Dams and Impounded Rivers - River Dynamics and Integrated River Management - page 405

T able 7.5 Effects of impoundments upon benthic macroinvertebrates (after Petts, 1984)
River, reservoir, location

Macroinvertebrate changes Source

River Elan, Craing Goch
Reservoir, UK

Reduced abundance and diversity Scullion (1982)

S. Saskatchewan River,
Gardiner Dam, Canada

Marked reduction of macroinvertebrates downstream
for over 100 km; 19 species of Ephemeroptera were
probably eliminated

Lehmkuhl (1972)

Green River, Flaming Gorge
Dam, USA

Number of taxonomic groups reduced and density of
benthos increased for over 100 km downstream

Pearson and Franklin
(1968)

Brazos River, Possum
Kingdom Reservoir, USA

Increased zoobenthos diversity for 80 km below the dam McClure and Stewart

(1976)

S. Platte River, Cheesman
Lake, USA

Reduced diversity but increased standing- crop for 32 km Ward (1976)

Invertebrate densities increased from 820/m 2 to 6727/m 2
within a 13 km reach

Upper Colorado River,
Navajo Dam, USA

Mullan et al. (1976)

River Tees, Cow Green
Reservoir, UK

Reduced diversity and increased biomass for only 400 m
below the dam

Armitage (1978)

Stevens Creek, Central
California, USA

Biomass more than doubled Briggs (1948)

River Svratka, Vir Valley
Reservoir, Czechoslovakia

Numbers increased by up to 3.5 times and biomass by
up to 2.8 times in comparison with the natural river

PeĖáz et al. (1968)

Mill Creek, Wisconsin, USA Many species disappeared and the fauna became

dominated by a few species: Simulium sp.,
Chironomidae, and Gammarus sp.

Hilsenhoff (1971)

Tennessee Valley, South
Holston Reservoir, USA

Increased numbers attributed to large population of
simuliids, chironomids.

Pfitzer (1954)

Guadalupe River, Canyon
Reservoir, USA

Diverse macroinvertebrate community established 24 km
downstream 5 years after dam closure

Young et al. (1976)

Clinch River, Norris Dam,
Tennessee, USA

Number reduced by 30%; Trichoptera and Ephemeroptera
replaced by chironomids and gastropods.

Tarzwell (1939)

(Odum, 1969). However, such adaptations require a long time-period. Within impounded rivers, the
fauna may become dominated by those species which can actively migrate into the substrate interstices for
protection against rapid increases in flow velocity (Radford and Hartland-Rowe, 1971; Trotzky and
Gregory, 1974; Ward and Short, 1978). In a reach immediately downstream of the Fengshuba Dam on the
East River in south China, only one species —Palaemonidae— survived because power generation produces
an artificial fluctuation of discharge and an abrupt increase and decrease of flow velocity. Excessive
velocity appears to be the primary limiting factor. Thus, the depleted fauna of the main channel below Glen
Canyon Dam, experiencing extreme flow-fluctuations, contrasts with the diverse benthic community found
in adjacent quiet-water areas, which includes gastropods, Diptera, Trichoptera, annelids, and amphi-pods
(Mullan et al., 1976).
Invertebrates, generally, are both sensitive to the environmental conditions in which they live, and able
to colonize newly-available habitats by upstream migration of adults and downstream drift of nymphs
and larvae. In an experiment the authors found that the taxa richness of macroinvertebrates increased from
17 species to 38 species and the number density increased from 60/m 2 to 1700/m 2 in just 20 days after
establishing artificial step-pools in a mountain stream with relatively uniform width and depth. However,
the community needs a longer time to reach an equilibrium adapting to the new environmental conditions
after upstream impoundment. Young et al. (1976) considered that the successful reorganization of the
benthic macroinvertebrate community to fill the new set of niches made available by changes of the
environment, required 5 years on the Guadalupe River, Texas, USA, after the closure of the Canyon
Reservoir.