2003 Seattle Annual Meeting (November 2–5, 2003)

Paper No. 10
Presentation Time: 3:45 PM

THE EFFECTS OF LARGE DAMS ON SALMON SPAWNING HABITAT IN THE PACIFIC NORTHWEST


DUBÉ, Kathy V., Watershed GeoDynamics, 14145 97th Ave NE, Bothell, WA 98011-6905, kdube@watershednet.com

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. 

Table 1.  Comparison of upstream and downstream characteristics in river systems with large reservoirs.

 

River system

Sediment characteristics upstream of dam

                               Immediately downstream of dam

 

 

Degree of peak flow reduction

Gradient

Confinement

Sediment characteristics

 

 

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.