Abstract: A retention gate (20) is pivotable about a horizontal pivot shaft (22) mounted near the gate's upper edge and is located behind the overflow sill (14) of a storage drainage opening (12), said retention gate in its closed position being in sealing contact by its lower edge (30) with the overflow sill and furthermore being loaded by a counterweight (42) mounted on the outflow side at a distance from the pivot shaft (22), said counterweight generating the closing force required against the inflow side pressure. The retention gate (20) when in its closed position runs from its lower edge in the downstream direction at an average slope of 20.degree.-60 .degree. relative to the vertical, and the counterweight (42) rigidly affixed to it is guided in such manner that it generates in each gate position a closing torque that approximately matches the torque generated by the amount of drained water required to keep constant the level (24).