilling over the barrage may occur each year at the end of the annual
flood. Therefore, a year-round freshwater lake will be formed upstream
of the Balingho Salinity Barrage, and an estuary with minimal freshwater
flow downstream.
4.2.2. Kekreti Storage Dam
As mentioned above, the purpose of a dam upstream of the Balingho
Salinity Barrage is primarily water storage and hydropower, with
hydropower a secondary factor. The original concept was to build a dam
at Kekreti in Senegal, about 790 km upstream. The Kekreti Dam will
provide water on a year-round basis for the irrigation of up to 70,000 ha
as well as to maintain water levels in the Balingho reservoir. However,
recently (HHL 1984) simulations have been conducted for the operation of
the Kekreti Dam without the Salinity Barrage.
The potential reservoir behind the Kekreti Dam has been the source of
simulations using two hydrologic models, RESIZE and ROSS, both developed
at HHL. Those simulations have shown that a reservoir capable of holding
9 3
3.5 x 10 m of water is the optimal size. At maximum size the
reservoir would have a mean depth of 10.3 m and a surface area of 338
2
km. Each year the reservoir would be reduced to a volume of 1.28 x
8 3 2
10 m, and area of 34 km , and a mean depth of 3.7 m. This would
2
create a drawdown zone of slightly more than 300 km •
The water in the Kekreti Storage Reservoir has three major potential
uses, irrigation, hydropower, and salinity control. These three uses are
somewhat compatible in that water released for hydropower can be used
downstream for irrigation and/or salinity control. For example, without
3
any irrigation, releases of 104 m /s for hydropower will hold salinity
penetration to no more than 135 km upstream (Balingho) on a year-round
basis. If 30,000 ha of irrigation is added to the system, releases for
3 3
hydropower would be reduced to about 95 m /s, but only 45 m /s of
3
that release would be available for salinity control; the 45 m /s would
hold the salt frontier at 179 km upstream (6 km downstream of Dankunku
added to the system, releases for hydropower would be reduced to about 95
3 3
m Is, but only 45 m /s of that release would be available for
salinity control; the 45 m 3 /s would hold the salt frontier at 179 km

104
upstream (6 km downstream of Dankunku sland). As full development of
irrigated land (70,000 ha) is achieved, the reservoir would be primarily
devoted to irrigation. However, almost full hydropower generation (97%)
could be obtained if an optional third turbine is installed in the
3
Kekreti Dam. Also, about 25 m Is of streamflow could be used for
salinity control to hold the 1 ppt frontier at 197 km upstream (23 km
downstream of Carrols Wharf). These results are highly affected by water
demands for irrigated rice. The values used in the ROSS model were
3
derived by AHT/HHL at just over 50,000 m Iha/yr. Using the lower
estimates of LRDC of about 25,000 m 3 Ihalyr, the same 70,000 ha could be
irrigated, with about 50 m 3 Is is available to hold the salt frontier
175 km upstream (10 km downstream of Dankunku Island). Table 4.4 shows
the various options of irrigation and salinity control.
The autonomous operation of the Kekreti Dam is not without some
compromises. During the wet season, most of the flow of the Gambia River
upstream of Kekreti will be diverted to filling the reservoir. As a
result, swamp or tidal rice will be affected by the operational policy.
But, river basin planners should gain flexibility in using their water
resources. Furthermore, in the early stages of basin development the
amount of irrigation will be small. This allows a considerable amount of
water to be used for salinity control.
The successful autonomous operation of the Kekreti Dam and Reservoir
9 3
depends on filling the reservoir to at least 3.0 x 10 m of water
each wet season. Simulations with the ROSS model have shown that two
times between 1971 and 1980 the reservoir would not be filled and thus
fail in the late dry season. The autonomous operation of the dam will
require careful manipulation of water uses versus storage depending on
each year's annual rainfall.
For the purposes of the impact discussion given below, the Kekreti
Reservoir is assumed to fill approximately to capacity each year, 70,000
ha of irrigation are in place, and the salinity frontier is maintained
about 180 km upstream on a year-round basis. Further, the reservoir
drawdown is assumed to be maximal.
4.2.3. Tandem Operation of Kekreti and Balingho Dams
The autonomous operation of the Kekreti Dam would not be able to hold
the salt frontier at Balingho on a year-round basis once a significant

106
(greater than 10,000 ha) amount of land is brought under irrigation. In
order to fill the Kekreti Reservoir during the rainy season, most of the
annual flood must be diverted away from the lower river. Under these
circumstances, the salt frontier will migrate upstream of Balingho.
Swamp rice cultivation between Balingho and the location of the salt
frontier will be lost.
In order to preserve the rice culture, the tandem operation of the
Balingho Salinity Barrage and Kekreti Storage Dam has been proposed. The
Salinity Barrage will hold the salt frontier at Balingho throughout the
dry season. During the wet season flood waters generated by tributaries
below Kekreti will permit the opening of gates at Balingho for an average
of one month per year to allow tides to propagate upstream for the swamp
rice.
The tandem operation of the two dams is not without some hydrologic
drawbacks. First, in order to maintain the water level in the Balingho
Reservoir at +1.3 GD, some water (2.20 x 10 8 m 3 /yr) must be used to
compensate for evaporative losses. This is water which would otherwise
be available for irrigation. Second, it is unlikely that the gates of
the Balingho Barrage could be left open for the entire period of swamp
rice cultivation without saltwater intrusion upstream of Balingho. Thus,
water that would be used to fill the Kekreti Reservoir must be spilled to
preserve the swamp rice crop.
Based on this operational policy, the hydrologic regime in the Gambie
River with the two dams used in the impact discussion below was as
follows: The gates of the Salinity Barrage will remain closed all year
except to spill excess flood water (open about 20-30 days per year).
Releases from the Kekreti Reservoir will be determined by competing
demands for irrigation, maintenance of the Balingho Reservoir, and
hydropower.
4.2.4. Guinean Dams
Three dams have been proposed for the Gambia River and its
tributaries. The Kouya Dam is the largest structure proposed for the
Gambia River Basin. The proposed site is on the Gambia River, six km
upstream of the confluence with the Liti tributary, or approximately 1034
km upstream of the river mouth. The Kouya Dam would develop the largest

107
reservoir on the Gambia River, with a total volume of 4.17 x 10 9 m 3 •
The reservoir would have a maximum surface area of 116 km 2
and a
minimum area of 80
2
km , creating a 36
2
km drawdown zone. The average
depth of the full reservoir would be 42 m. The sole purpose of this dam
is for power generation (although it is estimated that 3,000 ha could be
brought under irrigation).
The Kankakoure Dam site is on the Liti tributary, 10 km above the
confluence with the Gambia River. This dam would create a rather small
reservoir with a maximum volume of 1.30 x 10 8 m 3 and a surface area
283
of 16 km. Because of a rather large live volume (1.27 x 10 m,
most of the 16 km 2 would be part of the drawdown zone. The Kankakoure
Reservoir would have an average depth of 16 m when full. Similar to the
Kouya Dam, the sole purpose of the Kankakoure Dam is hydropower.
The Kogou Fou1be Dam would be a moderate to small structure located
on the Koulountou tributary about 150 km above the confluence with the
Gambia River; that confluence is about 576 km upstream. The reservoir
8 3
created by this dam would contain 4.50 x 10 m when full and have a
surface area of 38 km 2 • The size of the drawdown is uncertain, but
2
would probably not exceed 30 km. The average full reservoir depth
would be 12 m. The purpose of this dam is both hydropower and a modest
irrigation potential of up to 12,000 ha.
These dams are in the pre-feasibility stage; thus, their operational
policies have not been explored. However, some basic hydrologic
assumptions can be made in regard to their operation. The Kouya and
Kankakoure Dams would trap all of the wet season flow of the Gambia and
Liti Rivers. respectively. This water would then be released on a
relatively uniform basis the entire year. The flows would average 50
m 3 /s for Kouya and 16 m 3 /s for Kankakoure. Most likely the flows
would occur at twice that rate for 12 hours per day when power is needed,
and cease at night. Some serious concerns have been raised in regard to
filling Kouya Reservoir because its proposed live volume storage exceeds
the average annual flow of the Gambia River by 20%.
The Ross model was used to simulate the effect of the Kouya Dam on
the operation of Kekreti Dam. The preliminary results suggest that the
operational policy of Kekreti Dam would change very little with or
without Kouya Dam. Perhaps a 4% increase in power generation could be

108
achieved for Kekreti with Kouya Dam in place. But, the regulated
discharge from Kouya would enable a reduction in the size of the Kekreti
Reservoir by over one third. This reduction is achieved by regular
delivery of water to the Kekreti turbines without the need to store most
of the annual flood behind the Kekreti Dam. In order to achieve this
reservoir size reduction, the Kouya Dam must be deemed hydrologically
feasible and built simultaneously with Kekreti Dam.
The Kogou Foulbe Dam would trap no more than 25% of the annual flow
of the Koulountou River. That stored water would be released at a rate
3
of approximately 11 m j s. Because some of the water is proposed for
irrigation, it would be released both day and night. The overall effect
of this dam would be relatively minor compared to the existing flow
patterns • The annual flood would be slightly smaller and dry season
streamflows increased from almost a to a few m 3 js.
The operational policy of the three Guinean Dams used for the
discussion of impacts below is as follows: The combined effect of the
Kouya and Kankakoure Dams would totally regulate the annual flow
downstream of these dams. The Kogou Foulbe Dam would have only a small
effect on the current streamflow patterns of the Koulountou River.
4.2.5. Irrigation Network
A great many options exist for irrigation along the Gambia River.
Most of the discussion of impacts below assumes a fully developed network
of 85,000 ha (12,000 ha on the Koulountou River). However, in many
cases, impacts could be less severe in the early years of development
when the network is considerably smaller than 85,000 ha. Because most of
the impacts are discussed qualitatively, the reduction in their severity
with a smaller irrigation network has not been addressed. The primary
factor affecting the impacts is the operational policy of the dams, which
has not been established for different-sized irrigation networks.
4.3. Lower Estuary
The lower estuary zone is somewhat immune to the effects of the
proposed development program for the Gambia River. This partial immunity
originates from the high degree of interchange between the lower portions

109
of the Gambia River and the coastal oceanic environment. The waters of
the lower estuary zone have salinities about 34 ppt or approximately
equal to the ocean. These waters are well mixed by the semidiurnal tides
(two equal tides per day). The marine characteristics of the lower
estuary zone are reflected in the aquatic flora and fauna. The dominant
vegetation is a mangrove forest which extends up to 10 km back from the
river bank. Plankton, invertebrates, and fish are also characterized as
coastal oceanic or marine.
Essentially the upper limit of the lower estuary was defined as that
location where the influence of freshwater was no longer observed; in
practice this boundary migrates seasonally, but was roughly set at Mootah
Point, about 60 km upstream (see Figure 3.1). Thus the impacts of river
basin development on the lower estuary zone will be limited because this
zone derives many of its characteristics from the ocean, not the river.
The major impacts to the lower estuary will be:
• a change in the size of the lower estuary
• changes in tidal amplitude (tide heights)
• restructuring of some of the mangrove forests
• possible development of hypersalinity (increase in salinity
above those levels found in the ocean)
Each of these four major impacts will in turn generate additional
secondary or tertiary impacts.
4.3.1. No Development or Steady State
Without an active development program for the Gambia River Basin, the
lower estuary zone will remain relatively unchanged. Most impacts to
this zone come from anthropogenic nutrient loading, fishing, and
destruction of natural habitat. All of these impacts are an outgrowth of
normal human activities along the river banks. Although migration
patterns indicate that many people are moving from up-country to the
Banjul area, their impacts on the lower estuary zone have been relatively
minimal.
Upriver from Banjul, human settlement will probably remain minimal in
the future because of the unsuitable living environment found in the
mangrove forests. The only groups that appear willing to tolerate liVing
near the mangrove forests are fishermen who desire access to the river.

110
Without expansion of the fish industry, fishing communities will not grow
much in the near future. Only through a large increase in fishing
effort, as well as extensive changes in gear, will the fishing industry
vastly expand. With minimal additional human settlement along the river
banks in the lower estuary zone, impacts to the river will be trivial.
4.3.2. Kekreti Storage Dam
The impacts on the lower estuary zone from construction of the
Kekreti Storage Dam will be relatively minor and with proper management
could be somewhat beneficial. The most significant impacts caused by the
Kekreti Dam are the regulation of streamflows and the altered seasonal
pattern of streamflows.. The dam will allow controlled releases of
freshwater to maintain agriculture throughout the dry season. Thus,
flood season streamflows will be reduced while dry season flows will
increase. The generation of hydropower will further regulate streamflows
until a uniform 60-100 m 3 js is achieved (Harza, 1985).
The major consequence of the regulation of streamflows from the
operation of the Kekreti Dam is the possible reduction in the size of the
estuarine zones. Controlled releases of freshwater from the dam may
provide sufficient hydraulic head that saltwater penetration into the
Gambia River will be reduced on a year-round basis. Reduced saltwater
penetration will in turn diminish the size of the estuarine zones,
although the major effect of this process will be a reduction in size of
the upper estuary zone. However, the amount of water drawn from the
river for irrigation will determine if the hydraulic head is sufficient
to reduce saltwater penetration beyond the current state. Projections by
HHL indicate that with a moderate amount of irrigation (30,000 to 40,000
ha), saltwater penetration will be held approximately to Bia Tenda.
The construction of the Kekreti Dam may have a slight effect on the
suspended sediment loads reaching the lower estuary. During construction
a large amount of sediment will enter the river. Except during the flood
stage, all of this sediment should settle out of the water well above the
lower estuary zone. During the flood stage, increased streamflows may
carry some sediment large distances downstream, but the river is normally
carrying a heavy sediment load at this time of year; the increase in
suspended solids due to construction would probably not be detectable

111
above the then already elevated suspended solids loads. The opposite
effect will take place after the dam has been completed. The large
reservoir will act as a very effective settling basin to remove suspended
sediment. Thus, water discharged from the reservoir will have lowered
sediment loads. But the effect of sediment settling will probably be
masked by sediment additions to the river between Kekreti and the lower
estuary zone.
Whereas the construction of the Kekreti dam will have very little
effect on the suspended solids loads in the lower estuary zone, the same
will not be true for construction of irrigation networks. Once Kekreti
Dam is operational, a prolonged period of construction of irrigation
canals and bunds will begin in the lower portion of the Gambia River
Basin. Stringent erosion and construction practices should confine most
sediment and prevent it from entering the river, but these practices are
usually not followed.
Irrigation agriculture may cause a large increase in nutrient loads
to the river if heavy fertilizer application is adopted as part of the
farming practices. The excess use of fertilizer has been one of the
major causes of water pollution to lakes and rivers in many of the more
developed countries. Likewise, toxic substances in the form of
herbicides and pesticides can cause severe pollution problems. The
extent of the problem will depend upon the kind and the amount of
application, as well as the proper management of these chemicals. This
impact is discussed in more detail in section 4.5. Construction of the
Kekreti Dam should have little impact on finfish or shellfish fisheries
of the lower estuary. Fish studies showed almost no overlap or migration
of species from Kekreti to the lower estuary zone.
4.3.3. Kekreti Storage Dam and Guinean Dams
The addition of the Guinean Dams upstream from the Kekreti Storage
Dam will have essentially no impact on the lower estuary zone. Any
change in streamflows or suspended sediments from the Guinean Dams will
be stopped by the Kekreti Dam long before these impacts would reach the
lower estuary zone. The Kogou Foulbe Dam, located on the Koulountou
River, will have very little effect on the Gambia River. Most of the
water stored by this dam would be used for irrigation in Guinea and never

112
reach the Gambia River. But , this dam would s tore no morethan 25
percent of the annual flood of the Koulountou River and thus have only a
small effect on natural streamflow patterns.
4.3.4. Kekreti Storage Dam and Balingho Salinity Barrage
The Balingho Salinity Barrage will have the largest impact on the
lower estuary zone of the five proposed dams. The intent of this dam is
to block the penetration of saltwater upstream in the Gambia River;
similarly, the dam will block the downstream movement of freshwater and
hence eliminate the natural salinity gradient normally present in the
estuary. Essentially the natural estuary (with the gradual salinity
gradient from freshwater to saltwater) will cease to exist. The lower
estuary will receive a host of primary impacts because the physical and
chemical environment will be dramatically changed. These primary impacts
will create a suite of secondary and tertiary impacts.
Most of the primary impacts from construction and operation of the
Balingho Salinity Barrage will be a result of the elimination of the
salinity gradient in the estuary. The net result is that the Gambia
River will lose one of the five ecological zones, the upper estuary. The
physical and chemical characteristics in the lower estuary will expand
upstream to the base of the salinity barrage and will have coastal ocean
salinities from the mouth of the river to the barrage. If freshwater is
not allowed to pass over the barrage during the dry season, hypersalinity
may develop in the upper reaches of the estuary (hypersalinity is a
condition where salinities exceed that of normal seawater).
Hypersalinity will have a very adverse effect on the aquatic flora and
fauna, in that they cannot tolerate extremely high salinities for osmotic
reasons (McLusky, 1971).
The salinity barrage will also effect the tidal regime in the river.
Tidal mixing will be eliminated above the barrage while tidal amplitude
will be increased between 10 and 20% below the barrage. The increased
amplitude results from the reflection of tidal waves off the face of the
barrage (Harza, 1985).
The creation of a reservoir behind the salinity barrage will generate
impacts in the lower estuary zone. Streamflows in the lower estuary will
be both altered from present conditions and artificially regulated. The

113
amount of water flowing into the lower estuary will vary from such
factors as the amount of rainfall, evaporation, and the amount of water
drawn from the reservoir for irrigation (Harza, 1985). Under most
irrigation programs, no water will pass over the salinity barrage except
briefly at the end of the rainy season.
The reservoir will also affect water quality in that settling of
particulate material will reduce the suspended solids inputs to the lower
estuary. But, the bulk of suspended sediments in this zone will continue
to originate from tidal scouring of soft bottom sediments. During the
construction phase the suspended solids loads in the estuarine portions
of the Gambia River will be. greatly . elevated. The barrage will also
block much of the downstream drift of nutrients. Water released into the
lower estuary from the Balingho Salinity Barrage will amount to a very
small portion of the total volume of the estuary.
The secondary impacts generated by the Balingho Salinity Barrage will
primarily arise as a consequence of the elimination of the salinity
gradient. High salinity seawater will expand upstream from Mootah Point
to the base of the barrage. The more saline waters found between Mootah
Point and Balingho will cause a major shift in the species composition of
the mangrove forests (Twilley, 1985). The less salt-tolerant Rhizophora
racesoma will be replaced by smaller trees of the same genera as well as
other genera. Snedaker (1984) estimates that the mangrove forests from
Balingho to Banjul (the entire estuary) could eventually undergo
extensive species shifts. The mangrove species shifts could cause a
major impact to the coastal fisheries. Twilley (1985) has shown that
mangrove forests contribute the vast majority (85%) of organic material
to the estuarine and coastal environment. A reduction in the overall
mangrove productivity could in turn greatly reduce finfish and shellfish
productivity.
The lower estuary zone was found to be a region of high fish and
shellfish production (Dorr et al., 1985; van Maren, 1985). Much of this
fish production was supported by a detritus-based food web with mangrove
debris serving as the main source of detritus. Providing mangrove
productivity does not decline, the same food web should exist after the
barrage is constructed, but now will extend upstream another 80 km; if

114
the upper reaches of the lower estuary zone become hypersaline and/or
undergo maj or changes in the mangrove forest structure, fish production
and yield will decline considerably in the highly saline waters.
The Ba1ingho Salinity Barrage will be a major barrier to migration
patterns of several species of fish. Currently three species of shrimp,
one species of crab, and several species of finfish have migratory life
cycles which involve the segment of the Gambia River near Ba1ingho. For
the pink shrimp, the life cycle includes a period of post1arva1 growth in
the mangrove bo10ns. These post1arvae grow into juvenile shrimp that are
then harvested in the Ba1ingho segment of the river. The blocked
migrations present a serious threat to the estuarine and coastal
fisheries of The Gambia. The elimination of breeding grounds is a
serious potential impact. If life cycles of fish are broken, the fishery
will ultimately fail.
The tertiary impacts to the lower estuary zone from the salinity
barrage will arise primarily from man's activities along the banks of the
reservoir. As mentioned above, irrigation will probably be conducted by
intensive farming practices. The application of fertilizers and toxic
substances to fields will ultimately contaminate the water of the river.
These substances will find their way through the salinity barrage and
effect the lower estuary zone. Small amounts of nutrient pollution will
not present a major problem. However, even minute amounts of pesticide
pollution could render Gambia River fish unsuitable for human consumption
if tissue concentrations become high (approximately in excess of 0.1 to
1.0 part per million depending on the pesticide).
The Ba1ingho Salinity Barrage will serve as a transportation corridor
as well as dam. Motor vehicle traffic between the Casamance region of
Senegal and the northern markets will use the route across the river.
With this increased traffic will come the usual forms of pollution
associated with highly used roads. This pollution includes petrol, oil,
and grease in the water, as well as automotive trash such as old
batteries, tires, and metal debris. All of this material will cause
minor (local) pollution to the waters of the lower estuary zone. Large
spills of petrol or oil, which are toxic to most organisms, can cause
widespread ecological damage, but this is an unlikely event.

4.3.5.
115
Kekreti Storage Dam, Guinean Dams and Ba1ingho Salinity Barrage
The addition of the three dams in Guinea upstream from Kekreti will
have no effect on the impacts to the lower estuary zone, once the
salinity barrage has been constructed. The management of water within
the Gambia River system should be more precise with five dams rather than
only two. But the effect of water management on the lower estuary will
depend primarily on the release of water, if any, across the Ba1ingho
Salinity Barrage.
4.4. upper Estuary
The upper estuary zone was defined as the segment of the Gambia River
between Mootah Point and the present upstream limit of saltwater
penetration. The location of the saltwater-freshwater interface changes
seasonally because of the effect of the annual flood. During the end of
the rainy season, freshwater extends below the vicinity of the proposed
sa1inity barrage at Balingho. At the end of the 1983-84 dry season,
saltwater extended up to Kuntaur (Berry et a1., 1985).· The duration of
the annual rains, as well as the amount of rainfall determines the
streamf10ws in the river; higher streamf10ws in turn push the saltwater
farther downstream. This study used Bird Island just downstream from
Kuntaur (see Figure 3.1), as the approximate upstream limit of the upper
estuary zone.
The large annual changes in salinity throughout most of the upper
estuary zone yield a highly seasonal characteristic to this zone. The
species composition and abundance of almost all of the animals and
plankton was determined primarily by the salinity of the ambient water.
A marine flora and fauna were present during the dry season, while
freshwater flora and fauna dominated during the rainy season. Separate
flora and fauna occur because relatively few organisms can tolerate such
profound shifts in their salinity regime (McLusky, 1971).
Mangrove forests dominate the entire ecosystem function of the Gambia
River throughout most of the upper estuary zone. Mangrove detritus
apparently was the major source of organic material to the river
(Twilley, 1985), vastly exceeding inputs from other sources. Dynamics

117
are several impacts from the dam that will pervade the entire river
downstream from the dam. Most of these impacts deal with streamflows.
The primary purpose of the dam is to store water for controlled releases
to promote irrigated agriculture and hydropower. The systematic release
of water from the dam will alter and regulate streamflows in the Gambia
River. Changes in the streamflows will affect the location of the
freshwater-saltwater interface as well as reduce the extent of the annual
migration of this interface. Thus the operation of the Kekreti Storage
Dam will directly determine the size of the upper estuary zone. Another
factor which will affect the size of this zone is the amount of water
used for irrigation. Most estimates indicate that irrigation schemes
with 70,000 ha will consume most of the available freshwater.
Consumption of the freshwater will allow sufficient saltwater penetration
that the saltwater-freshwater interface will remain near Dankunku Island
and the upper estuary zone will be about one-third smaller than the
current size (HHL, 1984). Smaller irrigation programs should force the
saltwater-freshwater interface farther downstream through controlled
releases of water from the dam (Table 4.4). Thus, the operational policy
of the Kekreti Storage Dam will have an important effect on the upper
estuary zone.
The location of the freshwater-saltwater interface in the Gambia
River could have major consequences for the mangrove forests. With the
construction of the Kekreti Storage Dam and a very small irrigation
program, the interface could be held approximately near Yelitenda (HHL,
1984). This would place about 12% of the existing mangrove forests in a
permanent freshwater, tidal environment. Current hypotheses indicate
that once established mangroves can tolerate year-round freshwater
providing tidal action is present to prevent the trees from drowning
(Twilley, 1985). But the exact impact on the mangroves upstream from the
freshwater-saltwater interface is uncertain.
A portion of the existing upper estuary zone may be used as irrigated
cropland after the Kekreti Storage Dam is built. This practice would
result in some minimal loss of floodplain in this zone. But most of the
soils in the upper estuary zone will be unsuited for agriculture because
of potential acid-sulfate accumulation (Colley, 1985).

