Migrations of juvenile Chinook salmon, (Oncorhynchus tshawytscha), and steelhead,(Salmo gairdneri), from tributaries of the Snake River were monitored as far downstream as TheDalles Dam on the Columbia River in most years during the period 1966 to 1975. New damsconstructed 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 throughturbines at dams, predation, and delays in migration through reservoirs in low-flow years, andprolonged exposure to lethal concentrations of dissolved gases caused by spilling at dams duringhigh-flow years. Migrations of juvenile steelhead were generally later than those of Chinooksalmon and generally coincided with maximum river discharge. Lack of river runoff in 1973caused a significant number of steelheads to stop migrating and to hold over in reservoirs.Mortality of Chinook salmon and steelhead resulting from new dams has differed with respect toarea and cause. Magnitude and composition of seaward migration has changed from 3 to 5million wild fish in the mid-1960's to 8 to 10 million fish of both wild and hatchery origin in the1970's.