When there is a lot of rainfall over a short period of time, rivers begin to flood. Flooding occurs when the river rises above its channel and water covers the flood plain. Flooding typically happens once a decade or so, but any given decade can have two or more floods, or none at all. Especially large floods occur once or twice a century.
Floods can cause millions of dollars in damage.
They dominate the evolution of the surrounding landscape, by carving out new valleys and moving sediment out of the riverbed and out to the ocean. At the edge of the ocean, the fine portion of the sediment is washed out to sea by waves, and soon nothing is left but beautiful clean sand, which makes the kind of beaches everyone loves. The months between December and March are considered San Diego's rainy season, which usually brings about 12 inches of rain. When it rains several inches in a very short time, rivers in southern California begin to flood. In 1891, thirty inches of rain were measured over two days where the San Diego Wild Animal Park now exists.