Dams in River Systems - Warner College of Natural Resources ...
Dams in River Systems - Warner College of Natural Resources ...
Dams in River Systems - Warner College of Natural Resources ...
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<strong>Dams</strong> <strong>in</strong> <strong>River</strong> <strong>Systems</strong>: Effects on<br />
Stream Morphology, Riparian<br />
Vegetation, Fish Migration, and<br />
Entra<strong>in</strong>ment<br />
Amy Nowakowski, Paul Dante, Doug Falconi, Samantha Stiffler<br />
Department <strong>of</strong> Fishery and Wildlife Biology<br />
and<br />
Department <strong>of</strong> Geosciences<br />
Colorado State University<br />
Fort Coll<strong>in</strong>s, Colorado 80523
Abstract.−<strong>Dams</strong> have harmful effects on<br />
river systems, because they disrupt<br />
stream morphology, riparian vegetation,<br />
fish migration, and cause fish<br />
entra<strong>in</strong>ment. <strong>Dams</strong> generate water<br />
temperature changes, reduce sediment<br />
transport, and alter flow regimes with<strong>in</strong><br />
a river system. Changes <strong>in</strong> flow regimes<br />
have adverse affects on native riparian<br />
vegetation recruitment. <strong>Dams</strong> deny fish<br />
access to critical upstream habitats, and<br />
have the potential to cause fish<br />
entra<strong>in</strong>ment. Multiple consequences are<br />
associated with entra<strong>in</strong>ment, <strong>in</strong>clud<strong>in</strong>g<br />
isolation <strong>of</strong> populations, prevent<strong>in</strong>g<br />
genetic exchange, block<strong>in</strong>g access to<br />
essential habitat, <strong>in</strong>jury, loss <strong>of</strong><br />
recruitment, potential <strong>in</strong>creased<br />
hybridization, and both direct and<br />
<strong>in</strong>direct mortality.<br />
<strong>Dams</strong> are <strong>in</strong>timately tied to river<br />
systems <strong>in</strong> North America, with<br />
approximately 79,000 dams currently<br />
exist<strong>in</strong>g <strong>in</strong> the United States (ASCE<br />
2005). Water storage, flood prevention,<br />
hydropower generation, irrigation,<br />
<strong>in</strong>dustrial use, and recreation are touted<br />
as benefits to dam construction.<br />
Unfortunately, most dams were built<br />
before research <strong>of</strong> the potential negative<br />
effects was <strong>in</strong>itiated. The goal <strong>of</strong> this<br />
paper is to assess disruptions <strong>in</strong> aquatic<br />
and riparian habitats caused by dams.<br />
Disruptions <strong>in</strong>clude changes <strong>in</strong> abiotic<br />
factors, such as water temperature,<br />
sediment transport, and flow regime.<br />
Biotic factors, <strong>in</strong>clud<strong>in</strong>g riparian<br />
vegetation recruitment, fish migration,<br />
and fish entra<strong>in</strong>ment, are also negatively<br />
<strong>in</strong>fluenced by dam constra<strong>in</strong>ts.<br />
Colorado pikem<strong>in</strong>now. Photo by Falconi. 2005.<br />
<strong>Dams</strong> <strong>in</strong> <strong>River</strong> <strong>Systems</strong>: Abiotic<br />
Factors and Stream Morphology<br />
by Amy Nowakowski<br />
The occurrence <strong>of</strong> dams <strong>in</strong> river<br />
systems has been widely studied to<br />
identify the relationships between dams,<br />
stream morphology, and the function <strong>of</strong><br />
aquatic and riparian ecosystems (Baxter<br />
1977; P<strong>of</strong>f et al. 1997; Graff 1999). The<br />
goal <strong>of</strong> this paper is to explore<br />
correlations between dams, water<br />
quality, sediment transport, flow regime,<br />
and the result<strong>in</strong>g disruptions <strong>of</strong> the river<br />
ecosystem. It is imperative to understand<br />
how dams affect the natural processes <strong>of</strong><br />
river systems so watershed managers can<br />
strive to ma<strong>in</strong>ta<strong>in</strong> ecosystem <strong>in</strong>tegrity<br />
