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San Gabriel River
River
The headwaters of the San Gabriel River near Cattle Canyon. This stretch of the river is commonly called the East Fork.
Country	United States
State	California
Regions	Los Angeles County, Orange County
Tributaries
- left	East Fork San Gabriel River, Walnut Creek, Coyote Creek
- right	West Fork San Gabriel River
Cities	Azusa, Covina, Baldwin Park, El Monte, Whittier, Pico Rivera, Downey, Norwalk, Bellflower, Cerritos, Lakewood, Cypress, Long Beach, Seal Beach
Primary source	East Fork San Gabriel River
- location	Mount San Antonio, San Gabriel Mountains
- coordinates	34°20′35″N 117°43′30″W / 34.34306°N 117.725°W / 34.34306; -117.725
Secondary source	West Fork San Gabriel River
- location	San Gabriel Peak, San Gabriel Mountains
- coordinates	34°15′29″N 118°06′13″W / 34.25806°N 118.10361°W / 34.25806; -118.10361
Source confluence	San Gabriel Reservoir
- elevation	1,473 ft (449 m)
- coordinates	34°15′29″N 118°06′13″W / 34.25806°N 118.10361°W / 34.25806; -118.10361
Mouth	Pacific Ocean
- location	Alamitos Bay
- elevation	0 ft (0 m)
- coordinates	33°44′33″N 118°06′56″W / 33.7425°N 118.11556°W / 33.7425; -118.11556
Length	75 mi (121 km), North-South
Basin	640 sq mi (1,658 km2)
Discharge
- average	150 cu ft/s (4 m3/s)
- max	27,000 cu ft/s (765 m3/s)
- min	0 cu ft/s (0 m3/s)
Map of the Los Angeles and San Gabriel Rivers (watercourses in red). The San Gabriel River is on the right.

The San Gabriel River flows 75 miles (121 km) through southern Los Angeles County, California, USA. The river drains a long, narrow watershed extending from the mountains rimming the eastern Los Angeles Basin to the Pacific Ocean. 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. Although once free flowing and lined with marshes and grasslands for much of its length, today most of the river is a concrete channel, and impounded in places by flood control dams.

Course

The East, West and North Forks of the San Gabriel River, rising in the San Gabriel Mountains inside the Angeles National Forest, form the headwaters of the river. The East Fork, sometimes considered part of the main stem,^[1] rises in the shadow of Mount San Antonio as two smaller forks of its own, the Prairie Fork and the Fish Fork. The East Fork officially begins at the confluence of the Prairie Fork and a smaller tributary, Vincent Gulch.^[1] The Fish Fork and the main stem combine at the base of Iron Mountain in a canyon nearly 5,000 feet (1,500 m) deep. From there, the East Fork flows south then turns west, flowing into the east arm of San Gabriel Reservoir. The similarly sized West Fork starts near San Gabriel Peak near the Angeles Crest Highway and flows east before being impounded in Cogwell Reservoir, where it receives Devils Canyon Creek. The river continues to flow east and receives Bear Creek from the left before combining with the North Fork, which rises near Mount Islip.

San Gabriel Reservoir and Morris Reservoir, both formed by flood prevention dams built in the 20th Century, submerge most of the upper stretches of the main stem San Gabriel. It is not long after the river leaves the San Gabriel Canyon and exits from the mountains into the San Gabriel Valley near the city of Azusa. The normally dry riverbed proceeds southwest to the Santa Fe Dam, which impounds the river in the Santa Fe Flood Control Basin. After exiting the dam, the river flows south in a flood control channel roughly parallel to Interstate 605, also called the San Gabriel River Freeway, past Covina and El Monte, receives Walnut Creek and San Jose Creek from the left in quick succession, then proceeds into the Whittier Narrows where it is impounded behind Whittier Narrows Dam. Here, the river receives the Rio Hondo from the right, then splits in two immediately after: the main stem continues to flow south, while the Rio Hondo carries a portion of the water southwest to empty into the Los Angeles River near Downey.

The San Gabriel River meeting the Pacific Ocean in Seal Beach.

