Allowable instantaneous minimum river flows are established in the Columbia and Snake Rivers to ensure safe passage of anadromous fish during their migration to the spawning grounds. However, water storage during periods of low power demands (at night and on weekends) would be beneficial to the power producers. This storage procedure is called 'zero' river flow and is now permitted on a limited basis when there are few if any actively migrating anadromous fish present in the river system. Requests were made to extend 'zero' river flow into periods when anadromous fish were actively migrating and a study was initiated. Radio-tracking studies were conducted on the Snake River between Lower Monumental and Little Goose Dams to determine the effect of 'zero' river flow on the migration of adult chinook salmon, Oncorhynchus tshawytscha, and steelhead, Salmo gairdneri. From July through September, 1981, a total of 258 steelhead and 32 chinook salmon were radio-tagged. The rate of migration was used to determine differences between test and control fish and a gamma distribution model was used to describe the migration rate for radio-tagged fish. Estimates of the parameters of the model were used to statistically compare 'zero' flow and normal river flow conditions for the radio-tagged fish. The results show that the 'zero' flow condition delays the migration of adult chinook salmon and steelhead; therefore, extended periods of 'zero' flow to store water are not recommended when fish are actively migrating in the river system.