while fulfill<strong>in</strong>g management activities<br />
(Hart and P<strong>of</strong>f 2002).<br />
<strong>Dams</strong> change aquatic habitat by<br />
alter<strong>in</strong>g water temperatures. Deep water<br />
stored beh<strong>in</strong>d a dam becomes stratified<br />
<strong>in</strong>to layers <strong>of</strong> differ<strong>in</strong>g temperatures<br />
(P<strong>of</strong>f and Hart 2002). When water is<br />
released from the dam, the cold<br />
hypolimnion layer flows downstream<br />
(American <strong>River</strong>s 2002). Aquatic<br />
species can reproduce, grow, and survive<br />
only with<strong>in</strong> a particular range <strong>of</strong><br />
temperatures (Kendeigh 1961). Changes<br />
<strong>in</strong> water temperature can lead to a shift<br />
<strong>in</strong> species composition or density, if<br />
<strong>in</strong>vad<strong>in</strong>g species overtake native species<br />
due to their higher range <strong>of</strong><br />
environmental tolerances (P<strong>of</strong>f and Hart<br />
2002).<br />
Sediment transport through a river<br />
system is also greatly affected by dams.<br />
Most sediment enter<strong>in</strong>g a reservoir is<br />
stored beh<strong>in</strong>d dams, result<strong>in</strong>g <strong>in</strong><br />
sediment-starved conditions downstream<br />
(Graf 2002). This creates the potential<br />
for channel <strong>in</strong>cision, bank erosion,<br />
change <strong>in</strong> channel planform, and<br />
development <strong>of</strong> a coarse armor layer on<br />
the streambed as f<strong>in</strong>e sediments wash
downstream (Knighton 1998; Graf 2002;<br />
Pizzuto 2002). A river system with a<br />
conf<strong>in</strong>ed sediment flow <strong>of</strong>ten has<br />
reduced habitat diversity due to the<br />
decreased nutrient flux and loss <strong>of</strong><br />
habitat (American <strong>River</strong>s 2002; Graf<br />
2002).<br />
The flow regime is a fundamental<br />
variable <strong>in</strong> the river ecosystem which is<br />
significantly disrupted by dams, thereby<br />
alter<strong>in</strong>g ecological <strong>in</strong>tegrity (Figure 1).<br />
Flow regimes provide the stream<br />
environment with naturally vary<strong>in</strong>g<br />
fluxes <strong>of</strong> both water and sediment,<br />
which def<strong>in</strong>e the stream morphology and<br />
the function <strong>of</strong> the stream ecosystem<br />
(P<strong>of</strong>f et al. 1997). In a natural river<br />
system, the diversity <strong>of</strong> flow conditions<br />
provide habitat for aquatic species<br />
(Newberry 1995), and riparian species<br />
dependent upon overbank floods (P<strong>of</strong>f<br />
and Hart 2002; American <strong>River</strong>s 2002).<br />
<strong>Dams</strong> constra<strong>in</strong> a river’s flow regime by<br />
stor<strong>in</strong>g water <strong>in</strong> reservoirs, limit<strong>in</strong>g<br />
overbank floods, and regulat<strong>in</strong>g and<br />
m<strong>in</strong>imiz<strong>in</strong>g the magnitude, frequency,<br />
duration, and tim<strong>in</strong>g <strong>of</strong> natural flows<br />
(Baxter 1977; P<strong>of</strong>f et al. 1997; P<strong>of</strong>f and<br />
Hart 2002). Consequently, the food web<br />
and productivity <strong>of</strong> species adapted to<br />
dynamic flow conditions is considerably<br />
modified by dams (P<strong>of</strong>f and Hart 2002).<br />
Figure 1.─Flow regime is <strong>of</strong> central importance <strong>in</strong> susta<strong>in</strong><strong>in</strong>g the ecological <strong>in</strong>tegrity <strong>of</strong> flow<strong>in</strong>g water<br />
systems. The five components <strong>of</strong> the flow regime–magnitude, frequency, duration, tim<strong>in</strong>g, and rate <strong>of</strong><br />
change–<strong>in</strong>fluence <strong>in</strong>tegrity both directly and <strong>in</strong>directly, through their effects on other primary regulators <strong>of</strong><br />
