Dams in River Systems - Warner College of Natural Resources ...

Dams in River Systems: Effects on
Stream Morphology, Riparian
Vegetation, Fish Migration, and
Entrainment
Amy Nowakowski, Paul Dante, Doug Falconi, Samantha Stiffler
Department of Fishery and Wildlife Biology
and
Department of Geosciences
Colorado State University
Fort Collins, Colorado 80523

Abstract.−Dams have harmful effects on
river systems, because they disrupt
stream morphology, riparian vegetation,
fish migration, and cause fish
entrainment. Dams generate water
temperature changes, reduce sediment
transport, and alter flow regimes within
a river system. Changes in flow regimes
have adverse affects on native riparian
vegetation recruitment. Dams deny fish
access to critical upstream habitats, and
have the potential to cause fish
entrainment. Multiple consequences are
associated with entrainment, including
isolation of populations, preventing
genetic exchange, blocking access to
essential habitat, injury, loss of
recruitment, potential increased
hybridization, and both direct and
indirect mortality.
Dams are intimately tied to river
systems in North America, with
approximately 79,000 dams currently
existing in the United States (ASCE
2005). Water storage, flood prevention,
hydropower generation, irrigation,
industrial use, and recreation are touted
as benefits to dam construction.
Unfortunately, most dams were built
before research of the potential negative
effects was initiated. The goal of this
paper is to assess disruptions in aquatic
and riparian habitats caused by dams.
Disruptions include changes in abiotic
factors, such as water temperature,
sediment transport, and flow regime.
Biotic factors, including riparian
vegetation recruitment, fish migration,
and fish entrainment, are also negatively
influenced by dam constraints.
Colorado pikeminnow. Photo by Falconi. 2005.
Dams in River Systems: Abiotic
Factors and Stream Morphology
by Amy Nowakowski
The occurrence of dams in river
systems has been widely studied to
identify the relationships between dams,
stream morphology, and the function of
aquatic and riparian ecosystems (Baxter
1977; Poff et al. 1997; Graff 1999). The
goal of this paper is to explore
correlations between dams, water
quality, sediment transport, flow regime,
and the resulting disruptions of the river
ecosystem. It is imperative to understand
how dams affect the natural processes of
river systems so watershed managers can
strive to maintain ecosystem integrity
while fulfilling management activities
(Hart and Poff 2002).
Dams change aquatic habitat by
altering water temperatures. Deep water
stored behind a dam becomes stratified
into layers of differing temperatures
(Poff and Hart 2002). When water is
released from the dam, the cold
hypolimnion layer flows downstream
(American Rivers 2002). Aquatic
species can reproduce, grow, and survive
only within a particular range of
temperatures (Kendeigh 1961). Changes
in water temperature can lead to a shift
in species composition or density, if
invading species overtake native species
due to their higher range of
environmental tolerances (Poff and Hart
2002).
Sediment transport through a river
system is also greatly affected by dams.
Most sediment entering a reservoir is
stored behind dams, resulting in
sediment-starved conditions downstream
(Graf 2002). This creates the potential
for channel incision, bank erosion,
change in channel planform, and
development of a coarse armor layer on
the streambed as fine sediments wash

downstream (Knighton 1998; Graf 2002;
Pizzuto 2002). A river system with a
confined sediment flow often has
reduced habitat diversity due to the
decreased nutrient flux and loss of
habitat (American Rivers 2002; Graf
2002).
The flow regime is a fundamental
variable in the river ecosystem which is
significantly disrupted by dams, thereby
altering ecological integrity (Figure 1).
Flow regimes provide the stream
environment with naturally varying
fluxes of both water and sediment,
which define the stream morphology and
the function of the stream ecosystem
(Poff et al. 1997). In a natural river
system, the diversity of flow conditions
provide habitat for aquatic species
(Newberry 1995), and riparian species
dependent upon overbank floods (Poff
and Hart 2002; American Rivers 2002).
Dams constrain a river’s flow regime by
storing water in reservoirs, limiting
overbank floods, and regulating and
minimizing the magnitude, frequency,
duration, and timing of natural flows
(Baxter 1977; Poff et al. 1997; Poff and
Hart 2002). Consequently, the food web
and productivity of species adapted to
dynamic flow conditions is considerably
modified by dams (Poff and Hart 2002).
Figure 1.─Flow regime is of central importance in sustaining the ecological integrity of flowing water
systems. The five components of the flow regime–magnitude, frequency, duration, timing, and rate of
change–influence integrity both directly and indirectly, through their effects on other primary regulators of
integrity. Modification of flow thus has cascading effects on the ecological integrity of rivers (from Poff et
al. 1997; after Karr 1991).
Although dam removal is a
management option that can potentially
reverse the morphological and ecological
disruptions discussed above, little is
known about the response of rivers to
dam removal (Poff and Hart 2002;
Stanley 2002; Doyle 2005; Santucci
2005). Hart et al. (2002) provides a
simple representation of the potential
abiotic and biotic responses to dam
removal (Figure 2). The potential
responses to dam removal are time
dependent, and may take days to several
decades to respond. Therefore, it is a
great challenge to predict the ecosystem
response occurring after dam removal,
because river ecosystems involve a
complexity of interactions over a
multitude of spatial and temporal scales
(Hart et al. 2002).

