A pivotal gate system to be installed across the drain opening to a catch basin of a storm drain system and having a catch basin blocking gate that is adapted to rotate from a closed position extending completely across the drain opening at which to trap and prevent leaves, litter and other debris from entering the catch basin during dry periods and light rainfall to an open position removed from the drain opening at which to permit rainwater to enter the catch basin during periods of heavy rainfall. A water wheel is mounted for rotation within the catch basin. The water wheel is coupled to the catch basin blocking gate by way of a pulling cable. The water wheel rotates in response to impact forces caused by rainwater dropping downwardly on a plurality of scoops extending around the water wheel. A rotation of the water wheel generates a pulling force in the pulling cable to be applied to the catch basin blocking gate whereby to cause the blocking gate to move from the closed position to the open position.