<strong>in</strong>tegrity. Modification <strong>of</strong> flow thus has cascad<strong>in</strong>g effects on the ecological <strong>in</strong>tegrity <strong>of</strong> rivers (from P<strong>of</strong>f et<br />
al. 1997; after Karr 1991).<br />
Although dam removal is a<br />
management option that can potentially<br />
reverse the morphological and ecological<br />
disruptions discussed above, little is<br />
known about the response <strong>of</strong> rivers to<br />
dam removal (P<strong>of</strong>f and Hart 2002;<br />
Stanley 2002; Doyle 2005; Santucci<br />
2005). Hart et al. (2002) provides a<br />
simple representation <strong>of</strong> the potential<br />
abiotic and biotic responses to dam<br />
removal (Figure 2). The potential<br />
responses to dam removal are time<br />
dependent, and may take days to several<br />
decades to respond. Therefore, it is a<br />
great challenge to predict the ecosystem<br />
response occurr<strong>in</strong>g after dam removal,<br />
because river ecosystems <strong>in</strong>volve a<br />
complexity <strong>of</strong> <strong>in</strong>teractions over a<br />
multitude <strong>of</strong> spatial and temporal scales<br />
(Hart et al. 2002).
Figure 2.─A simple spatial and temporal context for exam<strong>in</strong><strong>in</strong>g potential ecological responses to dam<br />
removal. Prior to removal, upstream and downstream free-flow<strong>in</strong>g areas are separated by an impoundment.<br />
Dam removal <strong>in</strong>itiates a series <strong>of</strong> abiotic and biotic changes that vary among areas and occur at different<br />
rates. For example, the rate <strong>of</strong> sediment transport and channel adjustment is a function <strong>of</strong> the distribution <strong>of</strong><br />
sediment particle sizes and flow magnitudes, and the response rate <strong>of</strong> aquatic and riparian biota to these<br />
changes depends on their dispersal and growth rates. Key changes occurr<strong>in</strong>g with<strong>in</strong> each spatial and<br />
temporal area have been highlighted. For some processes, arrows <strong>in</strong>dicate net change as either <strong>in</strong>crease<br />
( ↑ ) or decreases ( ↓ ), though <strong>in</strong> other cases the change may be <strong>in</strong> either direction ( ↑↓) (from Hart et al.<br />
2002).<br />
Riparian Response to Upstream Dam<br />
Creation<br />
by Paul Dante<br />
Riparian zones are a vital l<strong>in</strong>k to our<br />
river systems, be<strong>in</strong>g described by<br />
Naiman and Décamps (1997) as “some<br />
<strong>of</strong> the most diverse, dynamic, and<br />
complex biophysical habitats on the<br />
terrestrial portion <strong>of</strong> the planet.” Most<br />
<strong>of</strong> the research <strong>in</strong>to the effects <strong>of</strong> dams<br />
on riparian zones has occurred only<br />
with<strong>in</strong> the last two decades; however<br />
they are still <strong>of</strong>ten overlooked by the<br />
government, as well as the public. As<br />
approximately 79,000 dams are <strong>in</strong> the<br />
U.S. National Inventory <strong>of</strong> <strong>Dams</strong> (ASCE<br />
2005), any degradation or destruction <strong>of</strong><br />
riparian zones will cont<strong>in</strong>ue to be a<br />
problem <strong>in</strong> the United States for a long<br />
time to come. In this paper I will discuss<br />
the importance <strong>of</strong> riparian zones and<br />
describe some <strong>of</strong> the negative effects <strong>of</strong><br />
river regulation on these zones.<br />
Riparian corridors (or riparian zones),<br />
are the <strong>in</strong>terface between terrestrial and<br />
aquatic systems described by Lowrence<br />
et al. (1985, <strong>in</strong> Wenger 1999) as “the<br />
complex assemblage <strong>of</strong> organisms and<br />
their environment exist<strong>in</strong>g adjacent to<br />
and near flow<strong>in</strong>g water.” Naiman et al.<br />
