Growth and development of the Pacific Northwest region in the 20^th 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.