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.
Floods can also occur when there is little or no rain.
Much of San Diego's water is the result of rainfall in the mountains. If it rains too much there, the water rushes down the mountains and into the ocean causing the rivers of San Diego to overflow. Large flooding is very hard to predict. Sometimes it will flood every year and other times it won't flood for forty years. This irregularity is caused by San Diego's climate.