San Gabriel River (California) - Watershed

Watershed

The San Gabriel River drains a watershed basin area of 713 square miles (1,850 km2) in eastern Los Angeles County and northwestern Orange County. It is the middle of the three major rivers of the Los Angeles Basin, with its watershed bounded on the west by the Los Angeles River and on the southeast by the Santa Ana River watersheds. To the north is the arid interior endorheic drainage basin of the Mojave Desert and Mojave River. The San Gabriel River mainly flows along the west side of its watershed. There are 37 major cities in the San Gabriel River watershed, 19 of which are actually situated on the river. In total, 26% of the watershed is covered by heavy development. Some tributaries of the river include Bear Creek, Walnut Creek, San Jose Creek, and Coyote Creek. The latter three are all large eastern tributaries that drain areas in excess of 40 square miles (100 km2). San Jose Creek flows nearly 20 miles (32 km) westwards from Pomona into the San Gabriel. Because most of the water from the mountains is stored in reservoirs and diverted for municipal use, the tributaries provide most of the flow below the Santa Fe Dam.

The northern part of the watershed is dominated by the San Gabriel Mountains, one of the Transverse Ranges, which were formed by seismic activity along the San Andreas Fault, a major fault system in turn created by the collision of the North American Plate and the Pacific Plate. Before these two plates collided, the San Gabriel River did not even exist. It was only after the San Gabriel Mountains rose about 75 million years ago, that the river first began to form. Changes in sea level, especially during the Ice Age, deposited thousands of feet of marine sediments in the flood plain south of the San Gabriel Mountains over which the San Gabriel River now flows. The San Rafael Hills, Puente Hills, and Chino Hills that cross the lower part of the watershed were formed by slippage of the Whittier Fault, part of a smaller fault system that formed the Peninsular Ranges.