118
During construction of the dam, an increase in the suspended sediment
load of the river is expected. Some of this sediment may reach the upper
estuary zone, although this is probably only a very minor concern. After
completion of the dam, most of the suspended sediment will settle out of
the water in the reservoir. Thus waters entering the upper estuary zone
from upstream could be slightly less turbid than without the dam. Again,
most sediment originates from tidal currents scouring the bottom.
Likewise, some soluble nutrients may be removed from the water in the
reservoir; water released over or through the dam may then have slightly
lowered nutrient concentrations. These impacts would have only very
minor effects in the upper estuary zone.
Considerably more important to the upper estuary zone is the release
of materials from irrigated fields immediately upstream from the
estuary. Improper farming practices can be expected to release
sediments, fertilizers, and toxic substances into the river. All of
these materials will degrade the quality of water in the river.
Increased sediments can degrade the river by covering many benthic
organisms and their preferred habitats with a layer of mud. Increased
nutrients can stimulate excessive algal growth; as the algae die and
decay, they consume a considerable amount of oxygen eventually creating
anoxia. Most organisms die in anoxic conditions. Toxic substances in the
form of herbicides or pesticides can readily contaminate finfish and
shellfish populations, rendering them unsuitable for human consumption.
Because pollution from toxic substances is dangerous even in the part per
million range, almost any release of these substances constitutes a
serious impact. The extent of pollution from farming practices will
depend greatly on the care and methods of the agriculture employed.
4.4.3. Kekreti Storage Dam and Guinean Dams
The construction of up to three dams in Guinea (upstream from
Kekreti) will have essentially no impact on the upper estuary zone beyond
those already generated by the Kekreti Storage Dam. The most important
impacts from the Kekreti Storage Dam are from streamflow regulation and
releases from irrigated cropland. These impacts will not change with the
addition of dams upstream from Kekreti unless the upstream dams
significantly alter the streamflows below the Kekreti Storage Dam; two of

119
the three proposed Guinean dams could reduce the quality and quantity of
water reaching the Kekreti reservoir.
4.4.4. Kekreti Storage Dam and Balingho Salinity Barrage
The construction of the Balingho Salinity Barrage will have an
enormous effect on the upper estuary. Essentially the upper estuary zone
will be eliminated by the barrage. Freshwater will fill the reservoir
immediately upstream of the barrage, while saltwater with a salinity
equal to that of coastal ocean water will occur just downstream of the
barrage. Because of this major alteration to the upper estuary zone, the
number of primary impacts to the river system created by the barrage will
vastly exceed the number of impacts from any of the other four dams. The
large number of primary impacts will in turn generate many secondary and
tertiary impacts.
The primary impacts created by the Balingho Salinity Barrage fall
into three general categories:
• impacts created from regulation of streamflows
• impacts associated with the reservoir
• impacts caused by physical barrier across the river
Impacts created from the regulation of streamflows are essentially the
same as those associated with the Kekreti Storage Dam. Streamflows will
be regulated both by releases of water from the Kekreti Storage Dam and
the barrage. The magnitude of the streamflows will depend on the water
demands for irrigation and hydropower.
The creation of a reservoir behind the barrage will generate the
largest number of primary impacts. The reason for the large number of
impacts is that the present Gambia River will be transformed from a
riverine system to a lacustrine system. The quiet waters of lacustrine
systems allow several processes to occur which are not usually observed
in a river. These processes include:
• the accumulation of sediments
• an increase in underwater transparency because of lowered
suspended sediment loads
• the development of vertical thermal stratification
• increased evaporation from the surface of the reservoir
• seasonal formation of anoxic bottom waters in the reservoir

122
The primary impacts discussed above will lead to several secondary
impacts in the upper estuary zone. Similar to the primary impacts, the
secondary impacts fall into two groups: those created by the presence of
the reservoir, and those generated by the elimination of the salinity
gradient. Impacts created by the presence of the reservoir arise
primarily from the change in the aquatic environment from riverine to
lacustrine. Impacts from the lack of salinity gradient are those due to
the elimination of the entire upper estuary zone.
The secondary impacts created by the formation of a reservoir
upstream of the barrage generally involve shifts from estuarine to
lacustrine species. Algal species will shift from the common marine
diatoms to freshwater species (Healey et al., 1985). Likewise the
zooplankton will change from marine species to freshwater species
(Healey et al., 1985). These shifts at the bottom of the food web will
cause shifts in the occurrence and abundance of fish species. The
lacustrine algal species should also be more productive than the existing
estuarine community (Beadle, 1981). Ultimately the reservoir will
produce more fish than the current riverine system. Estimates of yield
for the reservoir behind the Balingho Salinity Barrage range from 68 to
6,300 metric tons per year (see Chapter 6). This yield will be in
freshwater fish. But this increased fish production does not come
without considerable costs to estuarine environment fish and shellfish
production.
Concurrent with the increased yield in finfish, will be the
elimination of at least 10% of the present estuarine shrimp catch (van
Maren, 1985). The freshwater reservoir that displaces the upper estuary
zone will also eliminate many of the attached or relatively sesile
benthic invertebrates (van Maren, 1985). The largest secondary impact
will be the elimination of most of the existing spawning and/or nursery
habitat used by estuarine breeders. This could have a major impact on
the coastal oceanic shrimp fisheries.
The reservoir will also affect the rooted aquatic vegetation in the
upper estuary zone. The effect of this major impact will be the
elimination of all of the mangrove forests above Balingho, some 7,930 ha
or 12% of the total mangrove forest in the Gambia River Basin (Twilley,
1985). Mangroves cannot tolerate permanent freshwater inundation; the

123
trees will drown shortly after the tidal waves are eliminated above
Balingho (Twilley, 1985). The magnitude of this loss of mangroves is
greater than the simple destruction of 12% of the forests. Those
mangroves growing at the edges of the bo10ns above Balingho are the
largest and most luxuriant ones on the entire river. These trees often
exceed 30 m in height and contribute the major source of organic matter
to the estuarine segments of the river (Twilley, 1985). Mangrove
detritus flushed into the river exceeds the production of organic matter
by algae by almost one order of magnitude (Twilley, 1985). The
elimination of the flux of mangrove detritus from the upper estuary zone
into the Gambia River could cause a large reduction in the overall
productivity of the estuary and adjacent coastal oceanic environment.
The change in salinity regime downstream of the barrage will also cause a
major species shift in the mangrove forests in the river from 30 to
130 km (Snedaker, 1984) below the barrage.
The mangrove forests in the reservoir will probably be replaced by
nuisance aquatic weeds. Currently there are very few aquatic weeds in
the Gambia River (van Maren, 1985). Although weeds do exist in some
small bolons, their extent is limited either by the presence of saltwater
or scouring currents during the flood season. The alteration of the
river to reservoirs should provide the opportune environment for the
explosive growth of water weeds, as has occurred in other African
reservoirs (Freeman, 1974). The formation of large beds of emergent
aquatic weeds will greatly increase evaporative water loss form the
reservoir by the addition of evapotranspiration.
The physical barrier created by the salinity barrage will cause a
major impact through disruption of migration patterns, and loss of
spawning and nursery areas. A prime recipient of this impact is the pink
shrimp. This species has a postlarval and juvenile phase which includes
rapid growth while living in the mangrove bolons (van Maren, 1985). A
quantitative impact of the barrage is the direct elimination of the
shrimp nursery grounds upstream from Balingho, which is about 10 percent
of the total. A further impact is that much of the remaining nursery
grounds could be rendered unsuitable for the shrimp both from an overall
reduction in mangrove detritus and the possible formation of

124
hypersalinity. Studies in the Casamance indicate that as long as
salinities do not exceed 50 ppt, the shrimp life cycle should be
unaffected (LeReste, 1983). Once extreme hypersalinity develops, the life
cycle of the entire shrimp fishery could be disrupted. Similar problems
will be faced by crabs (van Maren, 1985) and several species of finfishes
including the extremely important bonga fishery (Dorr et al., 1985).
The construction of the Balingho Salinity Barrage and subsequent
development of the reservoir behind the barrage will generate several
tertiary impacts, or impacts associated with human activities. Most of
those impacts are discussed in the final conclusions of the socioeconomic
study (see Rural Development in the Gambia River Basin). Those human
impacts which will most directly affect the river are discussed briefly
below. The introduction of pollutants from farming practices used in
conjunction with irrigated cropland will be the most important tertiary
impact. As discussed earlier, these pollutants include suspended
sediments, excessive nutrients, and toxic substances. The threat to the
estuary from these forms of pollution is very high in that most of the
irrigated cropland will be just upstream from the reservoir or adjacent
to the reservoir. All nations of the world have suffered from these
pollutants despite the best attempts to contain them. Severe pollution,
especially from toxic substances, can render biota unsafe for human
consumption, and thus cause a large loss in food reserves and valuable
exports from associated fisheries. The potential for this impact
increases in direct proportion with the number of hectares (ha) of land
brought under irrigation. Thus, this will be a long-term impact which
will increase in severity over the course of the entire river basin
development program.
Another major tertiary impact is the resettlement of people on the
banks of the new reservoir. The freshwater reserves will undoubtedly
attract many people. These people will take advantage of the water for
many reasons, particularly food produced by the reservoir fisheries.
Small, new communities will develop with fishermen, farmers, boat pilots,
and ancillary services to fisheries and aquaculture. These new
communities will add considerably to the pollution of the river. Because
this is an influx of people to the edge of the river where previously no

125
population center existed, all of the pollution associated with this
influx will be new pollution to the river. As the lakeside communities
grow, the usual effects of human activities on watersheds will occur,
such as deforestation, loss of wildlife, altered routes of
transportation, releases of toxic substances, etc. All of these
activities will generate some impacts to the river. The primary impact
will be the addition of foreign materials into the water. These foreign
materials may range from relatively innocuous substances such as building
materials to extremely dangerous substances such as pesticides.
Along with all the negative impacts (associated with human habitation
along the edge of the reservoir), is the very positive impact of the
availability of increased food supplies from fishing as well as irrigated
agriculture. While the primary direction of this impact is to the people
from the river, there is a feedback mechanism which controls the
abundance of fish in the river. As the reservoir fisheries develop,
maximum sustainable yield will be achieved and thus the abundance of fish
in the river will be limited by the level of fishing effort. Thus, the
availability of fish will affect and alter local diets and rates of
consumption, thereby alter fishing effor t, harvest, and stock abundance.
This feedback relationship dictates the need for continuing assessment
and management of the reservoir fisheries.
4.4.5. Kekreti Storage Dam, Guinean Dams, and Balingho Salinity Barrage
The addition of three dams upstream of the Kekreti will have
essentially no additional impact on the upper estuary zone beyond those
already generated by the Kekreti Storage Dam and the Balingho Salinity
Barrage. All the impacts described in section 4.4.4. will occur with or
without the presence of the Guinean dams. The Guinean dams may serve to
refine the regulation of streamflows in the Gambia River, but effect of
this improved regulation will be trivial compared to the impacts already
imposed by the other two dams.
4.5. Lower Freshwater River Zone
The lower freshwater zone of the Gambia River was considered that
segment of the river which extended from the upstream extent of saltwater

126
penetration to the upstream extent of tidal fluctuations. Geographically
this zone roughly began near Kuntaur about 250 km upstream, and extended
to near Gouloumbou, about 510 km from the ocean (see Figure 3.1). The
major distinguishing characteristics of the lower river zone were the
presence of diurnal tidal waves and the absence of saltwater. The tidal
waves created very distinct reversals of current in the Gambia River four
times per day. The mixing caused by these current reversals was an
important factor in keeping the river waters from becoming stagnant. The
current reversals also moved plankton among different sections of the
river including many of the small bolons which branched from the main
. channel.
The lower freshwater zone had a very distinct seasonal nature because
of the annual flood. During the flood, streamflows were considerably
elevated over those of the dry season. The flood waters carried more
sediment than during the dry season and also had a different chemical
composition than during the rest of the year. For example, flood waters
were observed to carry elevated nitrate-nitrogen concentrations compared
to dry season river water (Berry et al., 1985). Conductivities and
alkalinities were also altered during the flood season. These chemical
shifts in river water composition were accompanied by a change in the
plankton of the lower river zone (Healey et al., 1985).
The dynamics of the lower river zone were considerably different from
those of the two estuarine zones. The absence of mangrove forests in the
lower river zone appeared to reduce the overall productivity of the
river. The high inputs of organic matter from the mangroves were not
available to support a rich and diverse aquatic flora and fauna. Inputs
of organic matter from the vegetation growing on the river bank were
still evident, but considerably lower levels of total phosphorus and
total nitrogen were found in the lower river zone compared to the
estuarine zone (Berry et al., 1985). Furthermore, high concentrations of
total nutrients were observed only during the flood season for a
relatively short portion of the year. Rates of primary productivity by
algae were high during the early stages of the annual flood (Healey et
al. , 1985) • The high rates of algal growth were accompanied by large
zooplankton crops (Healey et al. , 1985) • This annual pulse of plankton

127
growth was an important seasonal aspect of the total productivity of this
zone. The tidal action served to mix the plankton throughout the water
column, and thus stimulate overall aquatic production throughout a large
segment of the river.
4.5.1. No Development or Steady State
The lower river zone of the Gambia River will probably have a
moderate amount of growth even if the river basin development programs
are not implemented. This zone is one of the few areas that can support
some additional agriculture, providing freshwater reserves are available.
Most of this agriculture would be in the form of irrigated crops grown
adjacent to the river. This growth in agriculture will generate some
impacts to the river. The two primary impacts associated with increased
agricultural activity are the loss of floodplains and the alteration of
the nutrient regimes in the river. Floodplains will be lost because the
fertile soil of these areas will be the primary location of new cropland.
Nutrient regimes will be altered from the diversion of water from the
river onto agricultural fields. Nutrients will be removed as well as
selectively added to waters used for irrigation. Water will also be lost
from the river as it is consumed for irrigation purposes.
Some secondary impacts will result from increased agricultural
activity. A new aquatic environment will be created in the numerous
canals as the irrigation network expands. This new environment will
favor certain aquatic species, in particular aquatic weeds. The growth
of weeds could ultimately cause serious problems to the operation of the
irrigation network by clogging the canals and consuming a large amount of
water normally available for crops. The weeds will further promote water
loss from the river due to evapotranspiration.
The largest number of impacts associated with increased agriculture
production will arise from human actions in the fields. The most serious
impact from farming is the contamination of river water by nutrients from
excessive fertilizer and from toxic substances. The consequences of
these pollutants have been discussed above. The actual extent of the
impacts to the lower river zone will be determined by both the amount of
additional cropland brought into production as well as the extent of
settlement of people along the river. Thus these two processes will

128
determine the overall severity of the impacts to the lower river zone.
The processes will also generate the two additional tertiary impacts of
deforestation along the river and displacement of wildlife living in or
along the river. Although the focus of human activities will be on
agriculture, some increase in fishing pressure as a result of the larger
population can be expected. In particular, noncultivated floodplains may
experience additional fishing during the rainy season because a pirogue
is not required for access to them. Fishing pressure in the main channel
will probably not increase much above present levels.
4.5.2. Kekreti Storage Dam
The Kekreti Storage Dam will generate many impacts in the lower river
zone, despite the fact that it will be located more than 200 km upstream
from the end of this zone. Most of the downstream impacts generated by
the dam will arise from the irrigation network associated with the
development program rather than from the presence of the dam itself.
Thus, the extent of irrigation in the Gambia River will greatly influence
the degree to which the various impacts affect the river. While the
different types of impacts will remain the same, irrespective of the size
of the irrigation network, the intensity of all of the impacts will be
greatly affected by the size of the network.
The major primary impacts associated with the Kekreti Storage Dam
will be those of modification to streamflows and annual floods. The
alteration and regulation of the natural streamflow patterns in the river
will greatly reduce the seasonal nature of the lower river zone. The
historic annual flood will be reduced by more than 50 percent in volume
as will the inundation of numerous small floodplains. The annual flood
is the primary mechanism by which nutrients enter the lower river zone;
this nutrient pulse will be reduced and spread over the entire year.
Water released over the dam will also have a different thermal and
nutrient regime compared to current natural conditions. Nutrients will
be removed from river water as it is stored in the reservoir; waters
flowing over the dam will generally have lower nutrient concentrations
than at present. Likewise, suspended sediments will settle out of the
water in the reservoir and thus, clearer water should reach the lower
river zone after the dam is constructed. In contrast, during the

129
construction phase of the dam, suspended solid loads in the river will be
greatly elevated near Kekreti from the work in the river.
The post-Kekreti irrigation network in the lower river zone will be
the most extensive of all the five zones of the river. Of the total
85,000 ha that may be brought under irrigation, about 65,000 ha, or 75%
of the total, is planned for the lower river zone (Harza, 1985). This
extensive network will cause major changes to the river and river bank
environment along the Gambia River. River banks will be physically
modified and the result will be a large loss of floodplains. The
modified river environment will in turn cause a shift in the physical and
chemical regime of the river. Water used for irrigation will exchange
nutr.ients with the exposed soil of the agriculture fields. Water used
for irrigation will also become warmer as it lays in shallow irrigation
canals.
The alteration to the seasonal cycle of nutrients by the Kekreti
Storage Dam will cause some secondary impacts to the biota of the river.
The annual surge of algal productivity observed in the early stages of
the flood will probably not develop under the new flow regime. The
irrigation canals may in turn become the site of most of the aquatic
primary productivity, as aquatic weeds grow in an environment more suited
to their needs than the present river channeL Detritus-feeding animals
will be favored at the expense of plankton-feeders.
The tertiary impacts generated by the Kekreti Storage Dam to the
lower river zone will be similar to those associated with an increase in
irrigation. Thus, the same impacts will develop as with the
no-development scenario, but the magnitude of the impacts will be much
greater because of the larger irrigation program. These impacts include:
• pollution of river water from fertilizers and toxic
substances
• human resettlement along the river banks
• deforestation of the land adjacent to the river
• displaced wildlife from the river and river banks
• increases of commerce and transportation activities along
the river
All of these impacts can be expected to increase in direct proportion to
the amount of land brought under irrigation in the lower river zone.

4.5.3.
130
Kekreti Storage Dam and Guinean Dams
The impacts to the lower river zone from the combination of the
Kekreti Storage Dam and the Guinean Dams will be the same as those from
the Kekreti Storage Dam alone. The purpose of the Kekreti Storage Dam is
to provide freshwater reserves for both irrigation· and hydropower. The
Guinean Dams will only serve to augment those reserves and not to change
the basic regulation of streamflows in the Gambia River. Streamflow
patterns downstream from Kekreti will be slightly altered by the
construction of the Kogou Foulbe Dam and unaffected by the over two dams.
4.5.4. Kekreti Storage Dam and Balingho Salinity Barrage
The addition of the Balingho Salinity Barrage with the Kekreti
Storage Dam will generate a large number of impacts in the lower river
zone. As mentioned above, the largest impact of the dam at Kekreti is
the irrigation network which will be developed in the lower river zone.
The Balingho Salinity Barrage will allow a further development of that
irrigation network because saltwater penetration up to the agricultural
fields will be prevented by the barrage. Thus the irrigation network may
be maximally developed with the addition of the salinity barrage.
However, construction of that barrage is not without environmental costs
even in the lower river zone, which begins almost 80 km upstream from the
barrage site. Most of the primary impacts associated with this
development scenario are the same as those for the Kekreti Storage Dam
only scenario. These impacts, which were discussed in section 4.5.2.,
are:
0 regulated streamflows
0 altered annual streamflow patterns
0 altered suspended sediment regime
0 altered nutrient regime
0 loss of floodplains
0 alteration to river banks
0 increased evaporation from river
0 increased suspended sediment loads during construction
The Balingho Salinity Barrage will add four more primary impac ts beyond
those generated by the Kekreti Dam. These include:

131
• lack of tidal mixing above the barrage
• seasonal development of anoxia
• sediment accumulation in the bolons
• vertical thermal stratification
Because the barrage will not allow tidal waves to move upstream, tidal
mixing (an important process in the existing system) will cease beyond
Balingho. Without tidal mixing, waters in the river will become
relatively stagnant causing anoxia to develop during the warm months of
the dry season in the deeper portions of the river channel. Sediments
will accumulate in all of the small bolons and thus, prevent water
exchange between the bolons and the main river channel. Finally, without
mixing, the deep portions of the river channel (between 15 and 30 m deep)
will probably undergo vertical thermal stratification, further promoting
development of anoxia in deep waters. The withdrawal of water for
irrigation will also promote higher rates of evaporation and
evapotranspiration than under the current conditions.
The primary impacts to the lower river zone will have two major
consequences for the aquatic biota. First, the zone will expand down to
Balingho. Second, the waters of this zone will become stagnant without
tidal mixing. As a result, different flora and fauna will inhabit this
segment of the river. Aquatic weeds, which are only a minimal component
of the current flora, could become a major problem in the quiet waters.
The plankton and benthic (bottom-dwelling) invertebrates which depend
upon well-mixed water will cease to exist in this segment of the river.
Organisms which conduct annual migrations from the estuary into the lower
river zone will no longer be able to complete their life cycle. The
Balingho Salinity Barrage will become an impenetrable barrier to
migration; this impact will be somewhat minimal in the lower river zone
because no species were identified that migrate from estuarine to
freshwater portions of the river (Dorr et al., 1985; van Maren, 1985).
All of the tertiary impacts associated with river basin development
in the lower river zone will be the result of the implementation of the
irrigation network and the influx of people to the river basin. The
addition of the Balingho Salinity Barrage to the development program with
the Kekreti Storage Dam will not add any new tertiary impacts beyond
those discussed in section 4.5.2. Those impacts are:

132
• pollution from fertilizers and toxic substances
• human resettlement along the river
• deforestation along the river
• displacement of aquatic wildlife
• enhanced commerce along the river
• increased farming activities near the river
• increased public health problems associated with waterdependent
disease vectors
The addition of the salinity barrage may enhance the effects of
these impacts because the irrigation network can be expanded with
the barrage in place. But the types of impacts will not change from
those created by the Kekreti dam alone.
4.5.5. Kekreti Storage Dam, Guinean Dams, and
Balingho Salinity Barrage
The addition of three dams in Guinea upstream from the Kekreti
Storage Dam will have little effect on the lower river zone beyond those
already imposed by the Kekreti dam and the salinity barrage. The
addition of the Guinean Dams may allow an increase in the amount of
irrigated cropland, but the types of impacts will not change.
4.6. Upper Freshwater River Zone
The upper river zone is that portion of the Gambia River which is a
relatively slow-flowing river without tidal mixing. This segment of the
river remains freshwater throughout the entire year. The approximate
geographic boundaries of the upper river zone are from Gouloumbou,
approximately 525 km upstream, to the Senegal-Guinea border, about 965 km
upstream (see Figure 3.1). The river is relatively shallow throughout
most of this zone with occasional deep holes up to 15 m deep; extensive
floodplains are absent.
The upper river zone has an extremely seasonal nature, being
dominated by the annual flood. During the rainy season the river rises
as much as 13 m above the dry season level. The rainy season river not
only has very elevated streamflows, but the chemical characteristics are
altered from the dry season (Berry et al., 1985). The elevated

133
streamflows of the rainy season carry high suspended sediment loads. The
rainy season river also has somewhat elevated nutrient concentrations,
especially dissolved nitrate-nitrogen (Berry et al., 1985). Streamflows
in the river can increase significantly after one major storm in the
drainage basin. During the dry season the Gambia River here was observed
to dry up to pools (no net flow).
The Gambia River in the upper river zone is relatively unproductive
compared to the rich estuary. Algal production in the river is
relatively low, especially during the months of high suspended sediment
loads (Healey et al., 1985). Because the river is shallow, the volume of
water available for algal production is small, yielding low overall
productivity. Inputs of organic matter frotn the land into the river
appear relatively small because the land itself is not heavily foliated.
As a result, the river supports a somewhat meager fishery, although
fishing pressure may be high in areas near villages (Josserand, 1985).
4.6.1. No Development or Steady State
The eastern portion of Senegal, known as Senegal Oriental, has a
relatively sparse population, most of which lives in the Gambia River
Basin. This population has exploited most of the local existing
resources to their maximum, including freshwater reserves. Mineral
resources may be the principal untapped resource of the region (Moran,
1984). Population growth in the Senegal Oriental portion of the basin
will probably be only moderate, despite mineral exploitation, because of
limited freshwater supplies. Some additional farming and some commerce
can be expected, but this should proceed at a relatively slow pace.
Associated with this growth will be some impacts, but their effects will
also be relatively minor because of the limited extent of growth. Most
of these impacts are tertiary, a consequence of the increased human
activities. The only perceived primary impact to the river is a small
increase in nutrient concentrations in the river water from cropland
runoff. This small increase may in turn stimulate an increase in aquatic
primary productivity. Use of water for farming will also cause loss of
water.from evaporation off the fields and evapotranspiration from plants.
include:
The tertiary impacts associated with the no development scenario

134
• increased pollution from fertilizers and toxic substances
• increased pollution from mining activities
• human resettlement along the river banks
• further deforestation along the river banks
• displaced aquatic wildlife
• continued fishing
• increased commerce adjacent to the river
The magnitude of most of these tertiary impacts will depend upon the
level of human activity along the river. As mentioned above, because
freshwater reserves are already tapped to their limit, the potential for
additional human growth along the river is minimal.
4.6.2. Kekreti Storage Dam
The planned Kekreti Storage Dam site is approximately 790 km upstream
from the river's mouth at Banjul. The reservoir created by the dam will
extend up to 70 km upstream from the dam site and will fall almost
directly in the middle of the upper river zone. OVer 15% of that zone
will be altered from a riverine environment to a lacustrine environment.
The vast majority of the water in the river will end up in the
reservoir. The primary impacts to the Gambia River in the upper river
zone from this dam will be numerous and very large in magnitude. Most of
these impacts will be generated as a result of the alteration of the
river to include a reservoir, and not so much from the development of an
irrigation network. The major portion of the irrigation network will be
located in the lower river zone in The Gambia.
The Kekreti Storage Dam will completely alter the pattern of annual
streamflows. The seasonal flood and dry season will be eliminated and
replaced by a consistent year-round streamflow (Harza, 1985). The large
mass of water in the reservoir will be relatively stagnant. This
nonflowing body of water will cause some changes to the overall quality
of water in the river. The reservoir will serve as an ideal place for
suspended sediment to settle out of the water column, resulting in
clearer water. However, during construction a huge amount of sediment
will enter the river and be carried many kilometers downstream. This
increase in sediment from construction activity will probably persist