Figure 2.─A simple spatial and temporal context for examining potential ecological responses to dam
removal. Prior to removal, upstream and downstream free-flowing areas are separated by an impoundment.
Dam removal initiates a series of abiotic and biotic changes that vary among areas and occur at different
rates. For example, the rate of sediment transport and channel adjustment is a function of the distribution of
sediment particle sizes and flow magnitudes, and the response rate of aquatic and riparian biota to these
changes depends on their dispersal and growth rates. Key changes occurring within each spatial and
temporal area have been highlighted. For some processes, arrows indicate net change as either increase
( ↑ ) or decreases ( ↓ ), though in other cases the change may be in either direction ( ↑↓) (from Hart et al.
2002).
Riparian Response to Upstream Dam
Creation
by Paul Dante
Riparian zones are a vital link to our
river systems, being described by
Naiman and Décamps (1997) as “some
of the most diverse, dynamic, and
complex biophysical habitats on the
terrestrial portion of the planet.” Most
of the research into the effects of dams
on riparian zones has occurred only
within the last two decades; however
they are still often overlooked by the
government, as well as the public. As
approximately 79,000 dams are in the
U.S. National Inventory of Dams (ASCE
2005), any degradation or destruction of
riparian zones will continue to be a
problem in the United States for a long
time to come. In this paper I will discuss
the importance of riparian zones and
describe some of the negative effects of
river regulation on these zones.
Riparian corridors (or riparian zones),
are the interface between terrestrial and
aquatic systems described by Lowrence
et al. (1985, in Wenger 1999) as “the
complex assemblage of organisms and
their environment existing adjacent to
and near flowing water.” Naiman et al.
(1993) refined the definition to be the
area between the low water marks and
any areas above the high water mark
affected by the heightened water table or
regular flooding, as well as “the ability
of the soils to hold water”. Along with
the trapping of sediment and pollutants,

iparian zones moderate water
temperature, mitigate bank erosion and
attenuate peak flows (Leavitt 1998;
Wenger 1999). These attributes stabilize
the ecology conditions of the stream, as
well as help maintain water quality for
human and industrial use.
Dam systems degradate riparian
corridors for multiple reasons. Dams
fragment rivers by altering flow patterns,
limiting downstream movement of
sediment, and upstream and downstream
movement biological material. Guppy
(1906 in Merritt and Wohl 2006) and
McAtee (1925 in Merritt and Wohl
2006) noted the importance of plant
hydrochory (dispersal by water) for
riparian ecosystem health. Andersson et
al. (2000) and Merritt and Wohl (2002)
found that dam fragmentation of rivers
blocks transport, decreasing the
effectiveness of hydrochory of the seeds
and other vegetative propagules.
Bradley and Smith (1986) found that
cottonwood (Populus sp.) seed dispersal,
which is timed around natural peak
floods, had decreased effectiveness
under regulated flow that altered peak
timing. Merritt and Cooper (2000)
found that the spring flood peak of the
Green River of northwest Colorado was
completely removed by river regulation,
and as a result, riparian Populus
recruitment has been eliminated.
Riparian vegetation (Al-Kaisi 2000).
Along with removal of peak flow,
related changes in the water table may
affect riparian health. During the 37
years after the introduction of the
Flaming Gorge dam on the Green River,
there was a transition in riparian
vegetation that seems to indicate a shift
to shrublands from Populus, and an
increase of fluvial marshes (Merritt and
Cooper 2000) (Figure 1). This may
indicate a change in available water.
Riparian cottonwood and willow (Salix
sp.) dependence on shallow alluvial
groundwater make them extremely
susceptible to water table changes
(Amlin and Rood 2002; Rood et al.
2003). Reduction in water table height
can be lethal to seedlings, and thus limit
or prevent recruitment of new trees
(Rood and Mahoney 1990; Segelquist et
al. 1993).
Until regulated rivers are allowed to
return to more natural flow patterns,
riparian zones in the United States will
continue to degrade, and with them the
ecological health of these river. That is
why it is important for research to
continue to delve into the requirements
of riparian zones and for local, state, and
federal governmental agencies to take
this information into account when
establishing flow regulation.

