The U.S. National Marine Fisheries Service, under contract to the U.S. Army Corps of Engineers, monitored the seaward migration of juvenile salmonids through the Snake-Columbia river system from 1973 to 1983. We developed a procedure for estimating the numbers of steelhead Salmo gairdneri and yearling chinook salmon Oncorhynchus tshawytscha smolts passing McNary Dam each day in 1982-1983. The method utilized the fingerling collection and bypass system contained within the powerhouse. Freeze-branded groups of smolts released upstream from the dam were used to calibrate the efficiency with which migrants were collected over a range of powerhouse discharge levels. Application of collection efficiency calibration curves enables researchers and fisheries managers to generate daily estimates of smolt passage as a function of the amount of water discharged through the powerhouse and the number of fish collected in the bypass system.