Summary; This paper discusses the research process and results of a study designed toinvestigate how effective the fish screens on the Salmon River, Idaho were at diverting fish fromirrigation canals. A cost-benefit analysis of the fish screens as well as recommendations for theirimprovement is also included.The Salmon River, at the time that this paper was written, was home to a declining population ofChinook, sockeye and steelhead salmon. Of which the steelhead and Chinook population wasbeing supplemented by stocking. The general belief was that the salmon numbers weredecreasing as a result of hydroelectric development in the Columbia River Basin.In 1984, approximately 200 of the 220 irrigation canals were fitted with fish screens. The majority(16) of the irrigation canals without screens were located on Carmen Creek where, due to itsfrequent dewatering, the anadromous fish runs had been eradicated. To assess how effectivelythe screens were diverting fish from the irrigation canals and how many fish were lost to damagein irrigation canals, electro-fishing and mark-recapture activities were undertaken throughout the60's and 70's. This is the information that was used to evaluate the efficiency of fish screenoperation however the cost-benefit analysis was conducted by the author.This investigation determined that due to seasonal and yearly variability of weather conditionsand water levels as well as other influential factors, the information available was not adequate todetermine how many fish were saved at the screens. A series of recommendations were made toguide future acquisition of sufficient information.