In early 1938, before the dam was finished, Southern California was hit by record floods. The heaviest rains fell in the San Gabriel Mountains, swelling the San Gabriel River; on the night of March 2-3, 1938 a flood of poured out of the mountains and into San Gabriel Reservoir. San Gabriel Dam was able to knock about off the peak of the flood. Further downstream, Morris Reservoir was able to absorb roughly , reducing the flood to less than half of what it would have been if not for the dams.