The invention relates to a unit comprising a hydraulic turbine (1), a duct (5) leading a forced flow of water to the turbine, a duct (8) discharging the outgoing flow from the turbine and vanes (20) for guiding the flow through the discharge duct. Each guide vane (20) is able to rotate about an axis (x22) secant to the wall (84) of the discharge duct. Means (30) are provided for controlling the angular position of the vane (20) about its axis of rotation (x22). Each guide vane (20) can also be retracted into the wall (84) of the discharge duct (8), and means (21) are provided for adjusting the extent to which they are withdrawn into the wall.