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. High air pressure prevents convection and cloud formation, thus preventing rain. Intense rainfall occurs when this common state is altered by winter or tropical storms that bring moist marine air into the San Diego region. This happens especially frequently during El Ni o events. The phytoplankton serves as food for fish and other sea animals.