135
throughout the entire construction phase. The ultimate effect may be
permanent loss of aquatic life from segments of the river.
The large nonflowing reservoir will become vertically stratified
during a portion of the year and thus not mix readily from top to
bottom. The lack of mixing will provide the correct environment for the
formation of anoxia in the lower strata. This stratification and anoxia
will also alter the nutrient regime in the reservoir. particulate matter
will settle to the bottom, carrying nutrients. These nutrients cannot
readily mix back into the water because of the stratification. The lack
of suspended sediments and nutrients in the surface layers of the
reservoir waters will produce clearer waters than the current muddy river
waters. The large reservoir will also have extremely high rates of
evaporation during the dry season (AHT/HHL, 1984).
The development of a reservoir will have an enormous effect on the
river bank environment. The existing river banks in the vicinity of the
reservoir will be permanantly flooded. The reservoir will be subject to
annual drawdown, or the lowering of the reservoir water levels exposing
the reservoir banks. Downstream from the dam, the regulated streamflows
will prevent the annual inundation of the river banks and floodplains
during the flood season. This will eliminate the annual exchange of
nutrients and sediment between the river and the inundated areas. The
lack of annual flood will also cause the loss of many hectares of
seasonal floodplains. The constant moderate level of streamflow released
from the reservoir will cause substantial erosion to the river banks; the
river bottom appears sufficiently armored just downstream from Kekreti to
prevent significant erosion (Jasinski, personal communication).
Fundamental alteration to most of the physical environment of the
upper river zone (together, the reservoir and downstream area from the
dam constitute almost 80% of the length of the zone) will generate a
large number of secondary impacts. The reservoir itself will promote a
complete shift in the aquatic flora and fauna from the riverine
environment. Phytoplankton will find a suitable habitat in the surface
waters of the reservoir. The increase in primary productivity in the
reservoir will prompt a new food web to develop around the plankton
commlllli ty. This new lacustrine food web will ultimately be considerably
more productive than the existing riverine food web. Fish yields will be

136
higher (see Chapter 6.) and lead to productive fisheries. The pelagic
food web will not be the only portion of the biota affected by the
presence of the reservoir. The benthic community will be adversely
affected, assuming the development of anoxia in the bottom waters of the
reservoir (van Mar en , 1985). Neither fish nor many invertebrates can
survive anoxic conditions. Thus, useful biological production will be
limited to the upper oxygenated waters of the reservoir and their
underlying substrate. Additionally, water drawn from the anoxic portion
of the reservoir and discharged through the dam could have high hydrogen
sulfide content and thus adversely affect the aquatic flora and fauna
downstream of the reservoir. Shallow areas of tropical reservoirs are
also a prime environment for aquatic weed growth. Ultimately certain of
1
these weeds can overgrow the entire reservoir if left uncontrolled.
The weeds affect the mechanical operation of the dam, as well as supress
the amount of aquatic primary production in the water column. Also, they
promote water loss from the reservoir via high rates of
evapotranspiration.
Downstream from the dam several secondary impacts can be expected as
well. The alteration of the annual streamflow patterns will cause a
change in many of the aquatic species found in the river. For example,
fish which rely upon the seasonally available floodplains for spawning
sites will not be able to complete their life cycles and eventually be
eliminated from the area. Organisms which depend upon the annual
enrichment of the river waters during the flood season, will also not be
able to compete when the flood is eliminated.
The tertiary impacts associated with the Kekreti Storage Dam are
primarily associated with the development of a large reservoir in the
middle of the upper river zone. Most of the irrigation network developed
as part of the river basin program will be in The Gambia (80% of the
total irrigated hectares will be in The Gambia). But there will be some
IA large threat exists to the Gambia River from the introduction of
the water lily, Eichornia crassipes. This floating plant has been a
major problem in many reservoirs because it does not require a firm
substrate for growth. Floating mats of Eichornia can clog the reservoir
as well as provide habitat for disease vectors. The plant has been
observed in ornamental ponds near Banjul.

137
expansion of cropland in Senegal Oriental, thus the impacts associated
with expanded agriculture will be observed in the upper river zone.
Impacts from increased agriculture include the pollution of river water
from fertilizers and toxic substances, deforestation of the river banks,
displacement of aquatic wildlife, as well as a shift in occupations
toward farming.
The reservoir will also attract a large number of people to the edge
of the river to take advantage of the freshwater reserves. This human
resettlement will ultimately impact the waters of the Gambia River as
wastes and debris from human activities pollute the river. Likewise, the
population growth along the river and reservoir banks will increase
commerce in these areas and further serve to pollute the water. The
large body of standing water adjacent to a human population will provide
an ideal environment for the growth of water-dependent disease vectors.
The presence of a large reservoir in the upper river zone will also
have an extremely beneficial impact. The reservoir will be considerably
more productive than the existing riverine environment (see Chapter 6.).
The higher level of productivity may support a fishery which will provide
a new source of food to the region. Ultimately, the reservoir fishery
will provide additional food, employment, and income for many people in
Senegal Oriental.
The freshwater reserves and hydropower provided by the Kekreti
Storage Dam and reservoir will provide sufficient water and electricity
to support increased mining activities in eastern Senegal. The overall
impacts from improper mining activities can cause widespread
environmental damage to the Gambia River. The most common impact from
mining activities is pollution of water by acidic runoff and heavy metal
contamination. Because the waters of the upper Gambia River have
extremely low buffering capacity, the aquatic environment is highly
vulnerable to the effects of acid pollution. Acidic pollution of water
can almost completely destroy any intrinsic value of that resource.
Highly acidic waters (pH below 4.5) will not support most types of
aquatic life and are also dangerous for consumption by humans and
livestock. Furthermore, acid runoff usually mobilizes heavy metals,
which in turn, poison the water, making it even less suited for wildlife
and human use. The northeastern United States and Canada have suffered

139
Figure 3.1). The headwaters zone extends through relatively mountainous
terrain beginning about 965 km upstream to slightly over 1100 km
upstream. This zone is characterized by an extremely dendritic pattern
of small streams and rivers running northward into Senegal Oriental.
Many of these streams and rivers are intermittent, being pooled or dry
throughout much of the year. The river channels are typically scoured
out of bedrock.
Similar to the upper river zone, the headwaters zone is characterized
by a highly seasonal pattern of streamflows. During the dry season, most
of the river beds are either completely dry or reduced to pools (3 km or
less in length). The rainy season provokes the annual flood which
elevates water levels as much as 15 m above the river beds. The chemical
composition of the water in the Gambia River is also significantly
changed during the annual flood; flood waters are characterized by
increased suspended sediment loads and elevated nutrient concentrations.
The rivers in the Guinean Highlands are highly influenced by individual
rain events. Water levels in the river have been observed to rise as
much as one meter from a single large storm in the basin (Berry et al.,
1985). The runoff from a storm will also change the chemical composition
of the river for several days after the storm has passed.
The Gambia River in the headwaters zone is relatively unproductive.
The water is clear allowing sufficient penetration of light for
photosynthesis, but nutrient levels are low and thus probably limit
productivity (Healey et al., 1985). In addition, the volume of water in
the river is small except during the flood, when optical transparency
becomes low. When the river dries to pools in the late dry season,
resources in the small, unconnected pools are quickly depleted.
Fish often concentrate in these pools as the water recedes which
results in locally high densities (standing crops). As the dry season
continues, the numbers of fish decrease through fishing or natural
mortality, which combined with depletion of prey, reduces stocks to very
low levels of abundance. Those remnant populations which survive,
usually spawn during the next flood season. Some species may lose most
of the breeding population during the dry season, with only eggs or a few
young, surviving in pools or wet mud. The net result is that during much
of the year population densities are considerably lower than those found

140
in the pools early in the dry season. Therefore, presently the
headwaters zone has low fish production relative to the other portions of
the river.
4.7.1. No Development or Steady State
Even without the implementation of the river basin development
program, a moderate amount of development may still take place in the
headwaters zone. This development would include a small increase in the
amount of farming and some mining activities. Because rainfall in the
Guinean Highlands is more abundant than any other portion of the Gambia
River Basin (1 to 2 m per year), the potential for further agricultural
development exists. The current impediment to development in the
headwaters zone appears to arise more from lack of a transportation and
communication network than from any other factor. If this impediment
were overcome, a moderate level of development could be expected without
the construction of any dams. This moderate amount of development could
be expected to generate some impacts to the Gambia River.
Those impacts associated with farming include two primary impacts,
two secondary impacts, and five tertiary impacts. The primary impacts
from farming are the possible alteration of nutrient regimes in the river
and increased sediment loads. Water which runs off the agricultural
fields will carry a different nutrient load than runoff from undisturbed
soil. The agricultural runoff usually has higher concentrations of
nitrogen and phosphorus. The elevated nutrient concentrations could, in
turn, stimulate aquatic weed growth and thus create a secondary impact.
Excessive aquatic weed growth would cause another secondary impact of
high rates of water loss from the river due to evapotranspiration.
Tertiary impacts from agricultural activity would include:
• pollution from the use of fertilizer and toxic substances
on fields
• deforestation near the river
• higher levels of commerce in the vicinity of the river
• displacement of aquatic wildlife
• resettlement of human populations along the river
As has been mentioned above, all of these impacts cause a deterioration
in water quality in the Gambia River because many pollutants will be
added to the river by human activities.

141
Mining activities in the headwaters zone represent the largest single
threat to the Gambia River's water quality. The impact of pollution from
mining wastes could ultimately destroy the quality of water in a large
section of the river. The primary impact from mining is the improper
handling of wastes, which in turn create large amounts of extremely
acidic runoff. This acidic runoff will mobilize heavy metals and thus
pollute the river waters in two ways: acidification and heavy metal
contamination. The combination of these two forms of pollution could
render the river water useless for most purposes. Gambia River water in
the headwaters zone has extremely low buffering capacity, similar to the
upper river zone. This poorly buffered water is extremely vulnerable to
the effects of acid pollution. This would make the effects of pollution
from mining activities both severe and widespread throughout the
headwaters zone. The consequences of acid pollution to the Gambia River
would include the destruction of most forms of aquatic life in the river,
as well as making the water unsafe for consumption by humans and
livestock. Highly acidic waters also cannot be used for most forms of
agriculture. The heavy metal contamination -- especially aluminum which
is abundant as bauxite in the Guinean highlands (Moran, 1984) -- along
with the acid pollution serves to poison the water and exacerbate the
effects of the acid pollution.
4.7.2. Kekreti Storage Dam
The construction of the water storage dam at Kekreti would have no
effect on the headwaters zone of the Gambia River, because the dam would
be downstream of this zone. The only possible impact generated in the
headwaters zone by this dam would be additional mining activities if the
Senegalese decide to share the hydropower from the Kekreti Storage Dam
with the Guineans. The additional mining activities would increase the
magnitude of the impacts associated with mining discussed above, but not
add any new impacts.
4.7.3. Kekreti Storage Dam and Guinean Dams
All of the impacts to the Gambia River in the headwaters zone from
the river basin development program will arise from the construction of
three dams in the Guinean Highlands. One dam each is planned for the

142
Gambia, Liti, and Koulountou rivers. The dams on the Gambia and Liti
rivers are to be located at approximately 1,050 km upstream from the
river's mouth at Banjul. The impacts to the Gambia River from these
three dams will be very similar to those created in the upper river zone
by the Kekreti Storage Dam (see section 4.4.2.). The primary impacts
from each dam include the alteration and regulation of streamflows in the
rivers. These altered streamflow patterns will eliminate the seasonal
characteristics of the Gambia River downstream from the dams (about 2/3
of the total headwaters zone). The elimination of the annual flood will
also remove the seasonal floodplains from the river environment. The
lack of flood will cease the annual scouring of the river banks from high
streamflows, eventually causing an accumulation of soft sediment along
many of the river banks. This shift in bottom composition from rock to
mud will alter the benthic community to those organisms which prefer soft
sediments.
The development of reservoirs behind each of the dams will also have
an effect on the quality of water in the Gambia River. The calm, deep
waters of the reservoirs will allow the settling of much of the suspended
sediment load of the river water. Thus, river water discharged over the
dams will be clearer than river water currently flowing through the
headwaters zone, although current suspended sediment concentrations are
low. The loss of suspended sediment from the water column in the
reservoirs will promote an increase in the optical transparency of the
water. This beneficial impact will be most important during the rainy
season when the flood waters of the river normally carry a large sediment
load and are optically very opaque. The calm waters of the reservoirs
will also allow the development of vertical thermal stratification. The
presence of vertical thermal stratification will prevent mixing to the
bottom of the reservoir. The lack of vertical mixing will generate two
additional primary impacts: development of seasonal anoxia. in the bottom
waters of the reservoir, and a change in the nutrient regime of the water
column. The nutrient regime will change because, as particulate
materials sink to the bottom of the reservoir, they carry nutrients with
them. The lack of mixing will prevent these nutrients from being
returned to the water column.

144
• pollution by fertilizers and toxic substances from
agricultural practices
• pollution from mining activities
• deforestation from increased farming and mining
• resettlement of human populations along the river
• increased commercial activities near the river
• displacement or loss of aquatic wildlife
While the these same impacts will be observed with or without the
implimentation of the proposed development program, the extent and
severity of the impacts will be much greater after development.
Pollution from farming and mining activities will be much more extensive
after the development program is put into effect.
The three additional tertiary impacts which will be created by the
presence of the Guinean Dams will be beneficial impacts associated with
the presence of the three reservoirs. As mentioned above, the reservoirs
will be more productive than the existing riverine environment and
provide more fish. These fish will provide an additional local source of
food for the residents of the Gambia River Basin in the headwaters zone.
The expanding fishery will support ancillary service and supply sectors,
as well as provide direct employment for additional fishermen. Likewise,
the reservoirs will provide greater freshwater reserves to allow
expansion of the agricultural system also to provide more local food
sources. With the increase in domestic food sources, people can be
expected to migrate closer to the river and reservoirs to enjoy the
enhanced food and freshwater supplies. Unfortunately, this influx of
people toward the river can be expected to add to the overall pollution
of the river, as well as to increase human exposure to water-related
diseases.
4.7.4. Kekreti Storage Dam and Balingho Salinity Barrage
The construction of both the storage dam and salinity barrage will
have no effects on the Gambia River in the headwaters zone. The most
upstream effect of both dams will not reach past 850 km from the river's
mouth at Banjul, over 100 km downstream from the headwaters zone.

4.7.5.
145
Kekreti Storage Dam, Guinean Dams, and Balingho Salinity Barrage
The impacts to the Gambia River in the headwaters zone from the three
proposed dams in Guinea will not change with the addition of the two
downstream dams. Therefore, the impacts from all five dams will be the
same as those from the four-dam scenario. Those impacts were listed in
section 4.7.3., and are the same for this section.

148
TABLE 5.1
SUGGESTED MITIGATION MEASURES FOR ANTICIPATED IMPACTS TO THE
GAMBIA RIVER FROM PROPOSED DEVELOPMENT PROGRAM
IMPACT MITIGATION MEASURE
Physical-Chemical Impacts
Regulated Annual Flows
Altered Streamflows
Altered Thermal Regimes
Anoxic Bottom Waters
Altered Nutrient Concentrations
Increased Suspended Solids
Downstream of Dams
Increased Underwater Light
Increased Suspended Solids
During Construction of Dams
Modifications to River Banks
Loss of Flood Plains
Creation of Drawdown Zone
Increased Evaporation
No Tidal Mixing Above Barrage
Increased Tidal Amplitudes
Lack of Salinity Upstream
of Barrage
Lack of Salinity Gradient
in Estuary
Acid-Sulfate Soil Formation
Release water from reservoirs to
include controlled annual flood
Careful release of water from
reservoirs
Mix reservoirs to prevent stagnation
Mix reservoirs to prevent stagnation
None possible a
Reduce streamflows from cutting soft
banks
None needed b
Careful construction practices and
isolate sediments from river
Careful construction along banks and
controlled annual flood
Controlled annual flood
Restrict water withdrawl from
reservoirs
None possible
None possible
None possible
None possible
None possible
Careful water management

Sediment Accumulation in
Bolons
Formation of Hypersaline
Water
Biological Impacts
Species Shifts in Reservoirs
Increased Algal Production
Benthic Species Changes
Increased Fish Production
in Reservoirs
Aquatic Weed Growth
High Rates of
Evapotranspiration
Elimination and Restructure
of Mangrove Forests
Blocked Migrations
Elimination of Marine
Plankton Above Barrage
Greater Estuarine Fish
Production
Elimination of Marine
Invertebrates Above Barrage
Anthropogenic Impacts
Increased Use of Fertilizers
Pesticides and Herbicides
Increased Fish Harvest from
Reservoirs
Preference to Eat Fish
149
TABLE 5.1
(continued)
None possible
Allow limited freshwater discharge
past barrage
None needed
None needed
None needed
None needed
Mechanical control measures
None possible
None possible
None possible
None possible
None needed
None possible
Limited and careful application
None needed
None needed

151
5.1.1.1. Regulated annual streamflows. A major 0 bjective of the
development program is to provide nearly uniform annual streamflows for
the generation of hydropower and to provide water for irrigation. These
regulated streamflows would significantly reduce the annual flood from
the Gambia River. The natural historic flood has been crucial for many
aquatic organisms to maintain their life cycles. One method to mitigate
this impact is to regulate flows through and over the dams so that a
moderate annual flood would still occur. The logical time to conduct
this controlled flood is near the end of the rainy season when reservoir
levels should be high and even occasionally overflowing. Such a flood
would occur at roughly the same time as the natural flood. Using
freshwater reserves for a controlled flood might require a compromise in
the use of freshwater for irrigation and hydropower, for instance, when
there are insufficient freshwater reserves.
5.1.1.2. Altered streamflows. Regulation of the Gambia River will
change streamflows from a highly seasonal pattern of high flows during
the rainy season and low flows during the dry season, to a more even
annual pattern (Figure 5.1). Many organisms cannot tolerate very high or
low flows and thus key their life cycles to avoid those periods. For
example, insect larvae emerge from the river and lay eggs before the
rainy season (van Maren, 1985). The eggs can tolerate the high
streamflows much better than the larvae. The regulation of streamflows
will therefore favor some aquatic organisms at the expense of others. To
mitigate this impact, the release of water from the reservoirs should
attempt to match as closely as possible the existing pattern of annual
streamflows. Similar to the previous impact, this form of mitigation
will require compromises among many fresh water uses.
5.1.1.3. Altered thermal regimes. Calm waters of the reservoirs
will cause a lens of warmer water to develop during certain seasons on
top of the cooler deep waters. This vertical thermal stratification will
in turn prevent the mixing of essential elements throughout the
reservoir. Incomplete mixing will cause problems such as anoxia, which
is the absence of dissolved oxygen in bottom waters. Mitigation of this
impact requires mixing reservoir waters from the surface to the bottom.
This process can be accomplished by bubblers or some other mechanical
process. Wind mixing can also mitigate stagnation in reservoirs, but

153
winds rarely reach a sufficient force for a long duration to achieve
complete mixing. In practice, mechanical mixing is rarely invoked on a
large scale because of cost. Additionally, the extent of mixing required
to offset adverse conditions associated with stagnation is difficult to
predict in advance. Thus mechanical mixing might have to be available on
an "as needed basis."
5.1.1.4. Anoxic bottom waters in reservoirs. A major negative
impac t from the formation of reservoirs is the development of anoxic
waters in the bottom of reservoirs. This impact, which can be expected
throughout much of the dry season in all of the reservoirs, has an
extremely adverse effect on the aquatic flora and fauna. Mitigation of
this impact is the same as for the preceding one: mixing of the
reservoirs from the surface to the bottom. The waters of well-mixed
reservoirs carry sufficient dissolved oxygen to prevent the formation of
anoxia. Again, mechanical mixing is a costly and therefore a seldom-used
procedure. The development of anoxia in the bottom of reservoirs may
also cause adverse impacts downstream from the dams. If water is
released from the bottom of the reservoirs through the dam, anoxic
conditions may prevail several kilometers below the dam. These anoxic
waters will also destroy aquatic organisms just as. they do in the bottom
of the reservoirs. Downstream anoxia can be controlled either by
removing water from the top of the reservoir, or by aerating water drawn
from the bottom of the reservoir.
5.1.1.5. Altered nutrient regimes in reservoirs. The calm,
stratified waters of reservoirs will remove nutrients from the water
column by settling after they become absorbed to particulate materials.
Conversely, the irrigation network will add nutrients to river water by
use of fertilizers. The net result will be a change in the nutrient
regime throughout much of the Gambia River. The removal or addition of
nutrients in moderation will have little impact on the aquatic flora and
fauna, while large additions will have a major impact. Mitigation of
nutrient removal is probably not necessary and will be achieved by the
reservoir mixing scheme suggested above. The mixing process allows
nutrients normally trapped in bottom waters to return to the surface and
become available to algal photosynthesis. Mitigation of nutrient
additions to the river can be accomplished by careful application and

154
control of fertilizers on the irrigated crops. This concept is discussed
in more detail in section 5.1.3.1.
5.1.1. 6. Altered suspended solids loads. Dams and reservoirs along
the Gambia River will serve to allow much of the suspended sediments to
settle out of the river water. The irrigation networks, in contrast,
will add sediments due to certain farming practices. As a result, the
suspended solids loads of the Gambia River will be greatly altered from
existing conditions. The primary expectation is that the suspended
solids will decrease in most of the system, which requires no mitigation
in that this is a favorable impact. But, in some places (especially just
downstream of the dams), rapidly flowing water released over the dams
will transport high volumes of suspended solids. Reducing flows from the
dams is one method to mitigate this impact. Another method is to create
pools just downstream of the dams to allow the sediment to settle out of
the water column; these could require more or less annual dredging.
Reducing sediment input to the river from irrigated fields can be
accomplished by isolating the fields from the main river channel with
bunds and/or small levies and effective soil conservation practices.
Carefully managed farming practices will be the most effective method to
mitigate the impact of sediment release from the irrigated crops.
5.1.1.7. Increased underwater light transparency. The reduction of
suspended solids loads discussed above will have a very beneficial impact
in that water clarity will improve. The removal of sediment from the
river by settling will increase the clarity of water and thus result in
an overall improvement in river water quality and stimulate primary
production. This beneficial impact will not require any mitigation.
5.1.1.8. Increased suspended solids due to dam and irrigation
network construction. A large amount of sediment will enter the Gambia
River during all phases of construction of the dams. This sediment can
be disastrous for much of the aquatic flora and fauna, especially
attached or rooted organisms. The mitigation of this impact is
conceptually rather simple: the strict control of the release or
movement of any sediment into the river. But in practice it is often
difficult to get construction crews to comply with the need to control
sediment releases. Sediment movement can be controlled by erosion
barriers along the banks of the river and road grades, and isolation of

155
construction sites from flowing river water.
construction activities with minimum river
overall downstream transport of sediment.
The timing of high erosion
flows will also reduce the
5.1.1.9. Modification to river banks. Construction of the
irrigation network and dams, filling of reservoirs, and regulation of
streamf10ws will change the nature of the river bank environment. These
changes will arise both from physical modifications to the river banks as
well as from lack of seasonal inundation and drying. These altered banks
will affect much of the flora and fauna that use the banks as part of
their habitat. The primary method of mitigation of this impact will be
to keep the. river banks as close to natural conditions as possible. This
requires restricting construction and other development activities along
the river banks. The simulated annual flood described in section
5.1.1.1. will provide some inundation of river banks and also mitigate
this impact.
5.1.1.10. Loss of seasonal floodplains. Regulation of streamf10ws in
the Gambia River will eliminate the annual flood which in turn will
eliminate the inundation of seasonal floodplains. These floodplains have
been identified as vital areas as fish spawning sites as well for
providing nutrients to the river (Welcomme, 1979). The controlled flood
described in section 5.1.1.1. should provide the freshwater reserves to
inundate the floodplains for one to two months. An additional step to
mitigate this impact is to restrict the development of the irrigation
network so that some floodplains are kept in their natural state in all
zones of the river. In selected locations, floodplains could be
sequentially used for fish spawning sites when inundated and later as
cropland for recession agriculture, as they are now.
5.1.1.11. Development of draw-down zones. Removal of water from the
reservoirs will create regions of exposed reservoir bottoms. These zones
are unsuitable to aquatic organisms which cannot tolerate long-term
exposure to the air where they become dessicated. However, drawdown
zones may serve useful purposes either just after they are flooded (fish
spawning sites) or for nonaquatic purposes (cattle grazing). The only
method available to mitigate this impact is to limit the removal of water
from the reservoir. But this method cannot be readily applied because
demands for use of freshwater require release of water from the
reservoirs.

