Santa Ana River - Geology

Geology

Ancient igneous, metamorphic and sedimentary rock underlie and form the geologic base of the Santa Ana River. Most of the strata in the flat valleys and basins of the watershed are underlain by thousands of feet of sediment deposited by transient seas during climate changes and by erosion. Most of the mountains in and rimming the basin consist of granite batholiths only about 75 million years old. However, much of the rock overlying the highlands, above elevations of 8,000 to 9,000 feet (2,400 to 2,700 m), is ancient metamorphic rock up to 1.7 billion years old. This rock originally formed at the bottom of the ancient Pacific Ocean and was uplifted to the highest peaks of the mountains. Even in ice ages, glaciers have rarely occurred on Southern California mountains, so the rock has remained there for tens of millions of years without significant erosion.

Diverse and complex faulting and geologic instability have shaped the Santa Ana River watershed. The San Andreas Fault runs across the northern section of the watershed and was responsible for causing the uplift of the San Bernardino Mountains, part of the Transverse Ranges of Southern California. The Elsinore–Whittier Fault Zone crosses the Santa Ana River further downstream, near the Orange County/Riverside County line. This fault caused the rising of the Santa Ana Mountains, Puente Hills, East Orange Hills, Chino Hills, Loma Ridge, and the other mountain ranges and ridges that run northwest-southeast across the lower section of the watershed, comprising the coastal Peninsular Ranges. While the larger San Andreas Fault allowed the Transverse Ranges to rise to above 10,000 feet (3,000 m) in many places, the Peninsular Ranges are only about half that height.

During the last glacial period, when climate change during the Wisconsinian Glaciation caused rivers in Southern California to increase greatly in volume, the Santa Ana was able to cut across the Peninsular Ranges, creating the only gap across the range. During this period, the Santa Ana changed course multiple times, creating wind gaps in the Peninsular Range and occasionally entrenching into the channel of the ancestral San Diego Creek. The river later returned to its old course and abandoned the San Diego Creek channel, leaving it a wind gap across the Huntington Beach/Newport Beach mesa.