To study the feasibility of constructing Galloway Dam on the Weiser River in Idaho, the U.S. Army Corps of Engineers developed a linear, multiplicative, computer model (FISHBEN) to estimate fishery benefits. The primary purpose of the dam would be to store water to augment flows on the Snake and Columbia rivers from Hells Canyon Dam to the Pacific Ocean to increase the downstream survival of juvenile salmon, Oncorhynchus spp., and steelhead Oncorhynchus mykiss . Fifty years of past flow data were used to predict future flow conditions. Juvenile fish populations and their survival through river reaches, reservoirs, dams, transportation, and in-river fish passage were predicted and used to estimate overall fish survival. Adult returns were predicted, and economic values based on potential harvest were used to estimate monetary fishery benefits. Fishery benefits derived by FISHBEN were compared with engineering and construction costs for 23 alternatives, and the alternative deemed best for national economic development as defined by the Corps of Engineer's principles and guidelines for planning studies was recommended.