In 1998 a behavioral guidance structure (BGS; a steel wall 330m long and 17-24 m deep) was installed in the forebay of Lower Granite Dam on the Snake River, Washington. The purpose of the BGS was to change the horizontal distribution of downstream migrants approaching the south half of the powerhouse by guiding them toward the surface bypass and collector attached to the dam upstream of the north half of the powerhouses. The effectiveness of the BGS was evaluated with biotelemetry and hydroacoustics. The BGS was designed to be movable, thereby allowing a comparison between the horizontal distribution of the fish when the BGS was deployed as a diversion device and when the BGS was moved 800 m upstream of the dam and no longer influenced fish movements immediately upstream of the powerhouse. Radio telemetry and hydroacoustic techniques showed that about 80% of the fish migrating toward Turbines 1-3 were successfully diverted north. Radio telemetry data revealed that the mean residence times of chinook salmon, hatchery steelhead, and wild steelhead were 1.6, 1.7, and 2.4 times longer, respectively, when the BGS was out compared to when it was in. And overall fish passage efficiency was significantly higher when the BGS was in (93.7%) than out (91.2%).