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Fish screens have been developed on a trial-and-error basis. The purpose(s) for which screens are constructed are important in determining where they should be used, what types should be used, and if the costs are justified. Diversions for industrial, irrigational, hydroelectric, and domestic water uses provide many hazards to fish resulting in physical and chemical injuries, confinement, and diversion from natural destinations. Screens have been also used in censuring and controlling fish populations. Screen and by-pass designs are determined by the amount of water diverted, availability of excess of water, the swimming ability, behavior and size of the fish, and the quantity and size of debris which may reach the screen. The by-pass carrying fish past the screen is critically important. It should have an attractive velocity, substantial flow, and an entrance near the area where the fish are stopped. All screening devices are subject to damage by debris and heavy objects and must be protected by guards. They are affected by bed load movement of sands and gravels and must be protected against icing. They require a head differential sufficient to pass water through the mesh. The mesh openings must be small enough to prevent passage of the juvenile fish to be diverted. When requirements call for smaller mesh sizes, problems associated with filamentous algae are encountered. Because of their location, many screens require by-passes, which accumulate and concentrate fish, inviting predation. By-pass outlets should provide for dispersion or introduction into areas that discourage predator concentration, such as high velocities or upwelling. entrances into by-passes should provide smooth transition. See Burns (1966a), Heuber (1974), Rosenfell and Everhart (1953, and Sharma (1973) for more details and references on fish screens. In artificial guidance, the stimuli include bubbles, electric fields, and high water velocities. Although the literature shows that fish have an immediate response to bubbles (which may be a fright response), experiments with salmonid fish indicate that bubble screens are not effective in either stopping or guiding. There is evidence that fish will lead, to some degree, along lighted bubbles but this advantage is negated under conditions of darkness or turgidity. The literature discloses that a fright reaction may be engendered by sound, hanging chains and light. Pressure change is useful as a guidance mechanism, as it has been found that fish do not readily sound, even though instantaneous increases are not harmful. Feeding fish in lakes, however, are known to move vertically under darkness conditions but avoid deep areas under lighted conditions, indicating that the instinct to be guided by pressure can be negated by stronger stimuli. Chemical barriers that produce avoidance reactions may be used, but generally are not considered practical; however, certain chemicals cause complete rejection of an area of a stream or strong fright reaction. Barrier dams prevent passage by creating upper darting velocities, but also provide attraction velocities to the entrance located at the farthest upstream point. Broach (1968) describes modification of a spillway to prevent entry of undesirable fish. 6