Studies of Pacific salmon and anadromous rainbow trout (Oncorhynchus spp.) passage past hydroelectric dams on the Columbia River have been conducted since the first dams were constructed in the 1930s. Decades of research on juvenile salmonid passage led to the installation of turbine intake screens to guide juvenile salmon into bypass systems at most US Army Corps of Engineers (COE) dams on the Columbia and lower Snake rivers. These screens guide from 10 to 80% of the juveniles away from turbines, depending on time of year, species and stage of development. However, recent declines in salmon stocks have precipitated their listing for protection and recovery under the US Endangered Species Act. Regional goals established to recover these stocks have been set for each dam: passage of at least 80% of the juvenile migrants through a non-turbine route, and 95% juvenile fish survival. To achieve these goals, surface-oriented systems are bring developed that bypass fish around turbines and reduce forebay holding, delay and loss to predation. Three surface bypass concepts are discussed: deep slot, surface weir and corner collector. Physical design criteria include attraction velocity, bypass discharge volume, entrance depth and width, depth of interception, zone of separation and zone of influence. Hydroacoustic data indicate that use of existing surface bypass routes by juvenile migrants can be greater than 90%. Use varies based on vertical distribution of migrants in the immediate vicinity of dams, and is higher during daytime hours, Radio telemetry provides data on juvenile salmonid approach patterns into the dam forebay, preferred passage routes, and holding and delay behaviour under varying dam operating conditions. Both methodologies have strengths and weaknesses, but when used in conjunction they enhance our understanding of fish passage and behaviour. Integrating behavioural data with that of forebay hydraulics provides the comprehensive knowledge required to design effective surface bypass systems.
