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The San Gabriel River, highlighted in red (on the right). The Los Angeles River is highlighted in red on the left

The east fork of the San Gabriel River, in the Angeles National Forest.

The San Gabriel River flows 75 miles through southern Los Angeles County, California. It rises in the San Gabriel Mountains, flowing southwest into and through the western San Gabriel Valley before turning southward at the Whittier Narrows to enter the Los Angeles Basin. It forms the boundary between Los Angeles and Orange counties for a brief stretch before it empties into the Pacific Ocean between Long Beach and Seal Beach. It derives its name from the Spanish Mission San Gabriel Arcángel which was originally built in the Whittier Narrows (1771) before being moved to its present location in San Gabriel.

Like most rivers in southern California, the San Gabriel River today bears little resemblance to the river it was before the arrival of early Spanish settlers. It is dammed five times along its length: once along the West Fork by the Prescott F. Cogswell Dam, then twice more downstream of the forks in the San Gabriel Mountains to create reservoirs at the San Gabriel Canyon Dam, and at the former naval test site Morris Dam; at the Santa Fe Dam in the Santa Fe Dam Flood Control Basin in Irwindale; and with the nearby Rio Hondo (to which it is also connected by a short channel) at the Whittier Narrows Dam, between the cities of South El Monte and Pico Rivera. Its channel is lined with concrete for most of its length below the mountains. These alterations were made in response to disastrous flash floods in the late 19th and early 20th centuries. During periods of heavy rainfall, the Los Angeles District of the U.S. Army Corps of Engineers can shift flows between the Rio Hondo (a tributary of the Los Angeles River) and the San Gabriel River.

The San Gabriel River course is also the site for companion highways. In the lowlands it is adjoined by the San Gabriel River Freeway (Interstate 605) which replaced an older Rivergrade Road. Into the San Gabriel Canyon it is followed by State Route 39 to a terminus nearly 30 miles upstream.

As with the similarly modified Los Angeles River, the San Gabriel is a notorious symbol of environmental depredation, and efforts to restore its ecosystem have had only limited success due to water pollution and fertilizer runoff.

• State of California San Gabriel & Lower Los Angeles Rivers and Mountains Conservancy
• Amigos de los Rios (an organization whose goal is to create a necklace of parks connecting the San Gabriel River, the Rio Hondo, and Whittier Narrows)
• Public Law 108–42 (San Gabriel River Watershed Study Act) (an act proposed as H.R. 519 by Hilda Solis and S. 630 by Barbara Boxer)
• San Gabriel Watershed and Mountains Special Resource Study (the study authorized by Public Law 108-42)
• Documentary on the San Gabriel River, Ya Don't Miss the Water (a five-part video that documents the major ecological, political, and social problems of the community - approximately 2 million people surround and use the river's water)
• Online Computer Library Center - WorldCat search result: San Gabriel River Watershed