sentences:  Steep hills in the San Juan watershed are prone to collapse during heavy rainfall or seismic activity.Over the past 1.2 million years the uplift of the San Joaquin Hills, a small coastal mountain range generally following the Pacific coast of Orange County, created a physical barrier for streams flowing off the Santa Ana Mountains.Climate change during the Ice Ages periodically made Southern California much wetter, most recently during the Wisconsinian Glaciation (70,000 to 10,000 years ago), when the area was home to a temperate rainforest climate that would receive rainfall in excess of per year.