8188

• UCREFRP Catalog Number: 8188
• Author: Muth, R. T., et al.
• Title: Flow and Temperature Recommendations for Endangered Fishes in the Green River Downstream of Flaming Forge Dam.
• USFW Year: 2000.
• USFW - Doc Type: \
• Copyright Material: NO

Final Report

1-5

September 2000

these aquatic communities are quite different from those that existed prior to dam construction. They
developed naturally as a result of altered physical conditions or artificially as a result of active
management of reservoir fisheries, particularly stocking of nonnati ve fishes. Dams also have reduced
or eliminated movement of native fishes between river sections and thus resulted in population
fragmentation.

Riverine habitat does persist downstream of dams, but flow patterns, sediment loads, and
water temperatures in rivers are altered by water-release patterns dictated by power demands,
irrigation needs, flood control, and recreational uses.
These changes have had dramatic effects on
downstream physical conditions, and native aquatic
organisms that are adapted to the natural dynamic
flow, sediment, and temperature regimes typically
decline after dam closure. Sediment loads in rivers
immediately downstream of dams are reduced because
sediment carried by rivers drops out of suspension in
the slack water of reservoirs. Water released from
dams typically has very little sediment load and
consequently erodes any existing sediment deposits in
dam tailwaters. This process, called "armoring,"
eventually produces areas where the river bed
becomes eroded to cobbles, boulders, or bedrock, with
very little, if any, fine sediment deposits.

Tailwater - The portion of a river
downstream of a reservoir that exhibits water
conditions (such as temperature and clarity)
that are very similar to the conditions of the
water being withdrawn from the reservoir.
Releases from the hypolimnion of a reservoir
often provide clear, cold water in the
tailwater that can support cold-water sport
fisheries. The tail water for Flaming Gorge
Dam extends about 26 kIn downstream of the
dam.

Hypolimnion - The lowermost layer of
water in a thermally stratified lake that is
denser and colder than water in strata higher
in the water column. The density differences
between the hypolimnion and upper water
layers reduces mixing between the layers.

Changes in riverine thermal characteristics
may be one of the greatest physical alterations
precipitated by dam construction and operation (Ward
and Stanford 1979; Petts 1984; Stanford and Ward 1986a; Stanford et al. 1996). The temperature of
water released from dams tends to be colder in summer and warmer in winter than the temperature
of unregulated rivers because releases generally draw from the hypolimnion of reservoirs. These
changes in water temperature can affect river producti vity and the distribution, behavior, growth, and
survival of fishes. The net effect is to reset the species composition of a tailwater assemblage to one
similar to that found in cold, higher-elevation upstream reaches. Native fishes formerly present in
the area reappear in downstream ri ver sections only after temperatures recover from natural warming
or because of tributary amelioration. As a consequence, tailwater habitat downstream of
hypolimnetic-release dams often supports nonnative fishes adapted to cold-water conditions (e.g.,
trout) where native warm-water biota once existed (Holden 1979; Stanford and Ward 1986b; Carlson
and Muth 1989; Stanford et al. 1996). Resource managers have exploited this change in water
temperature to develop cold-water trout fisheries in tail waters of most dams in the basin.

Water depletion and river regulation can not only reduce annual flow volumes but can also
change seasonal and daily flow patterns, which can also be important. River flows downstream of
a hydroelectric dam generally exhibit much less seasonal variation but more daily variation than