Figure 1.─Model of channel response to flow regulation on the Green River in Browns Park from 1938
through the present, inferred from planform geometry. Frame (a) shows the pre-dam quasi-equilibrium
meandering channel prior to regulation. Stage I: frame (b) shows the short-term (1966–1977) narrowing
response of the channel to reduced flow; note the establishment of vegetation in the formerly active
channel. Stage II and III: frames (c) and (d) illustrate the longer-term (1977–1994) channel response, which
includes the development of bars, the stabilization of bars to form islands, and the continued widening of
the channel. Frame (e) is the hypothetical long-term form of the channel after several more decades of
widening, the coalescence of islands through channel infilling, and formation of a meandering channel
within the newly formed floodplain nested within the high banks of the former floodplain. (from Merritt
and Cooper 2000).

Instream Barriers: Preventing Fishes
From Accessing Upstream Habitats
by Douglas A. Falconi
The most important effect of all
instream barriers is to prevent upstream
movement of fishes (Baxter 1977).
Instream barriers have had the most
noticeable impact on diadromous fishes
(Table 1; Leggett 1977; Poff et al. 1997).
However, many riverine fishes that were
once considered to be residents have
now been observed to require many
kilometers of unobstructed stream to
fulfill their life cycle (Winston et al.
1991; Gowan et al. 1994; Gowan and
Fausch 1996; Fausch et al. 2002).
Although fish passageways have been
implemented at some dams to restore
partial connectivity, they may not be
adequate for all species to access
upstream habitat (Clay 1995; Beasley
and Hightower 2000; Moser et al. 2000).
Only removal of instream barriers will
restore connectivity.
Table 1.─Some common diadromous fishes of North America that are impeded by instream barriers and
native occurrence (modified from Lucas and Baras 2001).
Species
Common Name Scientific Name
Atlantic or
Pacific
Atlantic sea lamprey Petromyzon marinus Atlantic
Arcitic lamprey Petromyzon japonica Pacific
American Atlantic sturgeon Acipenser oxyrinchus Atlantic
shortnose sturgeon Acipenser brevirostrum Atlantic
alewife Alosa psuedoharengus Atlantic
blueback herring Alosa aestivalis Atlantic
American shad Alosa sapidissima Atlantic
delta smelt Hypomesus transpacificus Pacific
chinook salmon Oncorhynchus tshawytscha Pacific
Atlantic salmon Salmo salar Atlantic
coho salmon Oncorhynchus kisutch Pacific
chum salmon Oncorhynchus keta Pacific
pink salmon Oncorhynchus gorbuscha Pacific
striped bass Morone saxatilis Atlantic
white perch Morone americana Atlantic
On the East Coast, the presence of
instream barriers has prevented
American shad Alosa sapidissima from
accessing natal spawning sites (Rohde et
al. 1994; Beasley and Hightower 2000).
Beasley and Hightower (2000) observed
striped bass Morone saxatilis using the
denil passageway at the Quaker Neck
Dam, but American shad remained at the
base of the dam. When the Quaker Neck
Dam was removed in 1998, both striped
bass and American shad were observed
spawning in upstream habitats (Beasley
and Hightower 2000). For passageway
design, fish lifts have been most
successful for American shad (Barry and
Kynard 1986; Spankle 2005). The Essex
Dam fish lifts on the Merrimack River in
Massachusetts passed 5,283 American
shad, or 10% of the run, in 2002
(Spankle 2005). A common means of
passage for American shad on East
Coast rivers has been the use of

navigation locks (Moser et al. 2000;
Bailey et al. 2004). For example, on the
Cape Fear River in North Carolina, 3 of
66 (4.5%) radio-tagged American shad
used the denil passageway, whereas 28
out of 66 (42%) passed upstream using
the navigation lock (Moser et al. 2000).
Bailey et al. (2004) observed that 50% of
the American shad that returned to the
New Savannah Bluff Lock and Dam on
the Savannah River in Georgia also
passed upstream using the navigation
lock.
The effects of diversion dams and
instream barriers are not limited to
diadromous fishes. Many inland fishes
have also been severely impacted. In the
Colorado River Basin, instream barriers
are a main cause for the decline of
Colorado pikeminnow, Ptychocheilus
lucius (Minckley 1973; Tyus 1986).
White River resident Colorado
pikeminnow migrate over 600 km in the
Green River Basin to one of two
spawning areas year after year (Tyus and
McAda 1984; Tyus 1986, 1990; Irving
and Modde 2000). Instream barriers, like
the Taylor Draw Dam on the White
River, prevent individuals from
accessing potential spawning areas and
wintering habitat (Tyus 1986; Burdick
1995; Irving and Modde 2000).
Constructing passageways may be a
viable recovery option for this species.
Burdick (2001) observed several subadult
and adult Colorado pikeminnow
use a newly constructed vertical slot
passageway at the Redlands Diversion
Dam on the Gunnison River near Grand
Junction, Colorado.
Clearly, instream barriers have
negative effects on the native
ichthyofauna of streams by preventing
upstream access to spawning sites and
wintering habitats (Tyus 1986, 1990;
Beasley and Hightower 2000; Irving and
Modde 2000). Installing passageways
may help some species to use historic
habitat, but the efficiency of a
passageway is very difficult to calculate,
so the degree of success often remains
uncertain (Clay 1995). Only complete
removal will ensure that all species are
able to freely migrate upstream.
Salmon attempt migration over instream barrier
(LCS 2001).
Environmental Effects and Potential
Consequences of Fish Entrainment
by Samantha Stiffler
Entrainment of fish at power plant
and other water intakes has the potential
to adversely affect fish populations.
Jude et al. (1986, in Savitz et al. 1998)
described entrainment as the process of
an organism passing through a plant and
being discharged back into the
environment. Multiple consequences are
associated with entrainment: isolation of
populations, preventing genetic

