This is common on a time scale of several decades. Every fifty to one hundred years there are exceptionally large floods. Large floods ream out the steep upper portion of the valley and dump sediment in the wider lower portion, maintaining the flood plain. Sediment is deposited on beaches where waves wash it and take the fine matter out to sea, leaving clean beach sand. Floods are caused by unusually high precipitation over a short time.
Large floods can bring several inches of rain in a few days or weeks.
In 1891, thirty inches of rain were measured in two days near what is now the San Diego Wild Animal Park. Flooding can occur with little rain on the flood plain, provided that large amounts of rain fall in the mountainous regions. The intervals between large floods are highly variable ranging from 1 to 50 years. Floods are irregular because of our climate. Normally, the San Diego region is within a high pressure zone, extending inland from an offshore high.