(1993) ref<strong>in</strong>ed the def<strong>in</strong>ition to be the<br />
area between the low water marks and<br />
any areas above the high water mark<br />
affected by the heightened water table or<br />
regular flood<strong>in</strong>g, as well as “the ability<br />
<strong>of</strong> the soils to hold water”. Along with<br />
the trapp<strong>in</strong>g <strong>of</strong> sediment and pollutants,
iparian zones moderate water<br />
temperature, mitigate bank erosion and<br />
attenuate peak flows (Leavitt 1998;<br />
Wenger 1999). These attributes stabilize<br />
the ecology conditions <strong>of</strong> the stream, as<br />
well as help ma<strong>in</strong>ta<strong>in</strong> water quality for<br />
human and <strong>in</strong>dustrial use.<br />
Dam systems degradate riparian<br />
corridors for multiple reasons. <strong>Dams</strong><br />
fragment rivers by alter<strong>in</strong>g flow patterns,<br />
limit<strong>in</strong>g downstream movement <strong>of</strong><br />
sediment, and upstream and downstream<br />
movement biological material. Guppy<br />
(1906 <strong>in</strong> Merritt and Wohl 2006) and<br />
McAtee (1925 <strong>in</strong> Merritt and Wohl<br />
2006) noted the importance <strong>of</strong> plant<br />
hydrochory (dispersal by water) for<br />
riparian ecosystem health. Andersson et<br />
al. (2000) and Merritt and Wohl (2002)<br />
found that dam fragmentation <strong>of</strong> rivers<br />
blocks transport, decreas<strong>in</strong>g the<br />
effectiveness <strong>of</strong> hydrochory <strong>of</strong> the seeds<br />
and other vegetative propagules.<br />
Bradley and Smith (1986) found that<br />
cottonwood (Populus sp.) seed dispersal,<br />
which is timed around natural peak<br />
floods, had decreased effectiveness<br />
under regulated flow that altered peak<br />
tim<strong>in</strong>g. Merritt and Cooper (2000)<br />
found that the spr<strong>in</strong>g flood peak <strong>of</strong> the<br />
Green <strong>River</strong> <strong>of</strong> northwest Colorado was<br />
completely removed by river regulation,<br />
and as a result, riparian Populus<br />
recruitment has been elim<strong>in</strong>ated.<br />
Riparian vegetation (Al-Kaisi 2000).<br />
Along with removal <strong>of</strong> peak flow,<br />
related changes <strong>in</strong> the water table may<br />
affect riparian health. Dur<strong>in</strong>g the 37<br />
years after the <strong>in</strong>troduction <strong>of</strong> the<br />
Flam<strong>in</strong>g Gorge dam on the Green <strong>River</strong>,<br />
there was a transition <strong>in</strong> riparian<br />
vegetation that seems to <strong>in</strong>dicate a shift<br />
to shrublands from Populus, and an<br />
<strong>in</strong>crease <strong>of</strong> fluvial marshes (Merritt and<br />
Cooper 2000) (Figure 1). This may<br />
<strong>in</strong>dicate a change <strong>in</strong> available water.<br />
Riparian cottonwood and willow (Salix<br />
sp.) dependence on shallow alluvial<br />
groundwater make them extremely<br />
susceptible to water table changes<br />
(Aml<strong>in</strong> and Rood 2002; Rood et al.<br />
2003). Reduction <strong>in</strong> water table height<br />
can be lethal to seedl<strong>in</strong>gs, and thus limit<br />
or prevent recruitment <strong>of</strong> new trees<br />
(Rood and Mahoney 1990; Segelquist et<br />
al. 1993).<br />
Until regulated rivers are allowed to<br />
return to more natural flow patterns,<br />
riparian zones <strong>in</strong> the United States will<br />
cont<strong>in</strong>ue to degrade, and with them the<br />
ecological health <strong>of</strong> these river. That is<br />
why it is important for research to<br />
cont<strong>in</strong>ue to delve <strong>in</strong>to the requirements<br />
<strong>of</strong> riparian zones and for local, state, and<br />
federal governmental agencies to take<br />
this <strong>in</strong>formation <strong>in</strong>to account when<br />
establish<strong>in</strong>g flow regulation.
