Management of fish movement, either to direct them to the entrance of a fish bypass or to prevent their entry into areas where they can be injured, killed, or their migration delayed, is a common goal downstream of most dams in South America. Multidimensional numerical simulation is a useful tool which can provide detailed flow pattern information to help understand observed fish abundance and distribution in the tailraces of dams and to guide the decisions on locating fishway entrances. To this end, we developed a 3-D computational model to investigate flow characteristics near Tres Marias Dam located at river 3 KM on the Sao Francisco River, Brazil. The modeled spatial domain included a portion of the powerhouse (including the draft tubes), tailrace, and a river reach extending 3 KM downstream of the dam. We simulated a common operational scenario consisting of three active turbines on the right side of the powerhouse and three inactive turbines on the left side of the powerhouse. Domain discretization was conducted using Gridgen software and numerical simulation was performed using Fluent. From the model results we identified two dominant components in the flow pattern: a relatively straight discharge plume originating from the right-most turbine that was constrained by the right-side wall of the powerhouse and the generation plume from turbines 2 and 3 and a second component featuring a complex entrained primary eddy with two secondary eddys maintained by the discharge plume of the powerhouse. The complex primary and secondary eddys could constitute a low velocity refuge area where fish could concentrate even during hydropower generation. Finally, we concluded that local velocities were not a significant challenge for neotropical species, such as Pimelodus maculatus, Leporinus Renhardti and Prochilodus lineatus, since their prolonged swimming speeds were higher than the modeled water velocities.