156
5.1.1.12. Increased evaporation from reservoirs surfaces.
Impoundment of water in large reservoirs creates losses of water due to
evaporation. No practical mitigation of this impact is envisioned for
the reservoirs of the Gambia River. Increased evaporation is a natural
and unavoidable consequence of river basin development.
5.1.1.13. Lack of tidal mixing above the salinity barrage. The
Balingho Salinity Barrage will become an impenetrable barrier for tidal
waves moving up the Gambia River. These tidal waves and associated river
currents cause mixing within the river which is vital to stimulate
overaIl productivity (Twilley, 1985; Healey et a1., 1985). This maj or
impact cannot be mitigated unless the gates of the barrage are opened to
allow tidal waves to move beyond Balingho.
5.1.1.14. Increased tidal amplitude downstream from the barrage.
Reflection of tidal waves off the barrage will increase the range of the
tides inmediately downstream of the barrage from 10 to 20%. These
increased ranges will have a destabilizing effect on the mangrove
forests. The only methods available to mitigate this impact are the same
as those listed above in section 5.1.1.13., either to leave the barrage
gates open, or to eliminate the barrage from the proposed development
program.
5.1.1.15. Removal of saltwater from above Balingho. The salinity
barrage will prevent the intrusion of saltwater beyond Balingho.
Therefore, an 80 km segment of the river which is currently estuarine
during part of the year will become permanently freshwater. This will
cause a large shift in species composition, including associated fishes,
and the elimination of at least 12% of the mangrove forests. This major
impac t cannot be mitigated except by leaving the barrage gates open or
eliminating the barrage from the proposed development program.
5.1.1.16. Absence of a salinity gradient in the river. A fundamental
part of any estuary is the natural salinity gradient between freshwater
and saltwater. This gradient provides the multitude of environments and
stimuli which many organisms require either to survive or complete their
life cycles. The gradient also prOVides a natural buffer between the
entirely marine species and the freshwater species. Unless freshwater is
allowed to flow downstream, the salinity barrage will eliminate this
natural gradient. Mitigation of this impact is possible either by

157
year-round release of freshwater past the barrage, or by deleting the
barrage from the proposed development program.
s.LL17. Formation of acid-sulfate soils. The potential is very
high for the development of acidic soils above Balingho after the barrage
is constructed. Although difficult, it is possible to prevent
acid-sulfate soil formation by extremely careful water management
schemes; preventing the soils from ever completely drying-out will
eliminate the acid formation. This management scheme may not be possible
if the freshwater in the reservoirs is used for extensive irrigation. A
more realistic approach is to permit a certain fraction of land to become
acidic and accept that as an unavoidable consequence of basin
development. However, the irrigation potential of the barrage will be
greatly reduced by the acid soils. In many places the acid eventually
leaches out of the soil after several years (20) and the land becomes fit
for agriculture.
5.LL18. Sediment accumulation in bolons. Lack of tidal mixing and
scouring currents in the mangrove bolons above Balingho will allow rapid
accumulation of sediments. Eventually these narrow channels will become
totally stagnant and nonproductive. Their inability to conduct water
between the river and the swamp rice fields behind the bolons will be
deleterious to most of the rice fields. Also, the stagnant waters will
become unproductive as well as serYe as a breeding site f or numerous
human disease vectors. The only method to mitigate this impact is to
allow tidal waves to continue to move upriver.
5.1.1.19. Formation of hypersalinity downstream of the barrage. Once
the full irrigation network has been developed, freshwater will flow for
only a few days per year past Balingho. During dry years this freshwater
flow may be restricted to less than 10 days. The lack of freshwater
flushing and high rates of evaporation can serve to elevate the salinity
of water just downstream of the barrage to well in excess of seawater.
Extremely elevated salinities will cause a loss of aquatic flora and
fauna from the Gambia River, and ultimately result in considerable
lower productivity of the estuary. Hypersalinity has also been the
cause of the failure of shrimp populations in other rivers in West Africa
(LeReste and Odinetz, 1984). Mitigation of this impact is possible
through the release of freshwater to prevent the accumulation of highly
saline waters immediately downstream of the barrage.

5.1.2.
158
Secondary (Biological) Impacts
The primary impacts discussed above will collectively result in up to
eleven secondary impacts. These secondary impacts are the changes to the
aquatic flora and fauna in response to the primary impacts. In many
ways, the secondary impacts are those which the human residents of the
river basin most readily observe. The mitigation of all but one of these
secondary impacts arises from the identification of the primary impact(s)
causing the secondary impact, and mitigation of that primary impact(s).
5.1.2.1. Species shifts in reservoirs. Conversion of parts of the
Gambia River from riverine to lacustrine environments will result in a
complete change in the aquatic community. The calm waters of the
reservoirs will favor the growth of many species which currently are only
found in small pools of the river. In some cases, shifts in aquatic
species will result in desirable consequences as increased fish growth.
In other cases, the consequences will be very undesirable, such as the
growth of aquatic weeds. However, in general the shifts should result in
a food web which is more productive than the existing riverine food web
(see Chapters 4 and 6). This favorable impact does not require
mitigation.
5.1.2.2. Increased algae growth in reservoirs. The calm, clear
waters of the reservoirs will provide a much more favorable habitat for
small floating algae (phytoplankton) than the flowing, turbid waters of
the river. As mentioned above, these algae will serve as the base for a
pelagic food web which will in turn stimulate secondary (zooplankton) and
tertiary (fish) production in the reservoirs. This favorable impact does
not require mitigation.
5.1.2.3. Benthic species shifts. Just as the changes in the aquatic
environment from riverine to lacustrine favor growth of certain pelage
species, the changes also favor certain benthic (living on the bottom)
species. For example, the fast-flowing waters of the rapids favor many
species of mayfly larvae, whereas calmer waters favor other animals such
as mollusks (van Maren, 1985). Furthermore, the anoxic bottom waters of
reservoirs are unsuitable habitats for most benthic organisms. In
overview, this impact is considered neither favorable nor detrimental,
but as an inevitable consequence of river basin development. There is no
apparent mitigative action for this impact.

159
5.L2.4. Increased fish stocks in reservoirs. The final result of
the increased water volume, available habitat, and reorganized food web
in the reservoirs will be increased fish biomass and potential yield of
fish via reservoir fisheries. Standing crop per unit area and
sustainable yields of fish in the reservoirs will be much higher than in
most areas of the existing river. A possible exception is the Balingho
reservoir where displacement of estuarine fauna will occur. This
favorable impact does not require mitigation, but rather a management
policy to capitalize on the new resource. A detailed study of dam
management for the purpose of optimizing fish yields in African
Reservoirs is presented by Bernacsek (1984). These principles should be
completely mastered once a decision is made to build a dam and an
operational policy established.
5 .L2.5. Aquatic weed growth in reservoirs and irrigation canals.
The calm waters of the reservoirs and canals will favor the growth of
extensive aquatic weeds. These weeds can eventually render a reservoir
useless by covering the water with thick mats of weeds, and clogging
navigation channels and the dam gates. Also, the weeds can provide a
habitat for undesirable human disease vectors. A variety of control
measures have been tried throughout the world, none of which have been
particularly successful (Freeman, 1974). Mechanical cutting and removal
of the weeds is probably the most effective and ecologically the least
damaging mitigating measure possible but is only feasible on a local
scale. Another method of weed control is the application of herbicides.
But, while this is temporarily very effective, it tends to kill all
aquatic plants, including desirable phytoplankton species. The
application of herbicides also stimulates extremely rapid regrowth of
aquatic weeds. Reservoir drawdowns will provide a great deal of aquatic
weed controL The annual dessication of reservoir banks will affect a
great many of the weeds.
5.1.2.6. Evapotranspiration. Along with the growth of aquatic weeds
will come the loss of water from evapotranspiration. This loss of water
originates from the consumption of water for growth by the aquatic weeds
and release of water to the air by respiration. This impact can only be
mitigated by controlling weed growth, which is a rather difficult
process. While cutting of weeds may at least locally rid a reservoir of

160
excessive weed accumulation, it will also stimulate the growth of weeds
to replace those which have been cut. Mitigation of this impac t is
usually very difficult and most often not successful; therefore it
becomes an almost unavoidable consequence of river basin development.
5.1.2.7. Elimination or reorganization of mangrove forests. The
Ba1ingho Salinity Barrage will eliminate all mangrove forests above
Ba1ingho (12% of total), and cause species shifts in most of the forests
below Ba1ingho (Snedaker, 1985; Twilley, 1985). This will be the single
largest secondary impact. The mangrove forests have been identified as
the major source of organic material for the entire estuarine portion of
the river. The mangroves also provide valuable habitat for many
estuarine organisms (oysters, crabs, shrimp, juvenile and benthic fish),
both as a seasonal spawning area or throughout the year. Loss of the
forests will adversely affect all estuarine species, including the
possible demise of much of the estuarine and coastal fisheries.
Unfortunately there 1s no mitigative action for this major impact except
either to leave the barrage gates open or to delete the salinity barrage
from the proposed development program.
5.1.2.8. Blocked migration patterns. Dams built on the Gambia River
will present barriers to the migration patterns of several species. The
major effect of this impact will be from the salinity barrage which will
disrupt the migration of many estuarine species. In particular, penaeid
shrimp, crabs, and some fish migrate either between the upper estuary
zone and coastal ocean, or upper estuary and lower estuary zones (van
Maren, 1985; Dorr et a1., 1985). Migratory species were not found in the
freshwater sections of the river.
Because of the design of the salinity barrage, mitigation of this
impact cannot be achieved except by leaving the gates open or by
excluding the barrage from the development plans. A fish ladder will
serve no purpose in the case of the barrage, because no species can
tolerate the abrupt change from 34 ppt salinity to freshwater.
Furthermore, many of the migratory species in the estuary drift with
tidal currents and therefore cannot utilize a fish ladder. One suggested
solution to this impact has been the construction of a secondary estuary
connecting two bo10ns, one above and one below the barrage by a canaL

161
This artificial bolon would then contain all the essential elements of an
estuary, including a salinity gradient. While this measure may provide a
small amount of habitat for migratory species, it is probably not worth
the rather large expense and effort. Construction of an estuarine mixing
or buffer zone (such as a small holding pool) is also not practical,
given the large size and complexity of operating such a system. An
artificial estuary would provide only a small portion of the estuarine
habitat and would probably be insufficient to maintain the migratory
populations at their current size or at levels which would be
biologically or economically meaningful.
5.1.2.9. Elimination of marine plankton above Balingho. The
salinity barrage will prevent the upstream movement of all organisms,
including the drifting species (plankton) above Balingho. The lack of
marine plankton above Balingho will lead to the demise of the estuarine
food web almost immediately after the barrage is closed. This impact has
no mitigative measure except to leave the barrage gates open or to delete
the barrage from the proposed development program.
5.1.2.10. Altered estuarine fish yields. The lower estuary zone of
the Gambia River was identified as having the highest fish yields per
unit area of the existing five river zones (Dorr et al., 1985; Josserand,
1985). The expansion of the lower estuary zone from Mootah Point to
Balingho (by the salinity barrage) might be expected proportionately to
increase fish yields in this segment of the river. However, the
estuarine and coastal fisheries will be adversely influenced by the loss
of mangroves (Twilley, 1985) and elimination of migratory pathways.
Finfish and shellfish stocks would be reduced by loss of mangrove
production and habitat. As a result, some of this favorable enlargement
of the saltwater habitat of the estuary may be offset by losses in the
upstream areas and in loss of the saltwater-freshwater gradient and
migration routes. Any net advantage accruing from expanded habitat will
need strong management inputs to realize the net potential.
5.1.2.11. Elimination of marine invertebrates above Balingho. The
salinity barrage will prevent the growth above Balingho of any attached
invertebrate species. The change in the aquatic environment from
saltwater to freshwater will restrict all estuarine organisms to
downstream of the barrage. This impact will primarily be noticed from the

162
elimination of oysters. But this is not a major impact because most of
the existing oyster communities upstream from Ba1ingho suffered mass
mortality in the recent past. This minor impact cannot be mitigated
except by deleting the salinity barrage from the proposed development
program.
5.1.3. Tertiary (Anthropogenic) Impacts
The target beneficiary of the proposed development program is the
human population of the basin. Human patterns in living and working will
change as a result of the new resources generated by the development
program. These changes will create a vast array of impacts, including
impacts to the aquatic environment from human activities, which ate
discussed below.
5.1.3.1. Release of toxic substances and nutrients to the river from
farming activities. Development of the irrigation network will be
accompanied by intense agriculture practices. These practices will
include application of pesticides, herbicides, and fertilizers to the
crops. Ultimately these materials will find, their way into and pollute
the river water. Fertilizer' pollution will cause a problem by
stimulating excess algae growth and accelerating eutrophication.
Pesticide and herbicide pollution will contaminate the fish and water,
and even render them unsafe for consumption. Pesticide pollution is by
far the most serious impact because only a small fraction of the toxic
substances applied to the crops (much less than 1%), if allowed to enter
the river, will contaminate vast segments of the aquatic environment.
The mitigation of this serious impact can only be achieved by strict
controls on the use of these compounds including limited, selected, and
highly directed application, banning the most toxic substances, and
restricting the movement of water from the cropland back to the river.
Usually these forms of pollution defy mitigation even in the highly
developed countries of Europe and North America, and are considered
inevitable consequences of development programs.
5.1.3.2. Increased fish yields from reservoirs. The reservoirs will
provide increased fish habitat and production in comparison to the
existing riverine environment. This heightened production will in turn
provide increased yields and sources of food to local residents living

163
along the river, assuming proper development of these fisheries.
Furthermore, the increased yields will occur in locations where local
food production is currently low. This highly favorable impact requires
no mitigation.
5.1.3.3. Preference to eat fish. Increased yield and consumption of
fish causes shifts in occupations as well as in dietary habitats of
people living near reservoirs. These behavioral changes will have an
impact on the river in that reservoir fish will be exploited to supply
local demand for fish. But at the same time, exploitation levels' will
have to remain within the limits of sustainable yield to insure continued
availability of this resource. This highly favorable impact does not
require mitigation, but does require determination of the sustainable
yields of each reservoir, as well as effective catch assessment survey
and management programs.
5.1.3.4. Increased cropland. An important objective of development
is to expand the amount of irrigated cropland within the Gambia River
Basin. This expansion will have a very large impact on the Gambia River
through several processes including:
• destruction and alteration of river bank habitat;
• consumption and diversion of water from the river to crops;
• pollution of water from toxic substances, fertilizers, and
sediments;
• modification of segments of the river.
Mitigation of all of these processes is possible through restricting the
amount of cropland that comes in direct contact with the river. By
placing dikes or other barriers between the river and the irrigated
fields, the exchanges of materials between the river and the cropland
will be reduced. The extent of this impact is directly proportional to
the amount of land brought under irrigation. In addition to isolation of
cropland from the river, extremely careful agricultural practices will
also lessen the impacts from farming adjacent to the river.
5.1.3.5. Mining activities. Impacts from mining activities can be
both extremely adverse and widespread. Mitigation of the possible
impacts due to acid runoff and toxic metal contamination requires
stringent environmental safeguards. In particular, mine tailings should
be placed in an area where they will not come in contact with surface

164
water or groundwater. In the Gambia River Basin the tailings should be
buried during the rainy season to prevent acid pollution of runoff.
Water pumped from mines should never be allowed to flow into the river or
tributaries. Rather, this acid water should be placed in confined
catchment basins. Special safeguards should be taken to prevent acid
mine waste waters from entering the reservoirs either via surface or
underground routes. Pollution from mining activities has proven a major
problem in both developed and underdeveloped countries. As a result, the
best form of mitigation of this impact is to obtain detailed assurances
on pollution control from the mining companies before any mining is
permitted.
5.1.3.6. Human resettlement adjacent to the river and reservoirs.
Freshwater reserves of the reservoirs will attract a large number of
people to new communities along the banks of the river and reservoirs.
These communities will provide their share of pollution to the river in
the form of debris, human wastes, agricultural wastes, etc. These forms
of pollution are generally not too serious unless the riverside community
becomes large. Several . steps can be taken to mitigate these
miscellaneous forms of pollution. Reasonable living practices such as
solid waste disposal and livestock control can prevent the addition of
unwanted debris and animal wastes to the river; currently some villages
along the Gambia River appear to exercise these living practices. Should
this policy not control the pollution, then restriction of settlement
immediately adjacent to the river may be required. As more people move
toward the river, they tend to discard their wastes into the water.
Eventually a greenbelt adjacent to the river may be required to mitigate
this impact.
5.1.3.7. Changes in disease vectors. While a serious problem to
humans, this is a relatively minor impact to ecology of the river. As
human populations become established near the river, water related
vectors will increase. This will cause a slight shift in the aquatic
fauna as the disease vectors increase in relation to other aquatic
species. This impact may be .. self-mitigating .. because as disease vectors
become abundant, people may move away from the river
intolerable living conditions. Control of disease
pesticides or habitat modification will have a major
to escape the
vectors through
impac t on the

165
aquatic environment. Some pesticides could eventually render the aquatic
resources useless. Thus the mitigation approach taken for this impact
will have a major effect on the success of the development program.
5.1.3.8. Deforestation of river banks. Human populations in the
Gambia River Basin have consumed vast quantities of wood for cooking and
domestic building purposes (Checchi, 1981). This process is expected to
continue as people move toward the newly formed reservoirs, especially in
Guinea. Deforestation adjacent to the river can have an adverse impact
on water quality. Erosion of denuded soils will allow sediments to enter
the river as well as mobilize many nutrients. Mitigation of this impact
is relatively easily conceptually, but difficult to enforce. A greenbelt
of land should be maintained along the .banks of the river and
reservoirs. This forested strip would stabalize the banks and help to
buffer the river from adverse impacts of deforestation, as well as
provide valuable habitat for wildlife. This greenbelt should be
minimally 500 m wide.
5.1.3.9. Occupation changes toward fishing and related activities.
The increased fish yields of the reservoirs will generate new jobs in the
artisanal fishing industry. The net impact to the river will be an
active fishery which could eventually exceed the sustainable yield of the
reservoirs. Qverexploitation of fisheries is possible, but could be
mitigated by proper monitoring and management techniques. Many of these
techniques can be found in a study by Bernacsek (1984).
5.1.3.10. Relocated commerce routes. The formation of reservoirs
will add numerous roads within the river basin. New roads must be made
to replace submerged routes as well as to serve new riverside and
lakeside communities. Roads that run immediately adjacent to the river
and reservoirs will provide a source of sediment and pollution to the
water. This pollution will consist of automotive debris such as old
batteries and tires, as well as gasoline and oil spills. Mitigation of
this form of pollution can be achieved by keeping roads away from the
river and reservoirs. Roads should not be allowed to come within 1 km of
the river except where access to villages or fishing wharfs is required.
This policy will prevent many forms of pollution from entering the river
because it will reduce human access to the river.

166
5.1.3.11. Displacement of aquatic wildlife. A minor impact to the
river will be displacement of large wildlife species (such as
hippopotamus) with implementation of the development programs. These
large animals create a minor local effects through extensive grazing,
nutrient release, and habitat alteration. Partial mitigation of
displacement can be achieved by preventing local hunting and other forms
of human harrassment of wildlife.
5.2. Development Choices
Throughout the discussion of impacts (Chapter 4), five development
scenarios were considered for each of the five river zones. The
rationale behind this presentation was to provide decision makers and
planners with an in-depth understanding of what the consequences will be
to the different segments of the river as each development plan is
invoked. But from the aquatic resources perspective, one development
plan appears to make sense because it allows for maximum development
while preserving most of the aquatic environment of the river. That plan
is to develop the Kekreti project alone and then evaluate its success
before beginning work on other projects. Construction of the Balingho
Salinity Barrage should be delayed because it will invoke numerous
impacts which cannot be mitigated. These impacts will serve to greatly
reduce the quality of the estuarine environment of the Gambia River.
Ultimately the estuarine and coastal fisheries could suffer major damage
from the reorganization caused by the salinity barrage. However, the
rationale for building any dam hardly stops with environmental concerns
alone. Generally, dams are built for human benefit at the expense of the
environment. Thus, many factors could very reasonably outweigh these
views which arise only from consideration of aquatic resources alone. In
effect, this development choice is only offered as a large-scale means of
mitigating several impacts and not as a planning policy.
The Kekreti Dam should provide more freshwater resources than the
salinity barrage with considerably fewer environmental impacts. Thus,
the logical strategy appears to build the Kekreti Dam and develop those
new resources to the fullest potential. If this project proves highly
successful, then consideration can be given to additional dams. Numerous

167
questions about the environmental, sociological, and economic feasibility
of developing the Gambia River can be answered from the operation of the
Kekreti Dam. These questions should be answered before the entire river
from Mako to Banjul is disrupted, which would be the case if the Kekreti
and Balingho Dams were built together. With the elimination of the
salinity barrage from the immediate development program, the concept of a
bridge across the Gambia River at Yelitenda should be seriously
considered. A bridge would have minimal impacts to the aquatic
environment.
5.3. Management Strategies
The discussion of the mitigation of the impacts to the river
presented above has shown that many of the consequences of river basin
development are either unavoidable or beneficial and hence do not require
mitigation. But a carefully planned policy of controlled growth in
conjunction with the implementation of the river basin development
scheme, will go a long way toward protection of the aquatic environment
and resources. It is believed that OMVG should insist on protecting
these resources by strict coordination and regulation of the entire
development process. To achieve that end, two tasks mus t be
accomplished. First, it must achieve recognition as the ultimate
authority for planning within the basin. Local government support will
be required for OMVG to accomplish this task. Second, opportunistic and
wasteful exploitation of resources must be stiffled. The leaders and
citizens of the basin must be made aware that planned growth will best
serve their interests. Influential members of government and business
must be convinced that only planned growth is wise and acceptable. Once
these difficult objectives are achieved, specific steps toward managing
the aquatic resources of the Gambia River can be taken as follows.
5.3.1. Specific Management Policies
5.3.1.1. Controlled annual flood. Many of the aquatic organisms
depend upon the annual flood to complete their life cycle. The flood
also provides a stimulus for migration in many species. A controlled
flood should be conducted each year by allowing approximately 200 to

168
300 m 3 /s flow at Gou1oumbou in the river for 1 to 2 months. In
this flood will require careful planning and monitoring of
resources.
turn,
water
5.3.1.2. Mix reservoirs. Stagnation in the bottom waters of the
reservoirs will result in deterioration of water quality. If
economically feasible, the reservoirs should be mixed to maintain high
overall water quality. A fully mixed reservoir also provides a much
larger volume of water for fish growth, and thus increase carrying
capacity for standing stock. While this mitigating measure will serve to
greatly improve the quality of water in the reservoir, in practice it is
rarely invoked because artificial mixing is prohibitively expensive.
5.3.1.3. Control sediment release during construction. While some
sediment will undoubtab1y enter the river during construction, much of
the release can be avoided by implementation of effective containment
procedures. OMVG should demand that the construction companies take
every step possible to control sediment release. Contracts should have
these safeguards included as part of the basic construction process;
environmental monitoring will be required to assure that the safeguards
are invoked.
5.3.1.4. Control aquatic weed growth. Growth of aquatic weeds can
be expected to be a major problem even before the reservoirs are filled.
Consultants should be hired to evaluate this problem and to suggest
methods that would control the weed growth in all phases of construction
and operation. A variety of control measures should be planned.
Successful control techniques should be implemented continuously after
the reservoirs are filled.
5.3.1.5. Control agriculture practices. All of the managers of the
irrigated cropland should be educated in the adverse effects and
potential dangers of improper farming practices. In particular, releases
of toxic substances and fertilizers must be avoided. Farm managers who
fail to protect the environment should be replaced by new managers.
5.3.1.6. Separation of irrigated cropland from the river. Irrigated
fields should not be constructed with free drainage access to the river.
Dikes or causeways should be constructed between the fields and the
river. These barriers should be designed to restrict the flow of
sediment, organic and inorganic materials to the river from the

169
croplands. Substances such as toxic materials will continue to exit the
irrigated areas of the river via groundwater, but the surface flow at
least will be restricted by the physical barrier.
5.3.1.7. Mining safeguards. All mining activities should be
accompanied by the strictest environmental safeguards. Activities of
mining companies should be constantly monitored. Mining contracts should
include specific criteria to protect the environment. Companies which
fail to maintain those criteria should be asked to cease operations until
the criteria are achieved.
5.3.1.8. Keep roads and settlements away from river. Pollution of
the aquatic environment will be greatly diminished if the interaction
between humans and the aquatic environment is minimized. People should
be discouraged from settling directly on the river or reservoir banks.
Instead, settlements should be a few kilometers distance from the river
while still permitting access to the freshwater resources. Likewise,
roads should not run directly along the water's edge, but be located away
from the river.
5.3.1.9. Greenbelt along river. Protection of the river and its
associated wildlife will be greatly enhanced by a greenbelt up to 1 km
wide maintained along the banks of the river. This greenbelt would be
disrupted in specific locations for irrigated fields and fishing wharfs.
But for the most part the greenbelt would serve as a buffer between
human-generated pollution and the aquatic environment. It would also
serve as habitat for many forms of wildlife.