Figure 1.─Model <strong>of</strong> channel response to flow regulation on the Green <strong>River</strong> <strong>in</strong> Browns Park from 1938<br />
through the present, <strong>in</strong>ferred from planform geometry. Frame (a) shows the pre-dam quasi-equilibrium<br />
meander<strong>in</strong>g channel prior to regulation. Stage I: frame (b) shows the short-term (1966–1977) narrow<strong>in</strong>g<br />
response <strong>of</strong> the channel to reduced flow; note the establishment <strong>of</strong> vegetation <strong>in</strong> the formerly active<br />
channel. Stage II and III: frames (c) and (d) illustrate the longer-term (1977–1994) channel response, which<br />
<strong>in</strong>cludes the development <strong>of</strong> bars, the stabilization <strong>of</strong> bars to form islands, and the cont<strong>in</strong>ued widen<strong>in</strong>g <strong>of</strong><br />
the channel. Frame (e) is the hypothetical long-term form <strong>of</strong> the channel after several more decades <strong>of</strong><br />
widen<strong>in</strong>g, the coalescence <strong>of</strong> islands through channel <strong>in</strong>fill<strong>in</strong>g, and formation <strong>of</strong> a meander<strong>in</strong>g channel<br />
with<strong>in</strong> the newly formed floodpla<strong>in</strong> nested with<strong>in</strong> the high banks <strong>of</strong> the former floodpla<strong>in</strong>. (from Merritt<br />
and Cooper 2000).
Instream Barriers: Prevent<strong>in</strong>g Fishes<br />
From Access<strong>in</strong>g Upstream Habitats<br />
by Douglas A. Falconi<br />
The most important effect <strong>of</strong> all<br />
<strong>in</strong>stream barriers is to prevent upstream<br />
movement <strong>of</strong> fishes (Baxter 1977).<br />
Instream barriers have had the most<br />
noticeable impact on diadromous fishes<br />
(Table 1; Leggett 1977; P<strong>of</strong>f et al. 1997).<br />
However, many river<strong>in</strong>e fishes that were<br />
once considered to be residents have<br />
now been observed to require many<br />
kilometers <strong>of</strong> unobstructed stream to<br />
fulfill their life cycle (W<strong>in</strong>ston et al.<br />
1991; Gowan et al. 1994; Gowan and<br />
Fausch 1996; Fausch et al. 2002).<br />
Although fish passageways have been<br />
implemented at some dams to restore<br />
partial connectivity, they may not be<br />
adequate for all species to access<br />
upstream habitat (Clay 1995; Beasley<br />
and Hightower 2000; Moser et al. 2000).<br />
Only removal <strong>of</strong> <strong>in</strong>stream barriers will<br />
restore connectivity.<br />
Table 1.─Some common diadromous fishes <strong>of</strong> North America that are impeded by <strong>in</strong>stream barriers and<br />
native occurrence (modified from Lucas and Baras 2001).<br />
Species<br />
Common Name Scientific Name<br />
Atlantic or<br />
Pacific<br />
Atlantic sea lamprey Petromyzon mar<strong>in</strong>us Atlantic<br />
Arcitic lamprey Petromyzon japonica Pacific<br />
American Atlantic sturgeon Acipenser oxyr<strong>in</strong>chus Atlantic<br />
shortnose sturgeon Acipenser brevirostrum Atlantic<br />
alewife Alosa psuedoharengus Atlantic<br />
blueback herr<strong>in</strong>g Alosa aestivalis Atlantic<br />
American shad Alosa sapidissima Atlantic<br />
delta smelt Hypomesus transpacificus Pacific<br />
ch<strong>in</strong>ook salmon Oncorhynchus tshawytscha Pacific<br />
Atlantic salmon Salmo salar Atlantic<br />
coho salmon Oncorhynchus kisutch Pacific<br />
chum salmon Oncorhynchus keta Pacific<br />
p<strong>in</strong>k salmon Oncorhynchus gorbuscha Pacific<br />
striped bass Morone saxatilis Atlantic<br />
white perch Morone americana Atlantic<br />
On the East Coast, the presence <strong>of</strong><br />
<strong>in</strong>stream barriers has prevented<br />
American shad Alosa sapidissima from<br />
