The size, and therefore the cost, of screening facilities required at water diversion sites is primarily determined by the allowable approach velocity of water at the screen mesh. General screening criteria established by fisheries agencies specify maximum approach velocities. Biological factors affect the swimming ability of the fish. In addition to the biological factors, proper attention must be given to engineering factors including uniform velocity distribution at the screen facility. Providing basic screen facility hydraulics necessary for effective fish protection requires careful attention to channel configuration and frequently involves use of baffles and training walls to control direction of flow and magnitude of velocity.