6. ECONOMIC CONSIDERATIONS
6.1. Summary
Economic assessment of the current status of the aquatic resources of
the Gambia River and adjoining coastal waters as well as future
predictions has been couched in terms of the fisheries. This approach
was taken because fisheries are the only unit of the aquatic environment
that can readily be assigned an economic value and/or considered as a
source of employment for basin residents. The value of the current
fisheries is of concern because it is these fisheries which are faced
with an uncertain level of change from the proposed development programs.
The industrial fisheries provide economic return only to The Gambia
because they do not extend into Senegal or Guinea. The primary value of
finfish fisheries is through the collection of export taxes and fishing
fees. Most of the finfish fisheries are foreign-owned and export their
catches to other African nations. In 1978 these fees contributed over 1
million Dalasis to The Gambian economy. A decline in catches has lowered
that return to 300,000 Dalasis in 1982. Shellfish, in contrast to
finfish, are sold directly as an export for foreign exchange. The value
of the shrimp catch has increased over the past few years to over 5
million Dalasis in 1984.
Along with the industrial fisheries, the artisana1 catch is a major
factor in the local economy. The artisana1 catch has little value as a
source of foreign exchange because most of the catch is consumed locally
in the basin. But, field investigations and catch statistics have shown
that it has great value as a source of employment and domestically
produced food. Most of the artisana1 catch arises from the coastal
fisheries. In 1978 the coastal artisana1 catch was worth over 22 million
Dalasis, but declined to just over 12 million Dalasis by 1982. The
decline of these fisheries is not apparent, but a major part of the
problem is due to an inadequate marketing network for fish. Artisana1
fisheries exist on the Gambia River both in The Gambia and Senegal. The
value of these river fisheries has been difficult to determine because
much of the catch moves through the market as barter. Estimates of the
value of the riverine artisana1 fisheries (including the estuarine
171

catches) have been about 3 million Dalasis. Artisanal fisheries in The
Gambia and Senegal employ upwards of 17,000 people, some of whom work on
a seasonal basis. Artisanal fisheries of consequence were not identified
in Guinea.
172
The development programs proposed for the Gambia River would affect
the existing fisheries in an unknown manner. Without the construction of
any dams on the river, current trends of declining catches can be
expected. These declines are attributed to different causes for the
coastal and riverine fisheries. The coastal fisheries appear to suffer
from a gradual demise of equipment and an inefficient marketing system.
Bringing fresh fish to market has been a consistent problem in the past.
Stocks do not seem to suffer from overfishing, but that cannot be
determined without good stock assessment data. The riverine fisheries
are declining from apparent overfishing of shrinking populations. The
recent drought has inundated very few floodplains and thus reduced the
amount of natural spawning sites. A consistent level of fishing pressure
within the river has taken its toll on the declining riverine stocks.
But, the sustainable yields cannot be determined without stock assessment
data which currently do not exist.
The construction of dams in the Gambia River will for the most part
enhance the riverine fisheries.
habitats for new fisheries which
Reservoir could yield in excess of
The estimates of future yields are
most accepted index of prediction,
1984), predicts about 1400 metric
The reservoirs will provide good
may be substantial. The Kekreti
1000 metric tons of fish per year.
very difficult to predict, but the
the mo rphedaphic index (Bernacsek
tons of fish per year from this
reservoir. The three Guinean reservoirs will probably not be as
productive as the Kekreti Reservoir because of relatively low levels of
dissolved nutrients. The morphedaphic estimate for the three reservoirs
combined is 475 metric tons per year. For the Guinean and Kekreti cases,
almost all the yield from the reservoirs will be a net increase above
meager existing riverine fisheries. From the gross value of these
potential new fisheries must be subtracted the costs of developing and
managing the new resources. Thus, the economic estimates of the new
fisheries have very broad ranges in that management and development costs
are essentially unknown. Furthermore, the estimates of yield are

173
directly influenced by the volume of water and rate of drawdown in each
reservoir, which could vary by more than 30% among years.
The estimates of yields from the Balingho Reservoir are less precise
than the other four reservoirs. In this case, some losses to the
estuarine fisheries must be factored into the economic determination.
Those losses are extremely difficult to predict because the environmental
disruption to the estuary from the salinity barrage is a source of
debate. The fish yield from the Balingho Reservoir is predicted as 5600
metric tons per year using the morphedaphic index. This fish could be
worth as much as 8.5 million Dalasis. But, from that yield losses of 5.5
to 8.5 million Dalasis must be subtracted from possible degradation of
estuarine and coastal fish stocks. Thus net yield from the Balingho
Reservoir could be zero if losses are high. Should losses be high and
the yield not as high as predicted, the Balingho Salinity Barrage could
become a net drain on basin fisheries economics. On the contrary, if
yields are high and losses low, the Balingho reservoir could be a large
net asset. The primary conclusion is that this one dam offers a large
level of uncertainty in terms of Gambia River Basin fisheries.
6.2. Present Economic Status of Fisheries
This chapter summarizes available information on the current economic
value of fisheries in the Gambia River, and offers predictions of
economic yield from development projects on the river. An overview of
the existing fisheries' economics in the region has not been developed.
An impediment to developing that overview is that data on both industrial
and artisanal catch, effort, and value are incomplete. Although data
have been compiled since the mid-1970s, the results are not consistent
for any fishing sector or year. A major problem is the limited resources
available to the governments in order to conduct fishery surveys and data
analysis. While some of these limitations are technical, the majority
relate to inadequate budgets and manpower.
Included in this chapter is information on the existing industrial
fisheries, a summary of their recent catches and values to the local
economy, and the prospects for future expansion. No attempt was made to
predict sustainable yields because long-term data needed to do this were

174
not available. Senegal and Guinea are not included in this discussion
because there were no industrial fisheries on the Gambia River in these
countries.
The information used to compile this chapter was drawn from three
general sources: University of Michigan Gambia River Basin Studies; data
compiled in publications of the Fisheries Department (The Gambia); and
Josserand (1985). This section has been divided into two major parts:
an analysis of the existing fisheries and their economics, and predicted
effects of development activities on these fisheries. In both instances,
benefits to the local economy are obtained either (i) directly through
sale of fishery products or by taxation on fishing (fees), catch, and
revenue; or (ii) indirectly through employment in the fishing or support
sectors. In the upper areas of the basin (upper freshwater river and
headwater zones), fish products are most often consumed directly by
fishermen and their families or traded for other goods and services.
6.2.1. Industrial Sector
6.2.1.1. Finfish fisheries. Data from the Gambian Fisheries
Department and an evaluation of fisheries by Josserand (1985) show that
total landings in metric tons (taken from Table 3.10.) for The Gambia
were:
Year
1978
1979
1980
1981
1982
Total Landings
32,085
21,374
24,496
22,102
17,081
Of these totals, the industrial catch comprised 62%, 48%, 44%, 44%,
and 44%, respectively, each year.
Ca tch and economic value data from 1981 and 1982 were considered
fairly representative of fishing activity in the industrial sector, but
these are the only years for which catch records are relatively
complete. Total industrial fishing catch was 9,624 metric tons in 1981
and declined to 7,377 metric tons in 1982 (Table 6.1). Fishing fee
receipts to the Gambian government in 1981 totaled 448,732 Dalasis plus
an additional 60,000 Dalasis in export taxes (Josserand, 1985). Total
1981 fishing fee and tax receipts to the government of The Gambia

176
obtained from industrial fishing approached 500,000 Dalasis. The export
value of the 1981 industrial catch is not known but, Josserand (1985)
placed the export value of the 1982 catch at more than 3 million
Dalasis. This extrapolates to an export value of 4 million Dalasis for
the 1981 catch. Because these catches were sold by foreign-owned
companies in markets outside of The Gambia, little of their value was
returned to the local economy beyond the fishing fees and export taxes
paid to catch and export these fish. If the export value of the 1981
finfish catch was about 4 million Dalasis and the government received
500,000 Dalasis in fishing fees and taxes, this indicates that The Gambia
received about 12% of the value of the fish taken from its waters that
year by the industrial fisheries.
Using the 1981 relationship of 52 Dalasis in fees and taxes received
per metric ton of fish caught, rough estimates of the value of the
industrial fish catch to the local economy are:
Year
1978
1979
1980
1981
1982
1983 (projected)
Estimated Value
1,044,472 Dalasis
535,340 Dalasis
559,104 Dalasis
500,448 Dalasis
383,604 Dalasis
388,336 Dalasis
Although only estimated values, the range (0.3-1.0 million
Da1asis/yr) is probably accurate; tenfold increases or decreases are not
expected. These figures show that the value of the industrial catch (and
revenue to the Gambian government through fees and taxes on this catch)
in 1983 had declined to about one-third that of the 1978 yield from the
fishery. These estimates show that the decline in catch volume and value
from 1978-1983 is an indisputable and significant problem in the
industrial fishing sector. It is suspected that the problem relates more
to a decline in fishing effort resulting from lack of economic resources
to sustain the fishery, than to a depletion of fish stocks.
Recent estimates of coastal marine stocks or sustainable yields were
not available. But, during the late 1970s, maximum sustained yield (MSY)
in coastal waters near The Gambia was estimated at 4-8 thousand metric
tons for demersal species and 30-50 thousand metric tons for pelagic
species (Scheffers and Conand, 1976; King, 1979). Even if these

177
estimates were high and stocks have declined since that period (no
biological evidence was found to substantiate a decline), these figures
suggest that both the demersal and pelagic fisheries could support
considerable increases in fishing effort and harvest.
Prior to increased investments and exploitation, a study· should be
conducted to establish maximum sustained yield (MSY) for target coastal
fish species, both demersal and pelagic. Also, this study should
evaluate the relationship between economic investment and yield, and
establish the maximum economic yield (MEY) possible for these fisheries.
Given MSY and MEY, the coastal industrial fishing sector would be guided
in its efforts to increase its fisheries and economic return within the
biological and economic constraints of the system.
Concurrent with the possibilities for increased fishing pressure and
annual harvest of stocks from Gambian as well as adjacent coastal waters,
is the need to improve fishing gear and methods, product handling and
processing techniques, and marketing of the product. Much of the gear
currently deployed is antiquated or in need of repair which reduces the
efficiency and yield per unit of fishing effort. A significant portion
of the catch is lost to spoilage, bruising, or damage prior to or during
processing. Losses sustained to the catch in the artisanal sector prior
to marketing were estimated at 30%. Currently, the largest problem in
both the industrial and artisana1 fishing sectors in The Gambia are
losses sustained during transport to processors or markets because of
inadequate roads and marketing networks. Since 1978, the combined catch
from the industrial and artisana1 fisheries has dropped from 32,000
metric tons to 15,000 metric tons. Josserand (1985) attributed much of
this decline to problems associated in the handling, distribution, and
marketing of fish products.
6.2.1.2. Shellfish fisheries. Shellfish harvested from the Gambia
River and adjoining coastal waters include mollusks (oysters, cockles,
the large marine gastropod Symbium !!E.E.' locally referred to as "yeat,"
and cuttlefish) and crustaceans (crabs, lobsters, and shrimps). The bulk
of the oysters, clams, yeat, and cuttlefish were harvested by the
artisana1 fishermen and consumed locally. Export of these shellfish was
insignificant. Lobsters and crabs were caught by artisana1 fishermen,
with some of the lobsters sold to National Partnership Enterprises (NPE)

vicinity of the Gambia River, and most of the catch is consumed by the
fishermen or their families. Es timates are not available for the value
of this catch. Efforts should be made to explore development and export
of blue crab as an industrial fishery.
179
The primary and most important industrial shellfish fishery is that
of shrimp. Although several species comprise the oceanic catch, catches
in the Gambia River are confined to the estuarine reaches of the river
and are limited almost exclusively to the pink shrimp, Penaeus duorarum.
Van Maren (1985) presents findings on the distribution and biology of
pink shrimp in the Gambia River and adjacent coastal waters. Data on
shrimp catch and value are presented in Josserand (1985) and van Maren
(1985) .
Shrimp are captured in stake nets by artisanal fishermen in the
estuary of the Gambia River. The catch is sold to NPE which processes
and freezes the shrimp in their Banjul factory. Most of the frozen
shrimp are transported in refrigerated trucks to Dakar, Senegal for
export to Europe. Josserand (1985) indicated that about 227 metric tons
of shrimp were processed by NPE during July 1982-June 1983, and placed
the export value of this catch at 2-3 million Dalasis. Annual catch
increased to 412 metric tons for the July 1983-June 1984 period (van
Maren, 1985). Additionally, an unquantified but small amount of the NPE
shrimp product is sold locally. Estimates of maximum sustainable yield
of shrimp from the Gambia River itself are not available, but oceanic
stocks are likely underexploited at the present. Efforts should be made
to estimate both oceanic and riverine shrimp stocks and sustainable
yields, so as to establish annual catch quotas to more fully exploit this
valuable resource. The potentially severe impact of river basin
development on the biology, distribution, and abundance of estuarine
stocks is discussed extensively by van Maren (1985) and was summarized in
Chapter 4. Economic implications of this impact are discussed later in
this section.
6.2.2. Artisanal Sector
As with the industrial sector, the artisanal fisheries can be
subdivided into finfish and shellfish. In some instances, artisanal

fishing is closely linked to the industrial fishing sector as the case of
the shrimp fishery.
180
6.2.2.1. Finfish fisheries. The artisanal finfish fisheries on the
Gambia River can be separated into three categories: marine coastal
fisheries adjacent to the river mouth; estuarine fisheries; and
freshwater fisheries. The Fisheries Department of The Gambia has
compiled catch assessment data since the late 1970s. Beginning in 1980,
the department has conducted monthly catch surveys and an annual frame
survey on an intermittent basis to evaluate catch, ownership of gear
deployed, and nationality of fishermen operating in Gambian waters. Data
on total fishing effort or by specific gear were not compiled. In early
1982, Le Service des Eaux et Forets (Dakar, Senegal) initiated a limited
catch survey in the upper freshwater portion of the river in Senegal, but
more long-term data were required for use in this analysis. Additional
data on catch, effort, and economic value of fish taken from the Gambia
River were not located. A detailed description of the catch survey
programs in the basin was presented in Dorr et ale (1983). Josserand
(1985) conducted an economic analysis of Gambia River fisheries and
discussed the implications of river basin development on both the
industrial and artisanal fisheries.
By far, the major proportion of the catch and economic value of the
Gambia River Basin artisanal fisheries comes from the marine coastal
sector. The marine coastal catch has fluctuated from 6 to 13 thousand
metric tons annually between 1978 and 1982 (Table 6.1). These catches
have comprised 64-89% of the total artisanal fishery catch during these
years. Assuming an average 1984 market price of 2.0 Dalasis/kg, the
economic value of this catch has ranged from 22 million Dalasis in 1978
to 12 million Dalasis in 1982. Because the catch is sold on local
markets rather than exported, economic revenue from the locally-sold
marine coastal artisanal fishery catch is more than tenfold greater than
those obtained from the industrial fishery catch. In 1982, the economic
value of the marine coastal artisanal fishery to the local economy was 30
times that of the industrial fishing sector, and 5 to 6 times that of the
riverine artisanal fishery. Given the recent decline in marine coastal
catch, the current annual value of this catch is probably about 10
million Dalasis.

181
Indirect evidence. suggests that stocks presently targeted by the
marine coastal artisanal fishery could sustain a considerable increase in
fishing pressure and annual yield, although exact values of these
increases remain to be established. In addition, improvements in fish
handling, processing, and marketing techniques could significantly
increase economic return on existing levels of catch and investment.
These improvements relate to reductions in fish spoilage, more rapid and
efficient transportation to existing markets, and expansion to include
additional markets.
Most stocks presently targeted by the marine coastal artisanal
fishery will remain relatively unaffected by the barrage and dams
proposed for the river system, with the possibfe e:tception of Ethmalosa
fimbriata, "bonga" as it is known locally. The potential reduction in
stocks of Ethmalosa fimbriata poses the most immediate and severe threat
to the marine coastal artisanal fisheries from river basin development
activities. Dorr et al. (1985) discussed the dependence of bonga on
estuarine conditions in relation to spawning, nursery, and feeding
requirements. The economic implications of their findings are discussed
later in this section.
A valuable contribution of the marine coastal artisanal fishery to
the local economy is through employment. Josserand (1985) estimated that
2,600-3,000 fishermen are involved in this fishery and that an additional
10,000-15,000 ancillary jobs are generated by fishing activities in this
sector.
The present riverine artisanal fisheries can be divided into two
sectors: estuarine and freshwater. Catch assessment data for these
sectors in The Gambia appear in Fisheries Department databases compiled
for their Lower (estuarine) and Upper (freshwater) River Divisions,
respectively. Comparison of catch and yield data for these sectors from
January-December 1980 and July 1982-June 1983 (Table 6.2) reveals that
the estuarine catch (Lower River Division) declined from 2,406 metric
tons to 605 metric tons during the 1/2-yr interval between these
periods. However, catch from the freshwater portion (Upper River
Division) of the river in The Gambia remained nearly constant at about
630 metric tons per annum. If an average value of 2.0 Dalasis/kg for
estuarine fish and 1.5 Dalasis/kg for freshwater fish is assigned to this

182
catch, then the total riverine artisanal catch was valued at 5,758,500
Dalasis in 1980 and declined to 2,165,500 Dalasis for the 1982-1983
period.
Two factors could account for the stability of the freshwater
fisheries in the face of the decline in estuarine catch. First, although
both sets of catch data (Table 6.2) were compiled during a l2-month
period, the period of record began and ended at different points in the
annual hydrological regime. Dorr et ale (1985) documented that several
species of fish, including some targeted by the artisanal fishery (e.g.,
sardines, bonga, catfish) appear to move into and out of the estuary in
relation to the annual flood cycle. Part of the difference between the
1980 and 1982-1983 estuarine artisanal catches may be attributed to
seasonal differences in abundance, distribution, and catch of fish rather
than an overall decline in abundance of fish.
The second factor is that since 1980, when the boundary between the
Lower and Upper River Divisions was established at the interface of
freshwater and estuarine water near Kau-ur, the drought and reduced
streamflows have allowed saltwater to advance nearly to Kuntaur, a
distance of 61 km upstream from Kau-ur. The upper estuary is a region of
high productivity, relative to the freshwater river, and the effect of
this transferral of a high productivity region would be to restructure
the proportional distribution of catch between the two divisions. The
result would be an apparent reduction in the estuarine division catch and
a maintenance of the freshwater division catch, as was observed. A
detailed comparison of monthly catch and effort data is needed to
substantiate the apparent decline in estuarine artisanal fishery catch in
the face of stable freshwater fishery catch. However, without a doubt,
annual catch and economic yield of Gambian freshwater fisheries has
declined over the past 5 years.
The above figures show that the current economic value of the
estuarine artisanal fishery appears to have declined to about 1 million
Dalasis per year. The value of the Gambian freshwater fishery is also
valued at about 1 million Dalasis, annually. The total value of the 1982
artisanal catch of finfish to the Gambian economy was roughly 14.4
million Dalasis. Of this total, the artisanal marine coastal fishery
contributed about 12 million Dalasis, the inland or riverine artisanal

183
TABLE 6.2.
ESTIMATES OF ANNUAL INLAND ARTISANAL FISHERY CATCH (metric tons)
AND YIELD (kg/ha/y) FOR THE LOWER AND UPPER
DIVISIONS OF THE GAMBIA, WEST AFRICA,
DURING 1980 AND 1982-1983
Jan-Dec 1980b Jul 1982-Jun 1983c
Division Catch Yield Catch Yield
Lower River (Banjul to Kau-ur) 2,406 37.59 605 9.45
Upper River (Kaur to Koina) 631 126.20 637 127.40
Total 3,037 42.77 1,242 17.49
NOTES: a) Areas estimated as: Banjul to Deer Islands = 64,000 ha;
Deer Islands to Gou1oumbou, Senegal = 5,000 ha; total area
of river in The Gambia - 71,000 ha.
b) Estimates of catch from Lesack, 1982.
c) Estimates of catch from Josserand, 1984.
Unlveraity of 1I1chl••n, G_bla Rlver lI..ln Studle., 1985.

184
fishery about 2 million, and the industrial fishery about 0.38 million
Dalasis. The 1982 catch for each sector was 6,196 metric tons; 3,508
metric tons; and 7,377 metric tons; respectively (see Table 6.1). The
return per kilogram of fish was 2.0 Dalasis/kg, 1.6 Dalasis/kg, and 0.05
Dalasis/kg, respectively, for each sector. These data show that in terms
of fish biomass removed from the system, the economic return to the local
economy is highest for estuarine artisanal fisheries, followed closely by
the freshwater artisanal fishery, with industrial fisheries taking a
distant third to the others. But in 'contrast, local economic investment
in fishing gear, supplies, and services for the industrial sector is
minimal because the vessels are foreign-owned and operated. Therefore,
the approximately 500,000 Dalasis per year obtained by the local economy
from fishing fees and taxes on the industrial sector, is relatively free
of investment costs and represents nearly pure profit to the local
economy. At the same time, while the artisanal sector must make economic
investments in gear, supplies, and services, these fisheries are
technically simple and rudimentary in design and extent. Annual cost per
metric ton of fish caught by the artisanal fishing sector is not known
but is undoubtly low; the benefit/cost ratio could exceed 10:1. Although
investitures in equipment and supplies required for initial entry into
the occupation are relatively costly and would initially reduce the
bene£!t-cost ratio, most artisanal fishing is done by well established
family or other structured groups, and new entries into the sector are
infrequent, especially during present conditions of economic depression,
drought, and reduced riverine fish production. Therefore, present
investment costs associated with aquisition of new gear are minimal in
the artisanal sector. As a result, the benefit-cost ratio in this
fishing sector is near its maximum.
As with the coastal fisheries, the riverine artisanal fisheries
contribute to local employment. Based on 1983 frame survey statistics
(unpublished data, Fisheries Department, Banjul, The Gambia), about 1500
jobs are associated with the estuarine (Lower River Division) fisheries.
About 300 jobs are associated with the freshwater (Upper River Division)
riverine fisheries.
The situation in Senegal is considerably different from that of The
Gambia. Catch statistics have not been compiled for the Senegalese

185
portion of the river prior to 1984. But during February 1984 field
studies, Josserand (1985) noted that nearly all fish seen in the markets
in Tambacounda and Velingara were of marine origin. This observation
suggests that most fish caught in the western portion of the river are
sold or bartered and consumed locally. Artisanal fish catches from this
portion of the river appear to be small and of minor economic importance
outside of the immediate fishing community. The river enters the
national park in the southeastern region of Senegal and fishing between
this point and the border with Guinea is centered near Mako and
Kedougou. Based on personal observations and analysis of limited data
from the first six months of 1984, Josserand (1985) stated that the daily
quantity of locally caught fish sold in the Kedougou market probably does
not exceed 25 kg. Extrapolation of this daily market sale to annual
catch suggests that no more than 9 metric tons of fish were caught and
sold in this region. Assuming slightly lower yields for areas outside of
Kedougou, the total annual yield from Senegalese portions of the river
between the national park and Guinea that was sold on local markets was
probably less than 15 metric tons. Using a February 1984 market value of
300 to 400 CFAlkg (Josserand, 1985), the value of this catch was 2.7 to
3.7 million CFA. An unquantified but undoubtly significant portion of
total fish catch is probably consumed without ever reaching the markets.
Although artisanal fishermen were observed in the Guinean reaches of
the river, data on catch or marketing of fish were not available. It is
not possible at this time to make an accurate estimate of the catch or
economic value of fish taken from the Gambia River in Guinea. However,
the total market value of fish taken from the Guinean portion of the
Gambia River (excluding tributaries) is probably less than that of fish
caught and marketed from the Senegalese portion of the river, considering
that the river is reduced in size and human populations are no larger in
its Guinean reaches than in the vicinity of Kedougou, Senegal.
6.2.2.2. Shellfish fisheries. Shellfish harvested by artisanal
fishermen included oysters, cockles, yeat (Symbium spp.), cuttlefish,
lobsters, crabs, and shrimp. The bulk of the lobster, shrimp, and
cuttlefish catch is sold to industrial processors for export. Fishermen
and the local economy benefit from employment and sale of this catch to
the processors and thereby obtain a portion of the total value of the

186
catch. However, it is safe to assume that this yield to local economics
is considerably less than one-half the export value. Josserand (1985)
estimated the industrial export of crustaceans excluding shrimp as 60.7
metric tons for the period July 1982-June 1983 valued at 124,642
Dalasis. The above assumptions and figures suggest that perhaps 62,000
Dalasis may have entered into the local economy during the 1982-1983
period from sale of artisanally-caught shellfish to industry for export.
An additional but unknown quantity of lobsters, shrimp, and cuttlefish
were sold on local markets rather than exported. Most likely, .the input
to the local economy was nearly insignificant relative to the combined
total for the other artisanal fishing sectors (i.e., the combined
economic value of the finfish and other shellfish catch).
The remaining shellfish products which were caught and marketed
locally by the artisanal fishery included oysters, lobsters, cockles,
yeat, and crabs. Of these, oysters and yeat probably contributed most to
the local economy. Josserand (1985) suggested that perhaps 150 metric
tons of oysters was harvested and marketed locally each year. Most of
this product was smoked although some was marketed fresh. During
1983-1984, the price paid for oysters purchased from local vendors was
about 6 Dalasis/kg. At this rate, the total value of the oyster catch
approached 900,000 Dalasis per year. At nearly 1 million Dalasis
annually, the impact of the oyster fishery on the local economy must be
on the same scale as that of the industrial finfish catch and the inland
or riverine fisheries. If earlier calculations regarding artisanal
shrimp fishing and economics are accurate (2 million Dalasis or less per
year) then the contribution of the artisanal oyster fishery approached
50% or more of that contributed by the shrimp fishery to the local
economy. Because oysters require the presence of salinity and a
substrate (mangrove roots) for economically viable existence, this
fishery would be eliminated in all areas above the Balingho impoundment,
and in any hypersaline waters below the barrage. Although substantiating
data are lacking, the bulk ( 85%) of the present oyster harvest probably
occurs downstream of the Balingho. If these assumptions are correct, the
oyster fishery would sustain at least a 15% (150,000 Dalasis) reduction
in annual yield as a result of losses above Balingho, and possibly a much
greater loss downstream due to construction of the Balingho Barrage.