access<strong>in</strong>g natal spawn<strong>in</strong>g sites (Rohde et<br />
al. 1994; Beasley and Hightower 2000).<br />
Beasley and Hightower (2000) observed<br />
striped bass Morone saxatilis us<strong>in</strong>g the<br />
denil passageway at the Quaker Neck<br />
Dam, but American shad rema<strong>in</strong>ed at the<br />
base <strong>of</strong> the dam. When the Quaker Neck<br />
Dam was removed <strong>in</strong> 1998, both striped<br />
bass and American shad were observed<br />
spawn<strong>in</strong>g <strong>in</strong> upstream habitats (Beasley<br />
and Hightower 2000). For passageway<br />
design, fish lifts have been most<br />
successful for American shad (Barry and<br />
Kynard 1986; Spankle 2005). The Essex<br />
Dam fish lifts on the Merrimack <strong>River</strong> <strong>in</strong><br />
Massachusetts passed 5,283 American<br />
shad, or 10% <strong>of</strong> the run, <strong>in</strong> 2002<br />
(Spankle 2005). A common means <strong>of</strong><br />
passage for American shad on East<br />
Coast rivers has been the use <strong>of</strong>
navigation locks (Moser et al. 2000;<br />
Bailey et al. 2004). For example, on the<br />
Cape Fear <strong>River</strong> <strong>in</strong> North Carol<strong>in</strong>a, 3 <strong>of</strong><br />
66 (4.5%) radio-tagged American shad<br />
used the denil passageway, whereas 28<br />
out <strong>of</strong> 66 (42%) passed upstream us<strong>in</strong>g<br />
the navigation lock (Moser et al. 2000).<br />
Bailey et al. (2004) observed that 50% <strong>of</strong><br />
the American shad that returned to the<br />
New Savannah Bluff Lock and Dam on<br />
the Savannah <strong>River</strong> <strong>in</strong> Georgia also<br />
passed upstream us<strong>in</strong>g the navigation<br />
lock.<br />
The effects <strong>of</strong> diversion dams and<br />
<strong>in</strong>stream barriers are not limited to<br />
diadromous fishes. Many <strong>in</strong>land fishes<br />
have also been severely impacted. In the<br />
Colorado <strong>River</strong> Bas<strong>in</strong>, <strong>in</strong>stream barriers<br />
are a ma<strong>in</strong> cause for the decl<strong>in</strong>e <strong>of</strong><br />
Colorado pikem<strong>in</strong>now, Ptychocheilus<br />
lucius (M<strong>in</strong>ckley 1973; Tyus 1986).<br />
White <strong>River</strong> resident Colorado<br />
pikem<strong>in</strong>now migrate over 600 km <strong>in</strong> the<br />
Green <strong>River</strong> Bas<strong>in</strong> to one <strong>of</strong> two<br />
spawn<strong>in</strong>g areas year after year (Tyus and<br />
McAda 1984; Tyus 1986, 1990; Irv<strong>in</strong>g<br />
and Modde 2000). Instream barriers, like<br />
the Taylor Draw Dam on the White<br />
<strong>River</strong>, prevent <strong>in</strong>dividuals from<br />
access<strong>in</strong>g potential spawn<strong>in</strong>g areas and<br />
w<strong>in</strong>ter<strong>in</strong>g habitat (Tyus 1986; Burdick<br />
1995; Irv<strong>in</strong>g and Modde 2000).<br />
Construct<strong>in</strong>g passageways may be a<br />
viable recovery option for this species.<br />
Burdick (2001) observed several subadult<br />
and adult Colorado pikem<strong>in</strong>now<br />
use a newly constructed vertical slot<br />
passageway at the Redlands Diversion<br />
Dam on the Gunnison <strong>River</strong> near Grand<br />
Junction, Colorado.<br />
Clearly, <strong>in</strong>stream barriers have<br />
negative effects on the native<br />
ichthy<strong>of</strong>auna <strong>of</strong> streams by prevent<strong>in</strong>g<br />
upstream access to spawn<strong>in</strong>g sites and<br />
w<strong>in</strong>ter<strong>in</strong>g habitats (Tyus 1986, 1990;<br />
Beasley and Hightower 2000; Irv<strong>in</strong>g and<br />
Modde 2000). Install<strong>in</strong>g passageways<br />
may help some species to use historic<br />
habitat, but the efficiency <strong>of</strong> a<br />
