A gate system for an opening through which fluid flows, such as the opening to a storm drain typically found in the curb of an urban street. The system is biased to a closed condition to keep trash out of the drain during dry and low fluid flow situations, then automatically converts to an open condition during heavy fluid flow situations, and then returns to a closed condition when the heavy fluid flow condition abates. The system has a gate portion that rotates between an open position and a closed position adjacent the opening, being biased to the closed position, and a trip plate, which is also biased to a closed position. The trip plate has one or more pins that communicate with one or more grooves and/or detents in one or more adjacent bracket assemblies to hold the gate portion in the closed position until the fluid flow on or against the trip plate reaches a predetermined level such that the trip plate rotates from the closed position, releasing the gate portion and allowing the fluid flow to push the gate portion into an open position. After the fluid flow abates, both the gate portion and the trip plate rotate back to their closed positions automatically.