Existing oyster fisheries in unaffected portions of the estuary could be
187
expanded to compensate for upstream losses.
Cockles and cuttlefish were harvested by artisanal fishermen and sold
in local markets. The economic value of these catches, while
unquantified, was probably small in comparison with the contribution of
other shellfish and finfish products to the local economy. The extent to
which fisheries and markets for these products might be expanded to
compensate for losses in other artisanal fishing sectors is not known but
should be evaluated.
Drammeh (1982) noted in a summary of catch statistics recorded by the
Fisheries Department of The Gambia that about 96 metric tons of yeat (the
marine gastropod, Symbium spp.) were landed by the marine coastal
artisanal fishery during 1981. These marine gastropods were caught by
industrial fishing vessels in trawls for demersal finfish and sold to
artisanal fishermen who met the trawlers at sea and purchased the yeat,
which was then sold on the local markets. Assuming that the dressed
weight of this catch was about one-half the total weight (or about 50
metric tons) and sold for 3 Dalasis/kg, this catch would have been valued
at 50,000 Dalasis. Because the animal and the fishery are strictly
marine, they would not be directly affected by changes in the river.
Crabs are caught by artisanal fishermen but not widely marketed;
rather, they are consumed by the fishermen or their families. Crabs were
occasionally sold in the local markets but quantities available were
limited (no more than a few crabs at any given time) and supply highly
unpredictable. Given the extent of the blue crab fisheries in the United
States and elsewhere, further investigation should be made into the
potential for expanding the artisanal crab fishery.
The contribution of the artisanal shrimp fishery in relation to the
export of this product was discussed earlier in relation to the
industrial shrimp fishery. The total value of the 227 metric tons caught
during 1982-1983 by artisanal fishermen working under contract to the
major industrial exporter (NPE) was estimated to be 2 to 3 million
Dalasis by Josserand (1985) and 3.8 million Dalasis by van Maren (1985).
The monies received by the fishermen for their catch must have been
considerably less than these figures for NPE to have operated at a
profit. Van Maren (1985) noted that artisanal fishermen were paid about

6 Dalasis/kg for shrimp that were exported at about 12 Dalasis/kg, or at
a rate equal to about half the export value of the catch. Given these
figures, the annual contribution of artisanal shrimp fishing to the local
economy could be conservatively estimated at 1 to 2 million Dalasis per
year. The exact amount would depend on the volume of the catch,
processing costs, external market prices, and the value of the Dalasis
against other currencies. During the period July 1983-June 1984, NPE
processed 412 metric tons of shrimp, a 185-ton increase (81%) over the
preceding period. The export value and revenue to the local economy of
the 1984 shrimp catch will not greatly exceed 5 million and 2.5 million
Dalasis, respectively.
188
An additional consideration associated with the contribution of the
riverine shrimp fishery to local economics is the aforementioned
employment of nearly 1,000 people in jobs related to shrimp fishing for
NPE. Josserand (1985) estimated that about 60 man-years of employment
were generated annually in full-time artisanal shrimp fishing. To this
must be added employment and monies received by sectors servicing and
supplying these fishermen (excluding netting gear which is provided to
the fishermen by NPE). Profits realized to the artisanal fishing sector
and local economy, through the catch and sale of shrimp to industrial
processors, then spread throughout both the fishing and support sectors.
6.3.1. Overview
6.3. Development Implications and Tradeoffs
Economic considerations associated with the existing fisheries of the
Gambia River were discussed in section 6.2. Those findings form the
perspective in which the economic implications and tradeoffs presented
below are couched. The analyses presented in this section are based on
the anticipated reaction of the aquatic system to the impacts of specific
development schemes proposed for the river. These schemes were
identified and associated impacts discussed in Chapter 4 and are treated
herein in the same order: no development; Kekreti Storage Dam; Kekreti
Storage Dam and Guinean Dams; Kekreti Storage Dam and Balingho Salinity
Barrage; Kekreti Storage Dam, Guinean Dams, and Balingho Salinity Barrage.

189
Although development activities will result in alterations to many
aspects of the physical, chemical, and biological aquatic environment,
the economic repercussions of these activities will most obviously be
reflected in changes to the fishery resources in the basin. Also, it is
exceedingly difficult to place a direct monetary value on changes in
salinity, nutrient concentrations, and plankton or macrophyte
production. Rather, the economic impact of changes in these factors to
some degree are reflected as changes in measurable factors such as
fishery yield and value. In view of these considerations, this section
will focus on the economics of basin fisheries in relation to anticipated
environmental impacts.
Emphasis has been placed on evaluating the general scale, trends or
patterns, and relationships (e.g., respective sizes or economic
contributions) among the various fisheries. Considerable ranges exis t
for all values cited in this section. Part of this, particularly with
respect to future predictions, is due to the fact that population
parameters and stock estimates have never been determined for any target
species (finfish or shellfish) in the Gambia River or adjoining marine
coastal waters. Considerable additional variance must be added to the
estimates due to the inexact predictions of reservoir volumes,
streamf10ws, etc., which are unknown without a precise operational policy
of each dam. Without these data, all predictions must be considered
representative estimates not final values.
6.3.2. No Development
Assuming no anthropogenic impacts to the river system beyond
additional those changes already in effect (e.g., fishing, point-source
pollution through waste discharge and human activities, harvest of
mangroves for firewood, etc. ) , the existing fisheries should generally
continue to experience current patterns and trends in biological and
economic yield. Most analyses presented in this section are focused on
fisheries in The Gambia. Shellfish fishing does not occur in the
freshwater reaches of the river in Senegal and Guinea. Industrial
finfish fisheries were not found on the river outside of The Gambia.
Only limited data from 1984 (summarized in Josserand, 1985) were
available on artisana1 finfish catch in Senegal -- most of the river lies

190
within the national park where fishing is prohibited. No data were
available on artisanal fishing or catch on the Gambia River or its
tributaries in Guinea.
6.3.2.1. Finfish fisheries. Fishery Department statistics show a
steady decline in the industrial fishery catch in The Gambia from 20,089
metric tons in 1978 to 7,377 metric tons in 1982. Projected catch for
1983 was about 7,500 metric tons. The primary economic value of these
catches was derived from fishing fees and export taxes, because the fish
were exported for sale outside the basin. Total receipts to the Gambia
Government for these fish have declined from about 1 million Dalasis in
1978 to 0.3 million Dalasis in 1982.
A biological basis for the reported decline in industrial finfish
catch was not identified, but rather it appears that the decline in
annual catch was related to anthropogenic factors. These factors include
a decrease in fishing effort because of increased fishing costs which are
not offset by increased value of the product (mostly sardines). Also,
Josserand (1985) noted that constraints in distribution and marketing of
fish were limiting factors in the artisanal fisheries; the same may be
true for the industrial fisheries. Finally, only one company (Seagull
Cold Stores) has fished Gambian waters consistently since 1978. During
this period, total fishing effort has fluctuated considerably with the
entry and exit of several foreign firms on an irregular basis.
The prognosis for the industrial fishery from a biological standpoint
is good. Stocks should be able to support fishing effort and annual
cropping equal to that of 1978 or more, as some species may be capable of
sustaining even higher annual yields. This suggests that given proper
investment and management in the industrial sector, annual economic yield
via fees and taxes could approach 1 to 2 million Dalasis. If the Fish
Marketing Company (FMC) is brought into existence and catches are sold
locally, total annual economic yield might increase by several million
Dalasis, depending upon species caught, market value, and fishing costs.
In the absence of river development, annual catch and economic yield
from the estuarine and freshwater artisanal fisheries are expected to
continue at their presently depressed levels in relation to prior years.
An apparent major cause for this condition is the continuing drought and
reduced streamflows in the river, which have reduced primary and

192
indicates that shrimp stocks are presently not overfished. Caution must
be exercised to allow migrating juvenile shrimp to exit the estuary
during the onset of the annual floods, so that they can grow to maturity
and spawn in the ocean, thus sustaining local shrimp stocks. This can be
accomplished through the use of gear and mesh sizes which permit the
escape of undersized shrimp. Van Maren (1985) states that the coastal
oceanic stocks of shrimp appear stable and capable of sustaining
increased fishing pressure and annual yield. Given these considerations,
it can be expected that the annual economic yield of the industrial
shrimp export fishery should continue at 3 to 5 million Dalasis for the
next few years.
The July 1982-June 1983 industrial export of mollusks and crustaceans
(excluding shrimp) was 60.5 metric tons valued at about 123 thousand
Dalasis (Josserand, 1985). The stocks of all species exploited by these
fisheries may be capable of sustaining considerable increases in fishing
pressure and annual yield. Although industrial export of lobsters during
the year listed above was 0.2 metric tons (valued at 1,300 Dalasis),
these stocks also appear to be underexploited in relation to sustainable
yields. Josserand (1985) suggests that Gambian waters should sustain an
annual yield of 1,000 metric tons of mollusks and crustaceans to the
industrial fishing sector. The economic value of this catch could exceed
2 million Dalasis, annually.
Wi thin the artisanal shellfish fishing sector, the annual harvest of
oysters and yeat (the marine snail Symbium sp.) contribute most
significantly to the economic yield from this sector. The current annual
harvest of oysters may approach 150 metric tons and be valued at 900
thousand Dalasis. Although these are approximate estimates of catch and
value, the ranges shown are probably representative for the fisheries.
Stocks of oysters and yeat could likely sustain increased levels of
harvest. Market demand for these fish products, particularly for yeat,
may be the major limiting factor with regard to expansion of these
fisheries.
With respect to artisanal fishing for crustaceans, most shrimp and
lobsters are sold to industrial processors. The economic return on catch
of these species sold directly by artisanal fishermen on local markets is
probably insignificant in comparison with that received from export sales

193
of these products. As noted in section 6.2, an unquantified but likely
underexploited fishery in marine and estuarine waters is that of crabs.
The present economic yield on crab fishing is almost insignificant in
comparison with other fisheries. But, the extent and economic value of
crab fisheries for related species elsewhere in the world suggests
further investigation into the potentials for expanding the crab fishery
in Gambian waters.
All of the stocks of mollusks and crustaceans currently exploited by
artisanal fisheries may be capable of sustaining considerable increases
in annual harvest and economic yield. autside of increased knowledge on
the life history, distribution, and targeting of these stocks, the
factors currently limiting the economic value of these fisheries appear
to be more socioeconomic than biological in origin.
6.3.3. Kekreti Storage Dam
If construction activities and river alterations are limited to those
proposed (the Kekreti Storage Dam), existing fisheries in The Gambia and
Guinea will remain basically unaltered. Some redistribution of fishing
effort (e.g., emigration of fishermen from riverine areas adjoining the
reservoir) and marketing may occur, but riverine and coastal stocks in
these countries should be generally unaffected by the reservoir. This
assumes that a portion of the annual flood below the Kekreti Reservoir
will continue during the construction and operation of the dam.
An exception may be the narrow band or interface between the upstream
boundary of the reservoir and the Guinean headwaters. Some ecological
adjustment of existing riverine fish with the developing complement of
lacustrine species associated with the reservoir can be expected. If
anything, the yield and economic value of fish taken from this portion of
the river may increase with respect to existing levels. Because fishing
is currently prohibited in much of the river immediately below the
proposed dam site, any fish taken from this portion of the river
following construction of the dam would represent an increase in catch
and economic yield. However, this increase would be insignificant in
relation to the economics of the fisheries elsewhere on the river or in
the reservoir.

194
By far, the major economic effects of the Kekreti Storage Dam that
will accrue directly from the aquatic system, will stem from the
colonization of the reservoir by lacustrine species of fish. These
species will eventually comprise a spectrum of stocks available to
whatever artisanal reservoir fishery develops in the region.
The present yield and economic value of fish taken from the reach of
the river that will be displaced by the reservoir, while of local
importance, is almost insignificant in terms of basinwide fishery
economics. Those fishermen who presently rely on the riverine fishery
could undoubtly be supported by the developing reservoir fishery. An
attempt has been made to place bounds on levels of biological and
economic yield that can be expected from the reservoir fisheries as they
develop.
Four methods were used to estimate annual yield from the reservoirs
proposed for the Gambia River:
Method 1: based on the artisanal fishery;
Method 2: based on an established morphedaphic index;
Method 3: based on primary productivity rates;
Method 4: based on total phosphorus concentrations.
Method I assumes that yield per hectare for the reservoir will at
least equal that from the existing lower freshwater river. Methods 2 and
4 are based on factors that reflect the general nutrient content of the
water and therefore its potential biological productivity. Values for
these factors are those measured in the existing river at the various dam
sites. Method 3 extrapolates secondary production from primary
production. The models used in methods 2-4 were developed using
empirical data compiled during studies of rivers and reservoirs in Africa
(see Dorr et al., 1985, for discussion and application of these models
and estimates to this project).
However, caution must be exercised when interpreting this or any
other prediction presented in this section. The figures are calculations
of yield and economic value based on existing system catch statistics or
empirical models developed from other aquatic systems. Each. system,
including the Gambia River, requires unique models. Such system-specific
models usually will not yield highly accurate predictions when applied to
other systems. Also, because catches predicted for the Gambia reservoirs

195
are based on yields observed elsewhere in Africa, the figures do not
indicate levels of biologically sustainable yield or optimum economic
yield. Such estimates can only be generated through directed studies
such as stock assessment, biological monitoring, and catch-assessment
surveys conducted on the reservoir itself. Also, following initial
colonization, most reservoirs show initially high levels of fish
production which decrease and level off in subsequent years. This
process takes about 15 years in temperate zones and less than 8 years in
the tropics (Bernacsek, 1984). Therefore, in the long-term, annual
biological and economic yield from the Kekreti Reservoir may be reduced
from initial levels. Because the trophic age and stability of the lakes
and reservoirs from which the models used in this section to predict
biological production were developed are not known, fish production
(yield) estimates presented herein may be higher than those actually
realized over the life of the reservoir.
Estimates of catch and economic value predicted for the Kekreti
Reservoir (Table 6.3) are summarized as follows:
TABLE 6.3.
ESTIMATES OF YIELD AND ECONOMIC VALUE
(KEKRETI RESERVOIR)
Method 1 - 2,176 metric tons valued between 652 and 870
million CFA;
Method 2 - 1,488 metric tons valued between 446 and 595
million CFA;
Method 3 - 131 metric tons valued between 94 and 125
million CFA;
Method 4 - 28 metric tons valued between 8 and 11
million CFA.
University of Michigan, G..bia River Basin Studies, 1985.
For the Kekreti Reservoir, the estimates of total annual yield and
economic value of fish range from 28 to 176 metric tons with respective
values from 8 to 870 million CFA. The lower range represent estimates

196
based on current nutrient concentrations and primary production in the
river; evidence suggests that these will increase in the reservoir.
Reservoir waters should be at least as productive as those of the
existing river, and probably more so. Therefore, the upper end of these
estimates is more representative of annual yield and economic value of
fish production expected from the Kekretic Reservoir. Results of method
2, the morphedaphic index (MEl) are generally considered the best
predictor of yield. It should be recognized that each estimate can
fluctuate as much as 306 among years due to the change in the volume of
water in the reservoir.
The estimated 2.7 to 3.7 million CFA annual worth of river-caught
fish currently marketed in Kedougou, Senegal is considerably less than
the lower estimate of annual economic yield of 8 million CFA predicted
for the Kekreti Reservoir. Thus, the Kekreti Reservoir has great
potential to increase regional annual yield of fish and to stimulate the
local economy through its production and economic yield. This presumes
both the biological success of the reservoir as well as the
implementation and support of fisheries' development activities required
to judiciously exploit the resources of the reservoir.
6.3.4. Kekreti Storage Dam and Guinean Dams
The existence and operation of the Guinean dams above the Kekreti
Reservoir should have only minor economic impact on existing fisheries in
the portion of the Gambia River west of Niokola Koba Park.
As described in section 6.2.2.1, while the present catch and economic
value of fish taken from the Guinean portion of the Gambia River and its
tributaries is unquantified, it is undoubtly small, particularly in
comparison with the catch and economics of the river fisheries in Senegal
and The Gambia. Therefore, the annual yield and' economic value of fish
predicted for the Guinean reservoirs may be simply added to that
predicted for the Kekreti Reservoir (Table 6.4.) and summarized as
follows.
These figures indicate that the annual yield of fish from the Guinean
reservoirs could range from 93 to 476 metric tons valued at 28 to 190
million CFA. If these numbers for catch and value are added to those
predicted for the Kekreti Reservoir, the predicted annual yield from the

197
TABLE 6.4.
ESTIMATES OF YIELD AND ECONOMIC VALUE OF
GUINEAN RESERVOIRS
Method 1 - no estimate (no catch statistics were
available for Guinea);
Method 2 - 476 metric tons valued between 143 and 190
million CFA;
Method 3 - 93 metric tons valued be tween 28 and 37
million CFA;
Method 4 - no estimate (total phosphorus was not
measured for Guinean waters of the river).
Unlver.lty of H1chlgan, C.-bia River a..ln Studl•• , 1985.
-- reservoir system ranges from 121 to 2,652 metric tons. The value of
this yield ranges from 36 to 1,060 million CFA. Because these figures
represent the additive effects of wide-ranging estimates, the combined
range is very large. However, the upper and lower values for these
ranges are probably realistic limits which can be expected from this
reservoir system. Again, yields are highly influenced by the hydrologic
cycle which determines the amount of water in each reservoir. For
reasons discussed in section 6.3.3, actual catch and economic value
realized from the 4-reservoir sys tem will likely approach the upper end
of the estimated range of values, rather than the lower end of these
predictions.
6.3.5. Kekreti Storage Dam and Balingho Salinity Barrage
The addition of the Balingho Salinity Barrage to the Kekreti Storage
Dam will have profound and far-reaching effects on all riverine fisheries
below the headwaters region in Guinea. By far, the greatest proportion
of the impact of this development scenario on the economic resources of
the aquatic system, will be contributed by the addition of the Balingho
Salinity Barrage to the basin development scheme.
The biological impacts and economic implications of the Kekreti
Reservoir were discussed in Chapter 4 and above in section 6.3.3. The
addition of the Balingho Salinity Barrage to the river system should not

198
significantly affect the biology, production, and economic of the Kekreti
reservoir fisheries. This assumes that the manpower and economic
assistance required to develop the Kekreti fisheries will not be reduced
by the addition of the Balingho Salinity Barrage to the development
scheme. But, because the human and economic resources available to
develop new impoundment fisheries are limited, the addition of the
Balingho Salinity Barrage and development of its fisheries will probably
reduce the rate of development and economic yield that would be realized
from the Kekreti Reservoir, in the absence of the Balingho Salinity
Barrage. Because figures are not available on levels of manpower and
economy committed to fishery development in the basin, the extent of this
reduction 'in Kekreti fisheries development and economic yield cannot be
estimated with accuracy at this time.
The Balingho Salinity Barrage will have several major effects on
fishery biology and economics in the Gambian portion of the river and
adjoining coastal waters. First, the impoundment upstream of the barrage
will permit development of a freshwater reservoir fishery, but the
existing riverine finfish fishery will be eliminated within, and to some
extent above the impoundment, as will all shellfish fishing. Second, all
existing fisheries for estuarine species of finfish and shellfish
upstream from the barrage site will be permanently eliminated.
Additionally, fisheries for species (e.g., bonga, shrimp, and crabs)
which require estuarine conditions to complete portions of their life
cycles will be severely affected, and local populations of these species
will be reduced to varying extents. Finally, the vast bulk (80 to 90%)
of the annual organic input to the lower freshwater and estuarine reaches
of the river as well as adjoining coastal waters comes from the
floodplains, bolons, and mangroves. All of these habitats and their
contribution to total production will be lost to the aquatic system above
the barrage. Below the barrage, floodplain habitat will be eliminated if
no water is released from the river, mangroves will be eliminated in
areas of hypersaline water, mangrove forests will undergo major shifts in
species composition for a considerable distance downstream of the
barrage, and inland reaches of existing bolons will disappear. All of
these conditions will reduce downstream riverine production dependent on
nutrient inputs from these sources.

199
Predictions of the annual yield of finfish from the reservoir created
by the Balingho Salinity Barrage range from 68 to 6,325 metric tons
(Table 6.5) with an economic value between 0.1 and 9.5 million Dalasis,
based on present market prices (Josserand, 1985) for freshwater species
expected to colonize the impoundment. The larger figure for each
estimate was developed using Method 1 which assumed that yield per
hectare for the impoundment would equal that of the existing estuary as
determined from catch assessment data. However, those continued catches
are highly unlikely in that a rich estuarine environment will be replaced
by a reservoir.
But, before the above estimates for the Balingho, Reservoir can be
added to those of the Kekreti Reservoir, it is necessary to consider
losses and gains to existing fisheries in the area that will be affected
by the Balingho Barrage. The total net loss or gain to these affected
fisheries must be added (or subtracted) from the values predicted for the
Balingho Reservoir itself. Then, it will be possible to estimate the
total ultimate yield and economic value from the Kekreti-Balingho
impoundment system.
Industrial finfish fishing does not currently occur above Balingho,
but the catch of the freshwater river (Upper River Division) artisanal
fishery in The Gambia is valued at about 1 million Dalasis, annually.
Most of this fishery and its revenues will be replaced by the reservoir
and whatever fisheries develop in association with it.
At present shrimp, oysters, crabs, and perhaps some lobsters are
caught in areas above the barrage site and contribute to the total annual
economic value of these products. Summarized for Gambian waters, these
totals are:
• shrimp 5 million Dalasis
• oysters 0.9 million Dalasis
• lobsters and crabs 0.001 million Dalasis
The combined total annual value of these shellfish products is about
6 million Dalasis. The economic yield of the lobster and crab fishery is
almost insignificant with respect to the others. At least one-half (0.5
million Dalasis) of the annual oyster harvest occurs in downstream areas,
the hydrological regime of which will be relatively unaffected by the
barrage. The proportion of the shrimp catch that is taken above the

202
catch which has been valued at about 0.5 million Dalasis. This fishery
is confined to marine coastal waters and should remain relatively
unaffected by the barrage (although the diet and dependence of these
sardines on suspended organic material discharged from the river into the
ocean has not) but this should be established. With respect to artisanal
finfish fishing, the marine coastal catch is valued at about 2 million
Dalasis, and the estuarine river catch (i.e., the Lower River Division)
at about 1 million Dalasis, annually.
Both the marine coastal and in particular the estuarine artisanal
fisheries depend on species of fish, e.g., bonga (Ethmalosa fimbriata)
and catfishes, which either require or prefer brackish water during at
some stage in their life cycle. Detailed descrLptiqn of major fish
species in the Gambia River including their habitat requirements for
spawning, nursery areas, and feeding was presented by Dorr et ale (1985).
It is expected that marine coastal stocks of bonga (and the
associated fishery for these stocks) will suffer less from river basin
development than will the estuarine stocks and fishery on the river.
This is believed to be the case because bonga uses both estuarine and
marine environments for spawning and nursery areas. Also, stocks of
bonga (and associated fisheries) which exist at the extreme north and
south ends of the Gambian coastline are probably outside of the direct
influence of the river. Marine coastal conditions and estuaries north
and south of the Gambia River upon which these fish are dependent, should
remain relatively free from impacts of basinal development.
However, potential long-term effects of river impoundments on the
Atlantic coastal stocks and fisheries for bonga (and other aquatic
organisms for that matter) must not be underestimated. Although
individual development projects may have relatively localized or isolated
effects on the bonga, the combined impact of several development projects
within the region (e.g., Senegal River, Gambia River, Casamance River)
may become significant over time. An equally critical concern is the
extent to which estuarine and marine stocks of bonga are segregated (or
intermix) during spawning and at other times during their life cycle.
The Gambia River Basin Studies provided a clear evaluation of the
abundance, distribution, life history, movements, and spawning of
riverine bonga, but not on oceanic fish. However, the relationship

etween estuarine-dwelling and oceanic-dwelling bonga needs to be
explored.
203
About 60% of the 1981 marine coastal artisanal fishery catch was
comprised of species that also inhabit the lower estuary. Direct effects
of the barrage on the coastal stocks of these species will be minimal.
The remaining 40% of the 1981 catch was comprised of fish (e.g., bonga,
mullet,. and jortoh) also occurring in the upper and lower estuaries at
all stages in their life history, during this project (Dorr et a1.,
1985). These species inhabit both estuarine and marine environments,
although as noted for bonga, the extent of their dependency on estuarine
conditions is not fully understood.
Given the preceding considerations, less than a 10% annual reduction
in catch (valued at 1.2 million Dalasis) is expected in the artisanal
marine coastal fishery sector as a result of the barrage. Also, the
initial reduction might be offset over time by the reallocation of
fishing effort toward other target species.
The artisanal estuarine (Lower River Division) finfish fishery has
been valued at about 1 million Dalasis, annually. Bonga comprised 22% by
weight of the total 1981 catch from this fishing sector, but a smaller
proportion of the total economic value because the market value (0.5
Dalasis/kg) of bonga is considerably less than that for other species
(average price = 2.3 Dalasis/kg, based on prices as cited in Josserand,
1985). Other fish species that may require estuarine conditions and are
caught by this fishing sector made up less than 10% by weight of the
remainder of the 1981 catch. Given the total elimination of bonga and
other estuarine fish species by the Balingho dam and reservoir, the
estuarine catch should experience about a 30% reduction in catch by
weight; in terms of economic value the reduction will be even less. This
would amount to an annual loss of about 0.3 million Dalasis to this
fishery based on 1981 landings and prices ci ted above. In practice,
total elimination of these species from the residual fishery is unlikely.
Predicted annual losses to existing fisheries from effects of the
barrage can be summarized from the preceding discussion as follows:
• freshwater artisanal finfish fishery above the barrage - 1
million Dalasis

205
4 million Dalasis. This figure assumes maximum levels of reservoir yield
and minimum losses to existing fisheries, an improbable circumstance. If
the converse assumptions are made, a net loss of 8.4 million Dalasis
could be realized.
Values for annual fisheries catch and economic value predicted for
the Kekreti and Ba1ingho reservoirs combined are (assuming 10 Dalasis =
1,000 CFA).
The potential annual yield estimated for this 2-impoundment system
ranges from 96 to 8,501 metric tons. When expected losses to existing
fisheries are included, the total economic value of this yield ranges
from a net gain of 10 million CFA to a loss of 77 million CFA, annually.
'The wide range in this estimates results from the lack of information on
the feasibility (biologically and economically) of exploiting the
reservoir fisheries to their maximum while maintaining losses to existing
fisheries at minimum levels; best and worst case scenarios have been
presented. In actuality, the yield by weight and economic value realized
from the Kekreti-Balingho impoundment system will probably fall in the
upper range of figures cited above. But in terms of aquatic ecology,
fisheries, hydrology, and economics, the addition of the Ba1ingho
Salinity Barrage to the basin development scheme is a risky investment at
best, and at worst could result in serious ecological damage and economic
losses to the system.
6.3.6. Kekreti Storage Dam, Guinean Dams, and Ba1ingho Salinity Barrage
The addition of the Guinean dams to the Kekreti-Ba1ingho impoundment
system will have little effect on the fisheries and economic in these
lower reservoirs. The major economic impact will be the addition of the
annual yield and economic contribution of the Guinean dams to the total
yield and value of postdeve10pment fisheries in the basin. As before,
this assumes that adequate manpower and economic assistance will be
available to develop the Guinean component without subsequently reducing
resources available to develop the fisheries and aquatic resources of the
Kekreti and Ba1ingho reservoirs. It also assumes adequate water is
available to fill and maintain the reservoirs.
If the figures for yield and economic value predicted for all five
impoundment (Table 6.7) are summed, the following totals are obtained:

206
Because both the manpower and economic assistance available to
develop the potential reservoir fisheries as well as those already in
existence are limited, the rate of increase toward maximum economic yield
from the individual reservoir fisheries will be slowed as additional
projects are brought into the development plan. If development resources
and assistance are highly limited, total economic yield from the
5-impoundment system will be less than yields obtainable from a smaller
but more highly developed, exploited, and managed complex of impoundments.
TABLE 6.7.
ESTIMATES FOR YIELD AND ECONOMIC VALUE
(ALL 5 DAMS)
Method 1 - no estimate (no catch statistics were
available for Guinea);
Method 2 - 7,645 metric tons valued between 589 and 731
million CFA;
Method 3 - 1,215 metric tons valued between 49 and 119
million CFA;
Method 4 - no estimate (total phosphorus was not
measured for Guinean waters of the river).
Unlver.lty of Hlchlgan. Gambl. River Ba.ln Studle•• 1985.