passageway is very difficult to calculate,<br />
so the degree <strong>of</strong> success <strong>of</strong>ten rema<strong>in</strong>s<br />
uncerta<strong>in</strong> (Clay 1995). Only complete<br />
removal will ensure that all species are<br />
able to freely migrate upstream.<br />
Salmon attempt migration over <strong>in</strong>stream barrier<br />
(LCS 2001).<br />
Environmental Effects and Potential<br />
Consequences <strong>of</strong> Fish Entra<strong>in</strong>ment<br />
by Samantha Stiffler<br />
Entra<strong>in</strong>ment <strong>of</strong> fish at power plant<br />
and other water <strong>in</strong>takes has the potential<br />
to adversely affect fish populations.<br />
Jude et al. (1986, <strong>in</strong> Savitz et al. 1998)<br />
described entra<strong>in</strong>ment as the process <strong>of</strong><br />
an organism pass<strong>in</strong>g through a plant and<br />
be<strong>in</strong>g discharged back <strong>in</strong>to the<br />
environment. Multiple consequences are<br />
associated with entra<strong>in</strong>ment: isolation <strong>of</strong><br />
populations, prevent<strong>in</strong>g genetic
exchange, block<strong>in</strong>g access to essential<br />
habitat, <strong>in</strong>jury, loss <strong>of</strong> recruitment,<br />
potential <strong>in</strong>creased hybridization, and<br />
both direct and <strong>in</strong>direct mortality<br />
(USFWS 2001).<br />
Degree <strong>of</strong> impacts on populations<br />
depends on fish species and <strong>in</strong>take<br />
characteristics. Species specific effects<br />
depend on: motility; physiological and<br />
behavioral responses; vertical and<br />
horizontal distribution <strong>in</strong> vic<strong>in</strong>ity to the<br />
<strong>in</strong>take, and growth rate, which<br />
determ<strong>in</strong>es the period <strong>of</strong> vulnerability to<br />
entra<strong>in</strong>ment. Intake characteristics<br />
<strong>in</strong>clude: location and construction<br />
details, which control flow conditions <strong>in</strong><br />
the immediate vic<strong>in</strong>ity <strong>of</strong> the <strong>in</strong>take; size<br />
<strong>of</strong> structure; discharge volume; type <strong>of</strong><br />
release; and depth <strong>of</strong> turb<strong>in</strong>es (Boreman<br />
1977; Travnichek et al. 1993).<br />
Multiple studies have shown that the<br />
majority <strong>of</strong> organisms entra<strong>in</strong>ed are<br />
young-<strong>of</strong>-year or juveniles (Gray et al.<br />
1986; Jaeger et al. 2005; New York<br />
Power Authority 2005). Larval drift<br />
obta<strong>in</strong>s a maximum peak dur<strong>in</strong>g the<br />
early days or weeks <strong>of</strong> life, and primarily<br />
occurs at night, with a maximum shortly<br />
after dusk (Figure 1), and <strong>in</strong> some cases,<br />
close to dawn (Carter and Reader 2000).<br />
Determ<strong>in</strong><strong>in</strong>g the number <strong>of</strong> fish<br />
entra<strong>in</strong>ed is difficult; to determ<strong>in</strong>e<br />
percent loss, the average concentration<br />
<strong>of</strong> organisms <strong>in</strong> the water, mortality <strong>of</strong><br />
entra<strong>in</strong>ed organisms, and period <strong>of</strong><br />
vulnerability to entra<strong>in</strong>ment must be<br />
taken <strong>in</strong>to account (Boreman 1977). The<br />
effect on larvae entra<strong>in</strong>ed is also<br />
particularly difficult to determ<strong>in</strong>e<br />
because larvae abundance, mortality, and<br />
growth are difficult to estimate (Jensen<br />
1990).<br />
Figure 1.─Diel Variation <strong>in</strong> (a) entra<strong>in</strong>ment and (b) drift <strong>of</strong> 0+ fish, on two dates, <strong>in</strong> the <strong>River</strong> Trent,<br />
England. (BST = British Summer Time; from Carter and Reader 2000).