exchange, blocking access to essential
habitat, injury, loss of recruitment,
potential increased hybridization, and
both direct and indirect mortality
(USFWS 2001).
Degree of impacts on populations
depends on fish species and intake
characteristics. Species specific effects
depend on: motility; physiological and
behavioral responses; vertical and
horizontal distribution in vicinity to the
intake, and growth rate, which
determines the period of vulnerability to
entrainment. Intake characteristics
include: location and construction
details, which control flow conditions in
the immediate vicinity of the intake; size
of structure; discharge volume; type of
release; and depth of turbines (Boreman
1977; Travnichek et al. 1993).
Multiple studies have shown that the
majority of organisms entrained are
young-of-year or juveniles (Gray et al.
1986; Jaeger et al. 2005; New York
Power Authority 2005). Larval drift
obtains a maximum peak during the
early days or weeks of life, and primarily
occurs at night, with a maximum shortly
after dusk (Figure 1), and in some cases,
close to dawn (Carter and Reader 2000).
Determining the number of fish
entrained is difficult; to determine
percent loss, the average concentration
of organisms in the water, mortality of
entrained organisms, and period of
vulnerability to entrainment must be
taken into account (Boreman 1977). The
effect on larvae entrained is also
particularly difficult to determine
because larvae abundance, mortality, and
growth are difficult to estimate (Jensen
1990).
Figure 1.─Diel Variation in (a) entrainment and (b) drift of 0+ fish, on two dates, in the River Trent,
England. (BST = British Summer Time; from Carter and Reader 2000).

Legislation involved with
entrainment involves state and federal
requirements. Under section 316(b) of
the Clean Water Act, cooling water
intake structures are required to reflect
the best technology available for
minimizing adverse environmental
impacts (Winkle and Kadvany 2003).
This section evaluates water intakes and
documents permits after the evaluation is
approved (Edinger and Kolluru 2000).
In addition to state requirements, the
National Oceanic and Atmospheric
Administration Fisheries and the US
Fish and Wildlife Service often require
screening to protect fish species listed as
threatened or endangered. Justification
for these screenings result from the
removal of threatened or endangered
species by a diversion constituting
“take” under section 4(d) of the
Endangered Species Act (Moyle and
Israel 2005).
Various technologies have been
developed to attempt to reduce
entrainment. These barriers have been
divided into physical and behavioral
barriers. The Federal Energy Regulatory
Commission (1995, in New York Power
Authority 2005) report five types of
physical barriers that have been
deployed at hydroelectric stations: low
velocity fish screens, high velocity fish
screens, close-spaced and angled bar
racks, louvers, and barrier nets.
Behavioral barriers include: lights, such
as strobe and mercury, which have
variable response rates (McKinstry et al.
2005; Johnson et al. 2005), sound, air
bubble curtains, and electrical barriers
(New York Power Authority 2005).
Entrainment can be compared to
natural mortality; if the rate of
entrainment is small and less than
natural mortality, the impact would not
be appreciably noticeable. However, if
the rate exceeds natural mortality, the
population may not be sustainable
(Edinger and Kolluru 2000).
Percentage of entrainment from a single
intake may be low, but cumulative
impacts could be high (Edinger and
Kolluru 2000). Population-level
responses may be characterized by a
significant time lag, where effects would
not be seen for many years. In turn,
ecosystem-level effects, such as size and
structure of populations, are likely to
become evident with continued
population changes caused by
entrainment (Benstead et al. 1999).
Conclusion
One management option that has
potential to mediate the adverse effects
of dams in river systems is dam removal.
Dam removal can restore aquatic and
riparian habitats by reestablishing the
natural flow regime, allowing fish
migration, and eliminating entrainment
occurrences. However, abiotic and biotic
responses to dam removal are difficult to
predict because complex interactions
occur over different spatial and temporal
scales.
Removal of low-head dam on the Ashuelot
River, New Hampshire (USFWS 2005).

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