7. MONITORING AND INSTITUTIONALIZATION
7.1. Monitoring and Future Studies
The study of the aquatic resources of the Gambia River yielded, among
other things, a good description of the basic aquatic environment.
Included in that description was the distribution of the dominant aquatic
species in both time and space. But the study went beyond a simple
description of the aquatic environment in that a major effort was made to
reveal the key processes which drive the biological system. That effort
included defining tqe relationships between the physical-chemical,
environment and those species which inhabitat the river for a major
portion of the entire year. The ultimate objective, was to determine the
critical linkages among factors of the physical-chemical environment and
the biota. Once revealed, the factors and links became the diagnostic
base for the determination of impacts in the aquatic ecosystem from basin
development •
This study was successful in that major' speGies in the river and
estuary were identified as well as their basic requirements for
survival. But any project that is only one year in length, is limited in
the sense that a complete understanding of the system cannot be achieved
in so short a time. This fact points toward the continuation of studies
of the Gambia River using the results discussed above as a solid base for
the design of these related future investigations. As mentioned, the
Gambia River Basin Studies provided a good database of the location in
time and space of key species in the river. That information can be used
to divide the river into ecological segments so that future studies can
concentrate on a few representative locations in the river, Le., the
Balingho area, the Bai Tenda to Kau-ur area, the Kekreti area, the
vicinity of the Guinean dams, etc. The homogeni ty wi thin segments is
such that repetitive investigations of each segment should not be a high
priority.
Throughout the course of this study of the Gambia River, four items
emerged as requiring more investigation in the future. First, a better
understanding of temporal dynamics within the river is needed. This
investigation will require different approaches in different portions or
zones of the river. For example, in the estuarine zone the major
207

209
regime in the residual estuary. The large Rhizorpha spp. mangrove trees
exceeding 30 m in height, which exist only above Tendaba, will be highly
influenced by the barrage.
The fourth topic which requires future study is the assessment of
fish and shellfish stocks in the estuary and coastal environment.
Although good data were obtained on the relative abundance and
distribution of species in the estuary, these data do not support
estimates of sustainable yields. Those estimates must come from the
catch s tatistics of commercial and artisanal fishermen. The requirement
to estimate fish yields stems from the fact that fish catches appear low
in comparison to potential yields, especially in the coastal oceanic
environment· of 'The Gambia. The present and potential value of this
resource should be estimated with precision before it is influenced by
construction of the salinity barrage.
These four areas of study could be addressed in the overall context
of a monitoring program for the Gambia River. With the exception of the
fishery statistics and catch investigations, a program of basic study of
the Gambia River could include the concepts already examined, as well as
the procurement of additional baseline data on the basic nature of the
aquatic environment. The items identified above should serve in the
major framework for design of a monitoring program. This program should
begin as soon as possible so that data are collected before basin
development proceeds. Timely commencement of the monitoring program is
also required to train the scientists and technicians who will carry out
the field and laboratory work. A group of six African scientists was
trained in many of the Gambia River Basin Studies techniques using the
equipment and supplies brought from the United States to carry out the
one-year study. This manpower pool should be tapped before the training
is forgotten and/or the technical specialists are dispersed or otherwise
become unavailable. Opportunities also exist to bring scientists of the
original Gambia River Basin Studies River Resource Team back to the basin
in order to work with the African scientists in designing the future
monitoring program. Finally, the equipment and supplies left behind by
the Gambia River Basin Studies should be put to use before they
deteriorate from sitting in the tropical environment.

210
7.2. Parallel Studies
Any studies conducted of the Gambia River should be broader in
context than just the monitoring of aquatic resources. Because the
objectives of basin development are aimed toward large economic and
social goals such as food self-sufficiency, studies should include
alternatives to developing reservoirs for irrigation programs. For
example, groundwater reserves within the basin are poorly understood
(Harza, 1985). Yet, several highly successful farms in the Banjul area
rely primarily on groundwater. If groundwater reserves are sufficient,
irrigation could be achieved by pumped water rather than reservoirs; an
added benefit of this approach would be improved public health from the
cleaner water supply. Similar studies in the topics of fishery
development (see section 7.4), electrical generation and consumption, and
economic yields from irrigation are needed. A particularly large
deficiency in the base of information concerning the Gambia River Basin
is a precise estimate of the areal extent of the floodplains that line
the river. The information from these parallel studies can serve to
greatly aid in the planning of the development of the Gambia River.
The development of monitoring programs and parallel studies should
not be created in a vacuum, but rather have links to other West African
institutions. In the area of aquatic resources, there are several
agencies which have a record of successful monitoring of other rivers and
along the African Coast. The Oceanographic Institute (CRODT) just south
of Dakar has a long history of successful monitoring of the coastal and
riverine fisheries along the Senegalese coast. This institution can
foster a valuable exchange of ideas, data, equipment, and personnel for
use in the Gambia River monitoring program. Because a major objective of
the monitoring program includes fishery stock assessment, any
organisation with previous fish stock data will be a natural link to
OMVG. Many other similar organizations exist in West Africa to aid OMVG
such as ORSTOM, the Fisheries Department in Banjul, and Eaux et Forets in
Senegal. Eventually, parallel studies of several rivers could be
conducted to bring a broader understanding to the processes of tropical
rivers, estuaries and mangroves. Studies on the Senegal and Casamance
rivers have already been conducted by OMVS and CRODT.

211
7.3. Institutionalization
Successful development of the Gambia River Basin will depend upon the
ability of OMVG to plan, execute, and administer regional policies. The
problems of many of the countries of West Africa where planned growth is
often thwarted by parochial attitudes must be overcome if the development
program is to succeed. An ultimate authority must emerge and demonstrate
to the local units of government and developers that only planned growth
and carefully managed resources will be in the basin's best interests.
For these reasons OMVG should become the dominant unit of river basin
planning as well as accept responsibility for any lack of success of the
programs. Likewise, OMVG should be ·givE!n the charge of setting up and
conducting the monitoring program on the river. The monitoring program
should not be restricted to the aquatic environment, but incl.ude all
phases of study just as did the Gambia River Basin Studies. This
interdisciplinary approach stems from the same reasoning as used for the
Gambia River Basin Studies, the highly interrelated nature of all
activities in the basin. For example, poor agricultural or mining
practices will result in pollution that could readily contaminate river
water to the extent that it becomes useless for most purposes. Thus a
coordinated monitoring program should be initiated just as a coordinated
development program should be adopted.
Within OMVG an infrastructure already exists to develop a monitoring
program and to set water quality standards which should be maintained
during the development program. The Water Commission within OMVG has
been given the charge of assuring the quantity and quality of water
within the Gambia River. While a great deal of the responsibility of the
commission will go toward the equitable distribution and use of
freshwater reserves, water quality should hardly be ignored. Using
guidelines from other countries, water quality standards should be set
and implemented during and after development of the river basin. A
monitoring program will be the only mechanism by which those water
quality standards can be checked.
As a final step in institutionalization, OMVG should recognize its
limitations and defer those tasks to agencies where an existing
infrastructure already exists. For example, the Fisheries Department of

212
the Ministry of Water Resources in The Gambia already is conducting a
survey of Gambian fisheries . Rather than duplicate this activity, OMVG
should strengthen this survey and thus provide a better database for both
The Gambia and OMVG. This does not mean that OMVG should relinquish
authority for anyone aspect of the development of the river, but rather
rely on outside sources of help where possible. The ultimate objective
is a more complete understanding of the basin structure for more informed
planning and development while minimizing environmental impacts.
7.4 Future Fishery Monitoring and Management
A major part of the success of the development programs of the Gambia
River Basin will require recognition of potential fishery resources
followed by effective exploitation and management of these resources.
Three general steps are required to accomplish the above tasks:
• fish stock evaluation and biological monitoring;
• implementation of adequate catch assessment surveys;
• development of a resource policy and management program.
The Gambia River Basin Studies has the information and has completed
many of the steps needed to make preliminary evaluations of fish and
shellfish stocks existing in the river. In particular, estimates of both
existing and predicted postdevelopment stocks have been made. But work
beyond the scope of this project is needed to evaluate and project
changes in target stocks, after a specific development scenario has been
implemented. Without adequate and ongoing information on population
parameters of specific species, these stocks cannot be fished and managed
using modern techniques.
As mentioned in section 7.1, an aquatic monitoring program will be
required to provide information regarding the physical, chemical, and
biological conditions in the river and reservoirs. This program will
compile critical information that can describe environmental conditions
as well as the response of the river ecosystems to changes, particularly
those associated with development activities. In turn, the information
from the monitoring program is required to place fishery stock assessment
data into perspective, with respect to the environmental factors which
dictate the growth and survival of animal populations. Suggestions

213
regarding both stock assessment and monitoring programs are contained in
this report and Dorr et al. (1985).
Ongoing monthly and annual fisheries catch assessment surveys have
been in existence in The Gambia since 1980. These surveys, while
deficient in some critical areas such as compilation of data on overall
fishing effort or effort-by-gear, provide an excellent basis for the
implementation of an expanded catch assessment program. Much of the
groundwork and effort required to organize and implement such a program
has already been expended with considerable success. The existing catch
assessment survey program in The Gambia has reached a critical stage.
Additional assistance and guidance is needed in the areas of fiscal
,
support and development of data assessment objectives, to insure the
compilation of information according to identified analytical needs.
These needs must be established according to the goals set for river
basin development and overall production.
Catch assessment survey programs have not yet been established in the
Gambia River Basin of Senegal or Guinea; these programs will fill future
needs if the proposed reservoir development occurs in the upper river
basin. Not only must these survey programs be conceived and implemented,
but integration with the survey in the Gambian reaches of the river is
critical if a coordinated program of basin development is to be achieved.
Finally, there is a requirement to identify short-term and long-term
analytical objectives based upon the need to evaluate, develop, and
manage the fishery resources in the basin and adjoining coastal waters.
The river system should be considered an integral unit with an associated
master plan regarding the exploitation and management of fisheries, in
order that maximal and sustainable yields be realized from the resources.

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West Africa, 1983-1984," Great Lakes & Marine Waters Center
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Freeman, P .H. 1974. "The Environmental Impacts of a Large Tropical
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Harza Engineering Company International. 1985. ··Gambia River Basin
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Hydraulics Research Station. 1976. "Comments on the Feasibility of a
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King. H. 1979. "A Review of the State of Fisheries in the Gambia," Pub.
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LeReste, L. 1983. "Etude des variations annuelles de la production de
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Lesack, L.F.W.; Hecky, R.E. and Melack, J.M. 1984. "Transport of
carbon, nitrogen, phosphorus and major solutes in the Gambia River,
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'
APPENDIX I.
PRESENTATION OF IMPACTS BY TYPE

221
TABLE OF CONTENTS
A.l. PHYSICAL AND CHEMICAL IMPACTS •.••••.•
A.l.l. Alteration of Seasonal Flow Patterns in the Gambia
A.l.2.
A.l.3.
A.l.4.
A.l.5.
A.l.6.
A.l.7.
A.l.8.
A.l.9.
A.1.10.
A.l.ll.
A.l.12.
A.l.13.
A.l.14.
A.l.15.
A.l.16.
A.l.17.
A.l.18.
A.l.19.
River. . . . . . . . . . . . . . . .
Altered Streamflows in the River••••
Altered Thermal Regime within Reservoirs••.
Anoxic Conditions in the Bottom of Reservoirs
Altered Nutrient Concentrations in the Gambia River •
Altered Suspended Sediment Loads•••••
Increased Underwater Light Penetration in the
Reservoirs . . . . . . . . . . . . . . . .
Elevated Suspended Sediment Loads during Construction
Modification to River Banks by Soil Erosion and Lack
of Seasonal Inundation • • • • • • • • • • • • • •
Permanent Loss of Seasonally Inundated Floodplains••
Development of a Drawdown Zone in Each Reservoir • . . •
Increased Evaporation from the Reservoirs and
Floodplains••••••••••••••••
Lack of Tidal Mixing Upstream of the Barrage•...•••
Increased Tidal Amplitude Downstream from the
Salinity Barrage ••••••••••
Exclusion of Salt Water Above Balingho•••••
Lack of Salinity Gradient in the Gambia River Estuary
Formation of Acid-sulfate Soils • • • .
Sediment Accumulation in the Mangrove Bolons.••••
Formation of Hypersaline Water.
A.2. BIOLOGICAL IMPACTS••.
A.2.l.
A.2.2.
A.2.3.
A.2.4.
A.2.5.
A.2.6.
A.2.7.
A.2.8.
A.2.9.
A.2 .10.
A.2.ll.
Shifts in Aquatic Species Toward Limnetic (Lake-
Dwelling) Organisms. • • • • • • • •• • .•.
Enhanced Algal Production in Reservoirs • • • • • • . • .
Changes in the Benthic Invertebrate Species
Composition in the Reservoirs. • • • • • • • . • • • .
Increased Fish Production and Harvest in Reservoirs •
Explosive Growth of Aquatic Weeds • • • • • • • • •
Elevated Rates of Evapotranspiration•.••..•..
Elimination of Mangroves Upstream from the Barrage
and Alteration of Mangrove Ecosystem Structure
Below the Barrage•••••••••••••••
Disruption of Estuarine Migration Routes••.••
Elimination of Marine Plankton Upstream of the
Salinity Barrage ••••.••
Enhancement of Fish Production in the Lower Portion
of the Gambia River•.••••••••••••
Elimination of Invertebrate Communities Upstream
from Balingho. • • • • • • • • • . • • • • • • •

222
A.3. ANTHROPOGENIC IMPACTS ••••••••••••
A.3.1.
A.3.2.
A.3.3.
A.3.4.
A.3.5.
A.3.6.
A.3.7.
A.3.8.
A.3.9.
A.3 .10.
A.3.l1.
Increased Use of Herbicides, Pesticides and
Fertilizers•••••••••••••
Change in Traditional Fisheries Toward Lacustrine
Species. . . . . . . . . . . . . . . . . . . .
Change in Diet Based on Food Availability, Including
a Shift Toward Fish. • • • • • • • • • • • • •
Increase in Irrigated Cropland. • • • • • ••
Mining Activities • • • • • • • • • • • • • • •
Human Resettlement Adjacent to the River and
Reservoirs . . . . . . . . . . . . . . . . . . . . . .
Changes in Abundance of Disease Vectors •
Deforestation . • . • • . . • • • . • . . . • . .
Changes in Occupations.••••••••••••.
Altered Routes of Commerce and Transportation
Changes in the Distribution of Wildlife • • • •

223
APPENDIX I. PRESENTATION OF IMPACTS BY TYPE
The impacts to the aquatic environment of the Gambia River from the
proposed development program were described on a zone-by-zone basis in
Chapter 4. Table 4.2 demonstrates that all but one of the forty-one
impacts will occur at least two zones under certain development
scenarios. Several impacts may occur in all five zones under the
development scenario with all five dams constructed in the Gambia River
Basin. This appendix presents the forty-one impacts by type (primary,
secondary, and tertiary) rather than by zones. The intent of this
appendix is to avoid redundancy with Chapter 4 while providing
explanations of each impact to the nontechnically oriented reader.
Technical terms are either avoided below, or defined carefully on first
usage.
A.I. Physical and Chemical Impacts
Nineteen impacts to the aquatic physical and chemical environment
have been identified. These are considered primary impacts because they
concern the physical and/or chemical environment. The primary impacts in
turn create secondary (biological) and tertiary (anthropogenic) impacts.
A.LL Alteration of Seasonal Flow Patterns in the Gambia River
For thousands of years the Gambia River has followed an unregulated
cycle of flood (June through October) and drought (November through
June). The rainy season has initiated the annual flood which in turn
affects almost every organism in and near the river. After development,
streamflows will be regulated and tend to produce a more uniform annual
flow pattern compared to current conditions. The total net amount of
water flowing down the Gambia River will not change very much, but the
seasonal distribution of flows will be completely altered.
A.L2. Altered Streamflows in the River
Along with the regulated pattern of annual discharge will be
regulated flow of water in the river. Between each of the reservoirs the
streamflow in the Gambia River will be controlled by the release from

224
each upstream dam release. Thus, the large fluctuations currently
observed in the natural state of the river will cease to exist. Aquatic
organisms both in the river and on adjacent floodplains will be subject
to a regular streamflow.
A.L3. Altered Thermal Regime within Reservoirs
currently, the waters of the Gambia River are well-mixed from
streamflow and tidal waves. This mixed condition is evident from the
uniform temperature observed throughout all depths of the river, uniform
dissolved nutrient concentrations, and high levels of dissolved oxygen
(Berry et aL 1985). The large reservoirs associated with each of the
five dams will develop radically different chemical and physical
properties from the current well-mixed conditions. An altered thermal
regime is another result of impounding a major portion of the river
water. Water in the reservoirs may mix where it is shallow; mixing is
probable in places with less than four meters of depth. But, in other
places, water depths will be too great to permit surface to bottom mixing
throughout the year.
A.L4. Anoxic Conditions in the Bottom of Reservoirs
Tropical lakes and reservoirs are typically characterized by large
zones of seasonal anoxic (lacking dissolved oxygen) water near the bottom
(e.g. Lewis, 1983). The high temperatures and large inputs of organic
matter tend to yield high BODs (biological oxygen demand). (In other
words, large amounts of rotting plant material which accumulate on the
bottom of lakes consume all of the available dissolved oxygen.) The tall
water column prevents mixing to the bottom during certain seasons and
hence the bottom water become anoxic.
Only by mixing oxygen down to the bottom of the reservoir will anoxia
(oxygen deficiency) be alleviated. Anoxic waters tend to exclude most
forms of aquatic life and prolong the rates of decay of organic matter.
Reducing chemical conditions allow the conversion of oxidized sulfur and
iron to the more offensive iron sulfides and hydrogen sulfide.
A.L5. Altered Nutrient Concentrations in the Gambia River
Analysis of water samples for nutrients from the Gambia River have
shown river water as a dilute medium that undergoes large chemical

225
changes during the annual flood (Berry et al., 1985). The runoff
entering the river with the annual flood was enriched in nitrate-nitrogen
and soluble reactive silicon. After construction of the dams on the
Gambia River, two major impacts will produce greatly altered dissolved
nutrient concentrations. First, freshwater will not mix with saltwater
in the estuary. This will eliminate the enrichment of estuarine
saltwater with certain nutrients. Second, the annual flood will be
essentially eliminated by water retention in the reservoirs. As mentioned
above, primary productivity will probably be enhanced within the
reservoirs. These enhanced rates of productivity will serve to strip
many nutrients from the water column, especially because the reservoirs
will have thermal stratification. (The vertical thermal stratification
prevents vertical mixing. As algal cells die, they sink to the bottom of
the reservoir carrying nutrients with them. Eventually the upper layer
of water becomes depleted of nutrients.) The flow in the river
downstream of the dams will be from these nutrient-poor reservoirs,
especially if the discharge from the dam is from the upper water layer.
Ultimately the river will have water flowing in it with lower nutrient
concentrations than at the present.
A.1.6. Altered Suspended Sediment Loads
Annual floods usually carry very high suspended solids and bed
loads. The high velocity and streamflows of floods scour the bottom and
banks, carrying great loads of fine particulate material downstream.
Such floods have enormous effects on the basic ecology of the river and
adjacent banks. Riparian agricultural practices are usually tuned to the
annual inundation of lowlands with silt-laden waters that flood the river
banks. With the erection of the dams and barrages along the Gambia
River, the attempt will be to regulate streamflows to less than flood
conditions at all times. Water will pass down the river through a series
of reservoirs which are very effective settling chambers. Thus the
sediment load, and hence sediment distribution, will be radically altered
compared to present conditions. The high sediment loads of floods will
cease to exist. Similarly, the low sediment periods during the dry
season will also cease to exist. The discharge from the reservoirs
through the dams will pick up a sediment load typically associated with
medium flows and carry this load downstream.

226
The alteration to the suspended sediment loads will have a major
effect on aquatic primary productivity (photosynthesis by
phytoplankton). Less sediment load allows for deeper penetration of
light into the water column and enlarges the euphotic zone (Wetzel,
1975). Because most of the algal photosynthesis measured in the river
appeared light-limited (Healey et al., 1985), any increase in water
clarity will be accompanied by an increase in primary production.
A.l.7. Increased Underwater Light Penetration in the Reservoirs
\
Results of primary productivity (photosynthesis by algae)
measurements and nutrient determinations showed that algal productivity
was primarily limited by light (Healey et al., 1985). In addition, the
nonstable water column caused by tidal mixing and streamflow, aided in
suppressing photosynthesis. An increase in algal photosynthesis can be
expected in each of the five reservoirs. The water column in each lake
will stabilize and sediments will settle-out. This clarified water will
be a highly desirable environment for algae.
A.l.8. Elevated Suspended Sediment Loads during Construction
The construction of each of the five dams on the Gambia River and its
tributaries will involve an enormous amount of earth movement. Earthen
cofferdams may be built, banks will be cut down in some places and
built-up in others. Access roads will be cut to the construction sites
and large areas of forest will be cleared for the construction camps.
These and other activities will provide ideal environments for excessive
soil erosion. This erosion can occur either by water during the rainy
season or by wind during the dry season.
Once large quantities of fine particulate materials enter the rivers,
they can be transported large distances downstream. Immediately
downstream of the construction site, most forms of aquatic life can be
suddenly buried and destroyed. Many kilometers farther downstream, the
high sediment loads are capable of choking organisms which occupy benthic
habitats in calm waters where sediment rapidly accumulates.
Whereas these increased sediment loads are "temporary" in the sense
that they only occur during construction, their effects can be extremely
long-lasting. Construction of each dam may take three or four years.

227
Because the dams will not be built simultaneously, construction may
continue well over a decade. Soil erosion will doubtless continue for
several years after construction terminates until exposed soils
stabilize. Ten to fifteen years of extremely elevated sediment loads in
the river is enough to do irreversible damage to aquatic life,
especially just downstream of the construction sites.
A.L9. Modification to River Banks by Soil Erosion and Lack of
Seasonal Inundation
The river banks of the Gambia River provide an essential portion of
the physical habitat to many organisms. These banks are the interface
between land and water, and provide a unique habitat to many riparian
species. Exchanges of materials from the land to river and vice versa,
occur across the banks. The regulated hydrologic regime as a result of
dam operation and irrigation will cause widespread and permanent changes
to the river banks. These banks will no longer be sub ject to annual
floods but will receive a constant flow. While the periods of high
erosion will cease (floods), the reprieve from erosion during low flow
will also be eliminated. Thus a constant low level of soil erosion can
be expected throughout the year.
The river banks currently are inundated by two mechanisms. All banks
are immersed for several months each year by the annual flood. In recent
years this flood has not been large, yet for the most part, a major
portion of the banks were still inundated. Tidal flooding, is another
mechanism by which the river bank habitat is constantly kept wet or
moist. This process will be eliminated above Ba1ingho after construction
of the Salinity Barrage. The net result will be a radical alteration to
the physical structure of the river banks and a change in their
suitability as a habitat.
A.1.10. Permanent Loss of Seasonally Inundated Floodplains
Each year a large area of the low-lying banks of the Gambia River are
inundated by the freshwaters of the annual flood. These floodplains are
an integral part of the river environment (We1comme, 1979). They support
recession agriculture, provide fish spawning sites, and provide waterfowl
feeding grounds. The regulated water flow regime and the large
reservoirs will destroy most of these floodplains; some will be left

228
permanently dry, others will be permanently inundated, and some turned
into irrigated fields.
A.l.ll. Development of a Drawdown Zone in Each Reservoir
The operational policy of each dam will result in a large annual
cycle of the water depth within the reservoirs. As water is removed for
hydropower generation and irrigation, a large area of the bottom of the
reservoir will be exposed to the air. This dried-out bank is called a
drawdown zone or foreshore. This zone provides a rather poor habitat for
most forms of aquatic life. As a result, drawdown zones tend to be
relatively unproductive segments of the reservoirs. If, h9wever, some
foreshore grasses flourish and there is extensive foreshore farming and
livestock grazing, there can be vigorous nutrient release to the
reservoir.
A.l.12. Increased Evaporation from the Reservoirs and Floodplains
A major loss of water from the Gambia River Basin will be caused by
high rates of evaporation from standing water. These evaporative
processes also serve to concentrate dissolved substances in the water.
Evaporation will increase due to an increased surface area in the
reservoirs and irrigated fields. Total loss of water will be larger, and
soluble substance concentrations will be greater after filling of the
reservoirs. Because water supplies in the basin are extremely limited,
any additional loss of water has serious consequences.
The physical-chemical impacts discussed above pertain to the entire
Gambia River. There are seven impacts which will be associated just with
the Balingho Salinity Barrage. These seven impacts should be considered
in addition to the twelve already discussed.
A.l.13. Lack of Tidal Mixing Upstream of the Barrage
The Gambia River is tidal more than 500 km upstream. Up to 2 tides
may exist in the river at anyone time (Humphrey, 1974). Usually two high
tides and two low tides can be clearly observed in the river at once.
This tidal mixing is the most important factor affecting the distribution
of suspended and dissolved materials in the river (Berry et al., 1985).

