Okavango Delta Management Plan - Ramsar Convention on Wetlands

Subsequently, flow from the Thamalakane passes through a break in the fault downstream
of Maun into the Boteti River. There is a potential drainage route downstream to the
Makgadikgadi Pans, and into the Nhabe River and downstream to Lake Ngami, though this
has not been observed since 1989. Since 2000, the Boteti has at best reached Chanoga,
which is approximately 60 km downstream. The Boteti River downstream and its eventual
outfall to the Makgadikgadi Pans are excluded from the area of the river basin, and from
the <strong>Ramsar</strong> wetland area.
2.2.6.4 Flooding pattern
Sediment deposition, and swamp vegetation with associated peat aggradation raise the
channels above the flood plains. The transfer of flow between the river and flood plains is
highly dynamic depending on the period of the flood from upstream, antecedent ground
water conditions and the rainfall over the <strong>Delta</strong>.
This causes extreme spatial and temporal variations in the flooding pattern, which change
over at least four time scales. The longest is over geological periods of 10 000 to 1 000 000
years and at times the <strong>Delta</strong> has probably been a completely dry desert with sand dunes,
while at other times it has been completely flooded. Since around 1850 the flooding pattern
has moved from a very westerly to a central and now a very easterly distribution (McCarthy
and Ellery, 1998), with some recent evidence of a return to a westerly trend. There are dry
and wet periods as well with about 8 and 18 years interval (Tyson et al., 2002), which
causes the seasonally flooded areas to withdraw and expand. Finally the flooding pattern
can change from year to year caused by local factors such as vegetation blockages in the
streams causing damming and overflow of riverbanks. This large variation in flooding
pattern is one of the important and unique features of the <strong>Okavango</strong> <strong>Delta</strong>.
2.2.6.5 Infiltration
The calculation of the water balance within the <strong>Delta</strong> reveals that the infiltration to
groundwater is very large. In one floodplain system it amounted to 90% of the total inflow.
Groundwater levels that were 3–5 meters below ground rose with arrival of the water within
a few days up to the surface. As far away as 600 meters from the floodplain groundwater
levels rose 1.5 month after arrival of the flood. This was caused by lateral groundwater flow,
which became the dominant infiltration process after the first days and transferred at least
80% of the total infiltration to the surroundings. The impact area was calculated to be 3–6
times larger than the floodplain itself. This surrounding area of riverine woodland is a
dominant element of the vegetation throughout the <strong>Delta</strong>. It is probably strongly dependent
on this mechanism of flooding – infiltration – lateral flow. The lateral flow itself is in all
likelihood driven by transpiration from the riverine woodland. Early hydrological models
(Gieske, 1997, Dincer et al., 1987) neglected the groundwater-surface water interaction.
The new generation of models (Bauer et al., 2006, ODMP Mike SHE model) take this
process into account. It is a dominant hydrological process, which has large implications
for the understanding of the <strong>Delta</strong> ecology. For instance the proposed water development
projects like clearing of channels in the <strong>Delta</strong> of vegetation, upstream water diversions and
building of dams, are all actions that will reduce the duration of the seasonal flooding and
its spatial extent. The impacts will not be restricted to the floodplains but affect large areas
of woodland as well.
2.2.6.6 Island growth and salinity
In the <strong>Okavango</strong> <strong>Delta</strong> about 94% of inflowing solutes are retained within the <strong>Delta</strong> (Table
2-1). This could lead to an entirely saline environment, but that is not the case: the surface
waters have very low salinity with a freshwater biota. It has been deduced that the
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