Abstract: A shut-off valve is mounted below a fire hydrant to automatically shut down water flow in case the hydrant is knocked over by a vehicle. The valve has a cylindrical enclosure open at both ends. A rotor is rotationally supported within the enclosure and movable linearly by rotation between a first, valve-open, and a second valve-closed, positions, the rotor acting to stopper the valve when in the valve-closed position. The rotor has plural buckets which cause the rotor to rotate when water flows through the enclosure and this rotation causes a screw action moving the rotor into the valve-closed position. Rotation is normally prevented by a restrictor plate resting in recesses on the valve. In this case the hydrant is able to receive water flow through the valve. When the hydrant is displaced it releases the restrictor plate allowing the stoppering action.