229
Tidal mixing also affected the distribution of many planktonic and
pelagic organisms.
Completion of the Balingho Salinity Barrage will cause immediate and
permanent loss of tidal mixing from the barrage at 130 km upstream to
500 km upstream t or 370 km of the river; thus 72% of the current tidal
portion of the river will no longer have tidal mixing. Some mixing will
still occur t however t from the regulated currents of streamflows t but
this will be small compared to the combined current and tidal forces.
Loss of tidal mixing will create stagnation in many parts of the river t
especially the convoluted mangrove bolons. Stagnation will be
accompanied by altered sediment deposition patterns and areas of anoxia.
Lack of mixing will also favor thermal stratification of the water column
with the resultant surface nutrient depletion t as discussed in A.l.13.
Finally t lack of tides will cause many areas of floodplains which are
inundated twice per day to dry out t removing them as viable portions of
the river as well as induce the formation of acid-sulfate soils (see
A.lo17.).
A.l.14. Increased Tidal Amplitude Downstream from the Salinity Barrage
Simulations by the Danish Hydraulic Institute have shown that tidal
waves reflecting off the salinity barrage will result in up to a
o.4-meter increase in tidal range (RRI t 1982). This increase will be
greatest just below the barrage and diminish toward Banjul where there
will be no amplification. Tidal amplification will be greatest on spring
tides t about 0.2 m above current highs and dimution about 0.2 m below
current lows. They will be smallest during neap tides t about 0.1 m above
and below current highs and lows. Tidal amplification will cause an
increase in floodplain inundation t and some stress may be placed on
downstream mangrove forest because of slightly higher water levels.
A.l.15. Exclusion of Salt Water Above Balingho
The main purpose of the salinity barrage is to exclude saltwater from
the Gambia River upstream from Balingho. This exclusion will have
profound effects on the flora and fauna in that portion of the river
between Balingho and Kuntaur. Here the river will no longer be
estuarine for some eight months of the year t but become a permanent

230
freshwater reservoir. Marine and estuarine organisms will be excluded
from that portion of the river, and freshwater species will be able to
take over the reservoir. Evaluating the shift from estuarine to
freshwater in terms of "good" or "bad" serves no purpose. Rather, it can
be stated that there will be a major and permanent species shift
immediately after the barrage is constructed. It will take several years
for equilibrium in the new freshwater community in the impounded portion
of the river to become fully established.
A serious consequence of the exclusion of saltwater above Balingho
will be the loss of spawning grounds and habitat for migratory species.
Many fishes and invertebrates utilize the low salinity (less than 10 ppt)
waters of the present upper estuary as breeding and nursery areas. The
salinity barrage will remove approximately 80 km of this habitat from the
river and consequently reduce the production of these organisms.
A.l.16. Lack of Salinity Gradient in the Gambia River Estuary
All estuaries, by definition, have a source of freshwater which mixes
with saltwater (McLusky, 1971). This mixing process creates a salinity
gradient which usually ranges from freshwater (0 ppt salinity) to
full-strength seawater (34-35 ppt salinity). Many estuarine species are
adapted to live in waters with salinities which are intermediate to
freshwater and saltwater. Other species breed or have life history
growth stages in these brackish waters.
Under certain development scenarios, the basic operational policy of
the salinity barrage will result in the elimination of the salinity
gradient. In these circumstances, minimal freshwater will flow through
the barrage because all of the available freshwater would be used for
irrigation and to maintain the reservoir. In effect, the estuary would
cease to be an estuary as it is now, but rather the lower 130 km of the
Gambia River would become an extension of the sea. Without freshwater
flow, coastal salinities (34 ppt) would be found up to the base of the
barrage. As a result, many species will be excluded from the Gambia
River because the brackish water habitat will cease to exist. In
contrast, many coastal marine species will find the shallow, high
salinity environment of the lower river a favorable habitat. As
discussed below, the high salinity regions of the exist ing estuary have
been the most productive parts of the existing estuary.

231
A.l.17. Formation of Acid-sulfate Soils
Precedent has shown that under certain conditions t saltwater
swampland will become highly acidic after it is immersed in freshwater
and then is permitted to dry out (ColleYt 1985). This problem is
particularly severe where pyrite concentrations are high in the soils.
The acidity in these soils reaches alarming levels t often dropping the pH
to near 2.0. The result is a soil condition which can support neither
land vegetation t including crops nor any local aquatic bottom organisms.
The extent and intensity of the acid-soil conditions in the Gambia
River has been carefully considered. Yet t a finite prediction of the
severity of this impact has not been accepted. Some investigators
believe that proper management of the soils will prevent the acid
conditions from ever occurring in the first place. Others claim that
preventing these conditions from occurring is almost impossible based on
normal agriculture practices. A complete discussion of this impact can
be found in Colley (1985).
An additional result of the acid-soil formation is the acidification
of the surrounding river water. After the soils have turned acidic t
irrigation water could wash the acidity out of them in the form of
sulfuric acid. Some investigations indicate that enough acid could enter
the reservoir by this process to lower the pH to between 2.0 and 3.0 t
killing most if not all aquatic life. Other investigators suggest that
the pH will not fall below 5.0 t a dangerous but manageable level for many
organisms. Colley (1985) believes that the natural buffering capacity of
the water will hold the pH levels closer to 5.0; in saltwater t the pH
will not change unless enormous amounts of acid are released because of
the large buffering capacity of seawater. Whichever prevails in the
Gambia River t the final pH will depend upon a combination of the extent
of acid-soil formation t the volume of the reservoir t and the buffering
capacity of the reservoir waters. At this time one can only conclude
that a potential for the problem exists t and careful monitoring and
management may be required.
A.l.18. Sediment Accumulation in the Mangrove Bolons
The lack of tidal flushing in the Gambia River will prevent the
present semidiurnal water exchange into and from the mangrove bolons.

232
Tidal currents, which reach more than 3 km/hr, are the major process by
which materials are exchanged between the mangrove bolons and the river
channel. These same currents also serve to scour the bolons and retard
their filling by the rapid accumulation of organic and inorganic matter.
Once the tidal mixing processes are removed from the upper estuary,
materials will accumulate rapidly in the small, meandering bolons. These
will quickly silt-up and no longer serve as pathways for the exchange of
materials between the river channel and the floodplains commonly found at
the end of the bolons.
A.l.19. Formation of Hypersaline Water
Without freshwater flow, the segment of the Gambia River just
downstream from the salinity barrage may become hypersaline. This
condition arises when high rates of evaporation concentrate sea salts to
well above normal seawater, often up to three times that of the coastal
ocean. These extremely salty waters cause severe stress to both plants
and animals and eventually will support very little life. Hypersaline
conditions are especially detrimental to mangrove forests and often
result in their demise (TWilley, 1985). Furthermore, if water in the
Gambia River becomes more salty than the ocean, many migratory organisms
will receive confused signals about the direction of their migration,
perhaps losing clues entirely.
A.2. Biological Impacts
In the aquatic ecosystem, impacts from the river basin development
primarily occur to the physical and chemical environment. These impacts
in turn affect the aquatic flora and fauna. This section deals with the
anticipated impacts to the biological communities. These biological
(secondary) impacts are the response of the aquatic flora and fauna to
the alterations to their habitats. Man's perception as to changes
induced by river development is often through these biological impacts.
Eleven biological impacts are presented below. As with the physical and
chemical impacts, some biological ones are restricted to certain phases
of construction while others have geographic limitations.

A.2.1.
233
Shifts in Aquatic Species Toward Limnetic (Lake-Dwelling)
Organisms
The formation of the five reservoirs behind each of the dams will
create large lake (lacustrine) environments which currently do not exist
in the Gambia River Basin. These environments provide expanded habitats
to many species which either cannot live in the flowing water, or have
only meager populations in the quiet backwaters and pools of the river.
New aquatic communities will develop both in the wa ter column of the
reservoirs and on the bottom; they will be denser in the shallow water
areas. These new communities will create food chains composed of
different links compared to the existing riverine food chains. The net
result will be the development of potential lacustrine fisheries which
are often more productive (provide greater harvests) than the existing
riverine fisheries (Freeman, 1974).
This shift in the trophic structure of the food chain can hardly be
overemphasized. The fundamental structure of the biological community
will change all the way from the smallest plankton to the carnivorous
fishes. This impact will be the largest biological response to the
modification of the river by construction of the dams. While this impact
is extremely important, it is somewhat limited in that the changes to the
biological community will occur essentially only in the reservoirs; the
sections of the river flowing between the proposed dams will more or less
preserve their biological integrity as before construction. But, a
majority of the river water will be contained within the five reservoirs,
and one-fifth of the total river length will become lacustrine.
A.2.2. Enhanced Algal Production in Reservoirs
Lakes are usually more productive biologically than rivers (Wetzel,
1975). The waters of lakes are also clearer because suspended sediments
settle out of the water column. This greater optical transparency of
lakes enlarges the euphotic zone (portion of the water body that has
sufficient light for photosynthesis) which in turn increases primary
production. As a result, while the lake may not be more productive per
cubic meter of water than the river, the lake has a vastly larger volume
which will support algal growth.
Not only will the reservoirs support more algal photosynthesis, but
lakes will support different species than the riverine environment. The

234
effect of these alterations to the plankton community is a large shift in
the entire aquatic trophic structure as increased levels of algal
production are followed by an enlarged biological community. With
altered species composition comes a changed trophic structure, usually
with increased species diversity. These changes normally result in
augmented fish production at the top of the food chain. A somewhat
negative result of the increased algal production is the large amount of
decaying organic matter which rains down through the water column and
will accumulate in the bottom of the reservoirs; this material often has
a high BOD (biological oxygen demand) and ultimately leads to oxygen
deficiency
(anoxia) in the bottom waters of the reservoir.
A.2.3. Changes in the Benthic Invertebrate Species Composition in the
Reservoirs
As discussed above, the formation of the five reservoirs will cause a
major change in the trophic structure of the pelagic (pertaining to the
organisms living in the water column) food chain. There could also be a
major change in the composition of those organisms living on the bottom
(benthic organisms). The terrestrial vegetation that is flooded by the
reservoirs, the submerged trees and shrubs, offer ideal habitats for a
variety of aquatic insects (van Mar en , 1985). Wood-boring insects, in
particular, will find a suitable environment on the submerged vegetation,
as will a variety of crabs, mollusks, and other small invertebrates find
suitable environments among the newly submerged rocks and trees. This
enriched benthic fauna can serve as a primary food source for many
benthic-feeding fish. An abundance of microscopic as well as macroscopic
plants should provide a rich source of food for many bottom-feeding
organisms. But anoxic conditions in the deeper sections of the reservoirs
may exclude almost all species from those portions of the lake.
Nonetheless, a large and diverse benthic habitat will develop in the
shallow portions of the reservoirs. The submergence of terrestrial
vegetation and resultant invertebrate community expansion may not be
confined to the reservoirs. Irrigation canals also serve to stimulate
invertebrate growth, especially insects.

A.2.4.
235
Increased Fish Production and Harvest in Reservoirs
The lacustrine environment will provide substantially increased fish
production per unit area over the current riverine situation. This
increase will result from the altered trophic structure and increased
algal productivity of the reservoirs. This is without doubt the largest
beneficial impact of the development program on the aquatic resources.
The primary objective of the development scenarios proposed for the
Gambia River is an increase in domestic food production. Properly
developed and managed, the increased fish yields of the reservoirs will
go a long way toward meeting the goal of domestic food self-sufficiency.
The potential collectively for the river basin development program may
approach 7,600 metric tons annually, much of which is a net gain over the
existing riverine fisheries. Production estimates and the economic value
of these yields are discussed in detail in Chapter 6.
A.2.5. Explosive Growth of Aquatic Weeds
Precedent has shown that rooted and floating aquatic macrophytes
(large aquatic plants) can become major problems in tropical reservoirs.
The calm waters at the edges of the reservoirs provide a suitable habitat
for growth of macrophytes that did not exist or were sparse in the
flowing river. All of the major pest species exist in slow-flowing
backwaters of the Gambia River and its tributaries (van Mar en , 1985).
Their growth is currently kept under control by the lack of suitable
habitat. The banks of the Gambia River are steep in the freshwater
portion of the river, often descending almost vertically to a depth of 5
m below the surface. Currents during the annual flood scour these banks
and prevent any substantial growth of macrophytes. Once the reservoirs
are filled, large beds of macrophytes will grow out from the shoreline.
Some varieties of floating weeds will also develop into dense mats called
sudds. Whether attached or floating, these weeds will deteriorate the
usefulness of the reservoirs. The weeds compete with algae for limited
nutrient resources. Snails breed under these weeds and increase the risk
of schistosomiasis to residents living near the reservoirs. In some
reservoirs, the weed mats become so dense as to completely prevent access
to the water from the shore as well as make navigation almost impossible.

A.2.6.
236
Elevated Rates of Evapotranspiration
The primary reason behind the construction of the dams is the
conservation of freshwater normally lost to the sea as part of the annual
flood. Dams are extremely effective at storing and dispensing the water
for controlled uses. But water will be lost from the reservoirs through
evaporation and evapotranspiration from emergent plants. Water will also
be lost through evapotranspiration in irrigated crops. This loss is
rather unpredictable because evapotranspiration depends on a suite of
factors which include local meteorological conditions and physiological
status of the crops.
The six impacts discussed above pertain to the entire Gambia River
system. Their effects will be observed throughout the river basin and
will generally be permanent alterations to the aquatic flora and fauna.
There are five additional impacts which arise solely from the
construction of the Balingho Salinity Barrage. These impacts are limited
in their areal extent in that they only pertain to the Balingho Salinity
Barra'ge.
A.2.7. Elimination of Mangroves Upstream from the Barrage and Alteration
of Mangrove Ecosystem Structure Below the Barrage
The largest in areal extent and the most important impact in the
estuary will be the elimination of the mangrove ecosystems upstream from
the salinity barrage. Whereas there are only allegations that mangroves
can live in freshwater, it is certain they cannot live in water without
tidal fluctuations which will be stopped by the barrage. Large dikes in
Florida which impounded brackish water in mangrove swamps have been
observed to kill two mangrove species namely (Rhizophora and Avicenia)
(Twilley, 1985). The same effect will occur in the Gambia River.
Upstream from the barrage, about 12% of the mangrove forests along the
river will be eliminated. Primarily Rhizophora racemosa will be
destroyed, which are the most luxuriant mangroves along the Gambia River
with some specimens reaching more than 30 m in height.
Below the salinity barrage, salinities will increase to match and
probably exceed those of the coastal ocean after the flow of freshwater
is arrested by the dam. Tidal amplitude will also be enhanced about 20%
just below the barrage. These two impacts will combine to place stress

237
on the mangrove fores ts downstream of the salinity barrage. This s tress
will probably result in shifts of the mangrove community away from the
large Rhizophora racemosa trees toward smaller kinds. High rates of
evaporation may create hypersaline conditions, i.e., salinities in excess
of the coastal waters. These highly saline conditions ·often form salt
pans, areas of the intertidal zone where the salt dries onto the mud and
kills all forms of terrestrial vegetation. Thus the mangrove forest
structure will be changed from dense, continuous forest to more open and
discontinuous stands.
A.2.8. Disruption of Estuarine Migration Routes
Several species of fish and invertebrates conduct migrations into the
upper reaches of the Gambia River estuary. Bonga and other species of
fish larvae were collected in the upper estuary (Dorr et al., 1985).
Shrimp and crabs were abundant in the upper estuary nursery and feeding
areas at certain times of the year (van Mar en , 1985). The migration
routes of these species will be completely blocked by the salinity
2
barrage. A 700 km area of the present estuary will thus no longer be
available for migrants after the salinity barrage is complete.
A.2.9. Elimination of Marine Plankton Upstream of the Salinity Barrage
During the dry season and early wet season (December through June),
the Gambia River has measurable salinity (and is thus estuarine) as far
as 250 km upstream, near Kuntaur. This saltwater intrusion carries
predominantly marine plankton into the entire estuarine portion of the
river (Healey et al., 1985). This marine plankton serves as the food
base of an estuarine fauna which include estuarine and coastal marine
fish species. By eliminating the movement of marine plankton above
Balingho, these species will also be eliminated. Although this concept
appears evident from the logic of no saltwater, no marine fish, the
biological sequence of events is somewhat more subtle than which seems
obvious. Marine and estuarine fish species are commonly caught well
upstream from the extent of saltwater penetration. These were healthy
specimens and were harvested in relatively high abundances (Josserand,
1985). Evidently certain marine and estuarine species make a temporary
transition into freshwater for at least a few months each year. But these
species probably cannot survive if they were permanently removed from

238
their usual estuarine-based food chains and/or winter salinities for
breeding.
A.2.10. Enhancement of Fish Production in the Lower Portion of
the Gambia River
The lower estuarine portion of the Gambia River near Dog Island Point
was the most productive area of the river (Dorr et a1., 1985). The
experimental fish sampling as well as the results of the artisana1
fishery survey both confirmed that the lower estuary fisheries were
highest in yield and diversity of the entire river system (Josserand,
1985; Dorr et a1., 1985). This zone was consistently the most productive
for both finfish and shellfish during each of the four 'seasons sampled:
early rainy season, late rainy season, early dry season, and late dry
season.
The reason for the high levels of production were attributed to the
fact that the lower estuary is the most nutrient enriched segment of the
river and thus a strong attractant for many coastal species. As long as
physical and chemical conditions matched the conditions nearshore, marine
fishes and crustaceans (shrimps and crabs) move readily into the
estuary. With the construction of the salinity barrage, coastal marine
conditions will extend up to Ba1ingho, 130 km inland. This will create
an expanded habitat for the finfish and invertebrates, which grow in the
lower portions of the river. Presumably, with expanded habitat, will
follow expanded growth and production to the extent that the altered food
web will permit as affected by nutrient retention from the salinity
barrage. But, the productivity of the lower estuary is tied directly to
the health of the mangrove forests. Any decline in mangrove productivity
could readily be followed by a comparable decline in estuarine and
coastal fisheries.
A.2.11. Elimination of Invertebrate Communities Upstream from Ba1ingho
The Gambia River supports a moderate-sized invertebrate fishery in
the upper half of the estuary above Ba1ingho. This fishery includes a
seasonal penaeid shrimp, crab, and oyster fishery, the last two of which
appear vastly underexp1oited. These three invertebrate fisheries will be
eliminated above Ba1ingho once the salinity barrage is completed. The
fisheries are composed of estuarine and marine species, and will not

239
survive the transition to freshwater. Chapters 3 and 6 detail the
current and potential value of these fisheries.
A.3. Anthropogenic Impacts
Perhaps the most important impacts in the Gambia River basin due to
development plans are those concerning human activities. The newly
created resources from the five dams, their stores of water, and the
available hydroelectricity will have a profound effect on the way people
live, work, and conduct commerce in the basin. Many of these
anthropogenic impacts (tertiary) will not have a direct impact on the
aquatic environment of the river but rather indirect or secondary
effects. Eleven anthropogenic impacts have been identified which will
directly effect the aquatic environment and biota.
A.3.1. Increased Use of Herbicides, Pesticides and Fertilizers
Extensive irrigation for crops in the Gambia River Basin will entail
a shift from traditional rain-fed agricultural practices toward intensely
managed farming. Based on past experience with carefully managed farming
practices, yields can be increased and maintained by application of
fertilizers, herbicides and pesticides. These chemicals will ultimately
enter the water and become distributed throughout most of the aquatic
environment. The introduction of excess nutrients into the water column
becomes a problem when concentrations become high. In those cases, the
nutrients stimulate large, noxious growths of algae called algal blooms.
These blooms bring havoc upon the entire biological food chain with such
unpleasant effects as rotting algal mats, greatly reduced dissolved
oxygen levels, and fish die-offs (Likens, 1972). The introduction of
excess nutrients into water bodies accelerates eutrophication (natural
aging process), a severe problem that plagues many more developed
countries and heavily populated regions.
The use of even minute amounts of herbicides and pesticides
introduces toxic compounds and their residues into the environment.
These poisons become concentrated in the biota through the process of
bioaccumulation. Ultimately the fish and invertebrates of the river and
reservoirs become unsafe for consumption. These toxic compounds are

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especially dangerous in that many of them have long half-lives and
persist in the environment for many years after introduction.
A.3.2. Change in Traditional Fisheries Toward Lacustrine Species
The growth of lacustrine (lake) fish species in the reservoirs will
be followed by the development of artisanal fisher ies to exploit those
species. The current fisheries on the Gambia River are based on flowing
water systems and include techniques as floating gill nets, trap nets,
wiers at the discharge of floodplains, etc. The development of
fisheries in the reservoirs will use some different techniques to exploit
both the pelagic and benthic feeders. Small fishing communities and
temporary fishing camps will develop on the edges of the reservoirs. The
main consequence of this impact will be the highly beneficial use of the
increased productivity of the reservoirs. With careful management of
these new fisheries, the resource should provide a permanent source of
protein and employment for a significant fraction of the occupants of the
river basin.
A.3.3. Change in Diet Based on Food Availability, Including a
Shift Toward Fish
Parallel to the development of the reservoir based artisanal
fisheries will be increased supplies of fresh fish. Inland communities
such a Mako, Kedougou and Balaki will be located near these greatly
expanded resources of fresh fish. Given the opportunity to consume fresh
fish, rather than sun-dried or smoked fish, the residents of these
villages will probably develop a market for the daily catches from the
reservoirs. The demand for fresh fish will expand as dietary preferences
change as will the pressure on the artisanal reservoir fisheries.
Eventually the concepts of maximum sustainable fish yield must be invoked
in order to preserve the fishery for future harvest. Thus a feedback
between dietary preference and fish supplies must develop as the local
fisheries are exploited.
A.3.4. Increase in Irrigated Cropland
The ultimate expansion of irrigated cropland in the Gambia River
Basin to more than 85,000 ha will entail construction of an extensive
irrigation network. Pumping stations, irrigation canals, dikes, and

A.3.8. Deforestation
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After the reservoirs are filled, local populations will clear fores ts
adjacent to the river for fuel, building materials, and cropland.
Removal of forests will entail exposure of soil to erosion, thus
increasing sediment load in the river and reservoirs as well as reducing
the amount of organic material entering the aquatic environment. This is
an especially important impact in the mangrove area where a large
exchange of organic matter now occurs between the mangrove forests and
the water. Deforestation will also affect the exchange of nutrients
be tween land and water. Forests consume water and leach certain amounts
of nutrients as part of their normal metabolic processes.
A.3.9. Changes in Occupations
Most of the impacts associated with artisanal fisheries and
irrigation farming activities have been discussed above. The only
additional aspect is the shift of occupations toward these activities.
Both on seasonal and year-round bases, manpower needs in these fields
cannot be met by the current work force (Rhine-Ruhr, 1982). People will
have to be recruited into these occupations, especially farming in order
for the full 85,000 ha of cropland to be brought tmder irrigation. Over
the years, those impacts associated with artisanal fisheries and
irrigation farming will gradually increase. This includes the secondary
impacts from in-migration in the Gambian River basin and settlement
adjacent to the river and reservoirs.
A.3.l0. Altered Routes of Commerce and Transportation
New routes of commerce and transportation will develop as the old
routes are submerged by impoundments as well as when new villages are
established. Roads which are flooded by reservoirs will either be
rerouted or replaced by ferry service.
The Balingho bridge-barrage in par ticular will cause expanded
transportation through the trans-Gambia corridor. Those transportation
networks that run next to or across the river and reservoirs will add to
the pollution of those waters. Typical motor transport debris such as
old tires, batteries, and rusted frames, will end up in the water.
Additional pollution will arise from the inevitable oil and petrol that

244
enters the water; these compounds are highly toxic and usually cause mass
mortality several kilometers downstream from the point of pollution.
Expanded navigation, especially by large vessels, carries similar threats
to the environment.
A.3.ll. Changes in the Distribution of Wildlife
As humans move toward the Gambia River and its newly formed
reservoirs, wildlife will respond by moving away from the humans. Some
of this wildlife is aquatic including the hippopotamus, crocodile, and
manatee. These large animals have some effect on the river and its banks
by their natural behavior. Intense grazing along the river banks will
certainly alter· the nature of exchange of materials between terrestrial
and aquatic environments. Excretion by large numbers of animals can
significantly raise the dissolved nutrient pools.