Effects of Dams and Impoundments on Migrations of Juvenile Chinook Salmon and Steelhead from the Snake River, 1966 to 1975

Abstract

Abstract Migrations of juvenile chinook salmon, Oncorhynchus tshawytscha, and steelhead, Salmo gairdneri, from tributaries of the Snake River were monitored as far downstream as The Dalles Dam on the Columbia River in most years during the period 1966 to 1975. New dams constructed on the Snake River adversely affected survival and delayed migrations of juveniles. Significant losses of juveniles in 1972 and 1973 were directly responsible for record-low returns of adults to the Snake River in 1974 and 1975. Major causes of mortality were passage through turbines at dams, predation, and delays in migration through reservoirs in low-flow years, and prolonged exposure to lethal concentrations of dissolved gases caused by spilling at dams during high-flow years. Migrations of juvenile steelhead were generally later than those of chinook salmon and generally coincided with maximum river discharge. Lack of river runoff in 1973 caused a significant number of steelheads to stop migrating and to hold over in reservo...