THE EFFECTS OF LARGE DAMS ON SALMON SPAWNING HABITAT IN THE PACIFIC NORTHWEST
Growth and development of the Pacific Northwest region in the 20th century included the construction of large dams on many river systems for flood control and/or hydroelectric power production. At the time dams were constructed, the effects on anadromous salmonid populations were not considered to the same extent they are today. Large dams have many effects on the ecology of migrating fish populations, including changing the transport dynamics of sediment and large woody debris and alteration of downstream aquatic habitat.
Anadromous salmon bury their eggs in redds (nests) in gravel substrate (12-64 mm-depending on fish size). Most river systems in the Pacific Northwest are gravel and cobble-bedded, providing a variety of spawning habitats for different fish species. Large dams trap sediment and large woody debris and decrease peak flows, altering the downstream sediment supply, storage, and transport dynamics. While the general effect is a coarser grain size and reduction in the availability of gravel immediately downstream of the dam, studies of a number of river systems affected by reservoirs show that the effects in a specific river system depend on a number of factors, including stream gradient, confinement, presence of tributaries, and degree of peak flow attenuation (Table 1). The downstream extent of the coarsening also varies depending on the presence or absence of sediment supply below the dams.
|
River system |
Sediment characteristics upstream of dam |
Immediately downstream of dam |
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|
Degree of peak flow reduction |
Gradient |
Confinement |
Sediment characteristics |
|
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|
Lower Lewis River, WA |
-- |
Large |
0.06 % |
confined |
D50=40-60 mm |
|
|
Cowlitz River, WA |
gravel/cobble |
Large |
0.18 % |
unconfined |
D50=45-50 mm |
|
|
Upper Lewis River, WA |
-- |
Large |
0.5 % |
unconfined |
cobble/boulder |
|
|
North Umpqua River, OR |
D50=30-40 mm |
Large |
0.5 % |
confined |
D50=45-50 mm |
|
|
Elwha River, WA |
D50=60-80 mm |
Little |
0.6 % |
confined |
D50=110-160 mm |
|
|
North Fork Skokomish, WA |
cobble/gravel |
Extreme |
1.3 % |
confined |
cobble w/gravel |
|
|
Nisqually River, WA |
cobble/gravel |
Large |
8.0 % |
confined |
boulder/bedrock |
|
-- Sediment characteristics not noted. Gravel=4 64 mm; cobble=64 256 mm; boulder >264 mm. |
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