The performance of different kinds of vertical slot fishways for two different slopes was studied in a wide range of discharges. Water depths were measured in almost the whole surface of pools. A linear relation between dimensionless discharge and depth of flow, and the same flow patterns for each design were found. With an acoustic Doppler velocimeter (ADV), three-dimensional velocities were measured at several levels in the entire pool to detect the structure of the flow and quantify velocity distribution. Two different regions in flow patterns were found: a direct flow region characterized by maximum velocities; and a recirculation region, defined by low velocities and horizontal eddies. For a given slope, the velocity at any point of the pool (particularly at the slot) may be considered independent of the discharge and constant with the depth. Some suggestions on kinetic turbulent energy are also made.