Aliso Creek (Orange County) - Geology

Geology

Most of Southern California, including all of Orange County, was periodically part of the Pacific Ocean; the most recent epoch was approximately 10 million years ago (MYA). The Santa Ana Mountains, which now border the creek to the north and east, began their uplift about 5.5 million years ago along the Elsinore Fault. Aliso Creek formed about this time, running from the mountains across the broad coastal plain to the Pacific.

About 1.22 million years ago, the San Joaquin Hills along the Orange County coast began their uplift along a blind thrust fault (the San Joaquin Hills blind thrust) extending south from the Los Angeles Basin. As Aliso Creek was an antecedent stream, or one that had formed prior to the mountains' uplift, it cut a water gap through the rising mountains that today is Aliso Canyon. The same phenomenon occurred to the north with Laguna Canyon and San Diego Creek, and to the south at San Juan Creek. The uplift also caused Aliso Creek's largest tributary, Sulphur Creek, to turn north to join Aliso Creek instead of flowing south to Salt Creek. The Wisconsinian era was responsible for shaping the watershed to its present-day form, with deep side canyons and broad alluvial valleys.

During the last glacial period (110,000 to 10,000 years ago), especially in the Wisconsinian glaciation (31,000 to 10,000 years ago), the climate of Southern California changed radically from arid to wet, to a climate likely similar to the present-day Pacific Northwest. Prodigious rainfall gradually turned the small streams of the region into large and powerful rivers. It was this surge in volume that allowed Aliso Creek and other rivers to cut through the San Joaquin Hills. A 400-foot (120 m) drop in sea level escalated the process, allowing the rivers to flow more rapidly and have more erosive power. As sea levels rose after the Wisconsinian glaciation, the water gaps the rivers had cut through the San Joaquin Hills, including Aliso Canyon, became fjord-like inlets. Aliso Creek and these other streams deposited sediments into the inlets, turning them into flat-floored alluvial valleys with an elevation very close to sea level. Eventually, the sediment deposited met the coastline. By then, the rivers and streams had diminished to their original flow before the glaciation.

In the wake of the periodic inundation of Southern California by the ocean, most of the Aliso Creek watershed is underlain by several layers of marine sedimentary strata, the oldest dating from the Eocene (55.8–33.9 MYA) and the most recent, the Pliocene (5.33–2.59 MYA). These alluvial sediments range from 13 to 36 feet (4.0 to 11 m) in depth. Generally throughout the watershed, there are five major soil and rock outcrop types—Capistrano sandy loam, Cieneba sandy loam, Marina loamy sand, Myford sandy loam, and Cieneba-rock outcrop. The water table ranges from 6 to 20 feet (1.8 to 6.1 m) deep.