The San Gabriel River, however, continues to flow south, past Bellflower and Cerritos. It forms the boundary between Los Angeles and Orange Counties for a brief stretch before merging with Coyote Creek, one of its main tributaries, near Los Alamitos. The river eventually becomes tidal and empties into the outlet of Alamitos Bay between the cities of Long Beach and Seal Beach.

Watershed

The San Gabriel River drains an area of 640 square miles (1,700 km²) in eastern Los Angeles County and northwestern Orange County. It is the middle of the three major rivers of the Los Angeles Basin. The watershed is bounded on the west by the Los Angeles River and on the southeast by the Santa Ana River. To the north is the arid interior drainage basin of the Mojave Desert. The river mainly flows along the west side of the watershed.^[2][3] 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.^[4] 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 Morris Dam.

The northern part of the watershed is dominated by 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, part of the Transverse Ranges, 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 south of the San Gabriel Mountains over which the San Gabriel River now flows. The 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.

History

The upper reaches of the San Gabriel River in San Gabriel Canyon

In bygone times, the river ran freely across arid grasslands and through extensive marshes to the Pacific Ocean, flooding in the winter and spring then running nearly dry in the summer and fall. Once out of the mountains, the river's course would change frequently with every heavy inundation. Sometimes, the river would change course to run into the Los Angeles River in the west, and sometimes the Santa Ana River's floodwaters would travel westwards into the San Gabriel from Santa Ana Canyon. The River was historically in the tribal territory of the Tongva Native American group. Together with the Los Angeles and Santa Ana Rivers, the San Gabriel River provided sustenance for hundreds of members of this powerful coastal tribe, whose territory extended across the entire Los Angeles Basin.

In 1771, the Spanish founded Mission San Gabriel Arcángel near the present-day site of the city of San Gabriel, on the banks of the Rio Hondo, a distributary of the San Gabriel River. The river's modern name derives from the mission. The modern name of the Tongva, the Gabrielino, also was derived from the name of the mission. After California became part of the United States, Americans established the city of Los Angeles, and after the Los Angeles Aqueduct was constructed, the first development boom in the area began, creating many of the towns and cities that now line the San Gabriel River. Many, including Whittier, were named after their founders, and legend has it that Azusa's name was derived from "everything from A to Z in the USA", although it was actually obtained from a Native American word. In this period, agriculture was the primary economy of the region.

Devastating floods wreaked havoc along the San Gabriel River in the late 1800s and the early years of the 20th century. The most famous was the Los Angeles Flood of 1938, which produced the highest flows ever recorded in the river—some 27,000 cubic feet per second (760 m³/s) according to a U.S. Geological Survey river gauge near Los Alamitos. As a result, the U.S. Army Corps of Engineers began an ambitious effort to prevent flooding along the river in the lowlands. Much of the river downstream of Azusa was diked, channeled, lined with riprap or paved over with concrete. A cascade of 10 drop structures was constructed where the river empties out of San Gabriel Canyon to slow flood flows from the mountains. Check dams were constructed in upper canyons and the river itself was impounded in several artificial lakes. (See #River modifications.)

The increased flood protection afforded by the dams and channels indirectly led to a housing boom from the 1960s to the 1980s. Most of the lowlands and agricultural areas in the watershed were paved over to construct residential districts. Except for the Angeles National Forest (San Gabriel Mountains) and the Puente Hills between the San Gabriel Valley and the Los Angeles Basin, the remaining flat land in the watershed was filled with development. The river also provides a small amount of municipal water.

River modifications

Morris Dam on the San Gabriel River

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.

Crossings

The East Fork of the San Gabriel River, in the Angeles National Forest.

From mouth to source (year built in parentheses):^[5]

References

• "Study Findings and Alternative Concepts". San Gabriel Watershed and Mountains Special Resource Study. National Park Service. 2009-08-03. http://parkplanning.nps.gov/document.cfm?parkID=422&projectId=12203&documentID=28825. Retrieved 2010-01-06. 

External links

• San Gabriel Mountains Regional Conservancy
• 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