Legislation <strong>in</strong>volved with<br />
entra<strong>in</strong>ment <strong>in</strong>volves state and federal<br />
requirements. Under section 316(b) <strong>of</strong><br />
the Clean Water Act, cool<strong>in</strong>g water<br />
<strong>in</strong>take structures are required to reflect<br />
the best technology available for<br />
m<strong>in</strong>imiz<strong>in</strong>g adverse environmental<br />
impacts (W<strong>in</strong>kle and Kadvany 2003).<br />
This section evaluates water <strong>in</strong>takes and<br />
documents permits after the evaluation is<br />
approved (Ed<strong>in</strong>ger and Kolluru 2000).<br />
In addition to state requirements, the<br />
National Oceanic and Atmospheric<br />
Adm<strong>in</strong>istration Fisheries and the US<br />
Fish and Wildlife Service <strong>of</strong>ten require<br />
screen<strong>in</strong>g to protect fish species listed as<br />
threatened or endangered. Justification<br />
for these screen<strong>in</strong>gs result from the<br />
removal <strong>of</strong> threatened or endangered<br />
species by a diversion constitut<strong>in</strong>g<br />
“take” under section 4(d) <strong>of</strong> the<br />
Endangered Species Act (Moyle and<br />
Israel 2005).<br />
Various technologies have been<br />
developed to attempt to reduce<br />
entra<strong>in</strong>ment. These barriers have been<br />
divided <strong>in</strong>to physical and behavioral<br />
barriers. The Federal Energy Regulatory<br />
Commission (1995, <strong>in</strong> New York Power<br />
Authority 2005) report five types <strong>of</strong><br />
physical barriers that have been<br />
deployed at hydroelectric stations: low<br />
velocity fish screens, high velocity fish<br />
screens, close-spaced and angled bar<br />
racks, louvers, and barrier nets.<br />
Behavioral barriers <strong>in</strong>clude: lights, such<br />
as strobe and mercury, which have<br />
variable response rates (McK<strong>in</strong>stry et al.<br />
2005; Johnson et al. 2005), sound, air<br />
bubble curta<strong>in</strong>s, and electrical barriers<br />
(New York Power Authority 2005).<br />
Entra<strong>in</strong>ment can be compared to<br />
natural mortality; if the rate <strong>of</strong><br />
entra<strong>in</strong>ment is small and less than<br />
natural mortality, the impact would not<br />
be appreciably noticeable. However, if<br />
the rate exceeds natural mortality, the<br />
population may not be susta<strong>in</strong>able<br />
(Ed<strong>in</strong>ger and Kolluru 2000).<br />
Percentage <strong>of</strong> entra<strong>in</strong>ment from a s<strong>in</strong>gle<br />
<strong>in</strong>take may be low, but cumulative<br />
impacts could be high (Ed<strong>in</strong>ger and<br />
Kolluru 2000). Population-level<br />
responses may be characterized by a<br />
significant time lag, where effects would<br />
not be seen for many years. In turn,<br />
ecosystem-level effects, such as size and<br />
structure <strong>of</strong> populations, are likely to<br />
become evident with cont<strong>in</strong>ued<br />
population changes caused by<br />
entra<strong>in</strong>ment (Benstead et al. 1999).<br />
Conclusion<br />
One management option that has<br />
potential to mediate the adverse effects<br />
<strong>of</strong> dams <strong>in</strong> river systems is dam removal.<br />
Dam removal can restore aquatic and<br />
riparian habitats by reestablish<strong>in</strong>g the<br />
natural flow regime, allow<strong>in</strong>g fish<br />
migration, and elim<strong>in</strong>at<strong>in</strong>g entra<strong>in</strong>ment<br />
occurrences. However, abiotic and biotic<br />
responses to dam removal are difficult to<br />
predict because complex <strong>in</strong>teractions<br />
occur over different spatial and temporal<br />
scales.<br />
Removal <strong>of</strong> low-head dam on the Ashuelot<br />
<strong>River</strong>, New Hampshire (USFWS 2005).
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