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perimeters) is also a major adverse impact to the benthic community through dislodgement (Anderson
and Lehmkuhl 1968), stranding (Trotzky and Gregory 1974), or overall changes to daily cycles (Perry
and Perry 1986). However, proper management of flow modification at hydropower facilities may
protect and enhance the community downstream (Morgan et al. 1991).
Studies conducted by Blinn et al. (1989) with diatoms from the tailwater of Glen Canyon Dam,
a hydropower project on the Colorado River in Arizona, demonstrated temperature effects on
community structure with a threshold between 12 and 18 OC. They further suggest that changes in
diatom community structure, associated with changes in temperatures as a result of a change in
operations, could impact higher trophic levels such as macroinvertebrate grazers. Angradi and Kubly
(1993) suggest that fluctuation in discharge and resultant exposure to the atmosphere downstream from
Glen Canyon Dam, lowers the resistance of the epiphytic community, decreases the rate of
recolonization, and decreases gross primary production. A longer term study of the Upper Rhne
River, France (Doldec, et al. 1996) describes a pre-project benthic community dominated by
rheophilic species (Trichoptera, hydropsychids such as caddisflies) which changed to a community with
mostly lentic and thermophilic species (e.g., flatworms, gastropods, leeches, crustaceans) after
impoundment. Changes were observed at all sites (both upstream and downstream from a diversion
dam, by-passed section, and power station) and were attributed to a progressive warming of the water
and newly created habitat.
Petts (1984) provides a good discussion of the ecological impacts of reservoirs in relation to
effects on life cycles (hatching, growth, and emergence) of invertebrates and how changes to the
invertebrate community can effect the fisheries. Cues to life stages, such as temperature requirements,
water level, and substrate types) are altered with the construction of the impoundment and community
structure of the tailwater can change dramatically. Ecologically, changing the macroinvertebrate
community can impact the fisheries and result in a very different biological community compared to the
"pre-reservoir" community.
Reservoir tailwaters, while exhibiting water quality processes observed in unregulated rivers,
differ due to water quality influences from the upstream impoundment and varied flow due to operation
of the dam. Both positive and negative impacts can be associated with the impoundment of a river and
effect in the downstream area or tailwater. Habitats can be either degraded or improved, water quality
conditions can decline or be enhanced spatially and temporally (compared to inflow conditions).
Physical, chemical, and biological processes are often inseparable resulting in the need for an ecosystem
approach to the management of tailwaters that may often be limited by project operations.


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