Facilities that draw water in large quantities from natural sources are commonly equipped with mechanistically driven barrier screens for removing indrawn debris and captive fish. Owing to the mistransport of debris into the works of the facility by the cyclic action of conventional screening apparatus, amny plant operators favor the refitting of intake systems with alternative devices called dual-flow screens, whose manner of operation precludes the deposition of debris downstream of the screen location. Fish conserving apparatus, otherwise suited to the flows and mechanics of a conventional screen, is often attached without alteration to the screen panels of a dual-flow machine in the hopes of rescuing entrapped fish. As opposed to contrary beliefs, the full-scale experiments reported here show why the flow patterns and water speeds associated with a (double-entry) dual-flow screen are more adverse to live fish recovery than flows through a conventional screen. Owing to flow separations at the entries of these devices and the resulting concentration of flow over a restricted portion of the screening, fluid speeds comperable to flows of 30 and 45 cm/s through conventional screens increased to 90 and 140 cm/s at free-flow regions of the screenfront. In experiments with two species of juvenile fishes, survival was nil. Flow trajectories were mapped by streak photography, and details of fish behavior were recorded on video tape and 35mm film. Equations for flow distributions were also resolved from two-component vector measurements of water velocities.