San Gabriel Dam is a rock-fill dam on the San Gabriel River in

Los Angeles County Los Angeles County, officially the County of Los Angeles, and sometimes abbreviated as L.A. County, is the most populous county in the United States and in the U.S. state of California, with 9,861,224 residents estimated as of 2022. It is the ...

California California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...

, within the

Angeles National Forest The Angeles National Forest (ANF) of the United States Forest Service, U.S. Forest Service is located in the San Gabriel Mountains and Sierra Pelona Mountains, primarily within Los Angeles County, California, Los Angeles County in southern Calif ...

. Completed in 1939, the dam impounds the main stem of the San Gabriel River about downstream from the confluence of the river's East and West Forks, which drain a large portion of the

San Gabriel Mountains The San Gabriel Mountains ( es, Sierra de San Gabriel) are a mountain range located in northern Los Angeles County, California, Los Angeles County and western San Bernardino County, California, United States. The mountain range is part of the Tr ...

. It is located directly upstream from the

Morris Dam Morris Dam (officially the Samuel B. Morris Dam) is a concrete gravity dam across the San Gabriel River in the U.S. state of California. It impounds Morris Reservoir, directly downstream of San Gabriel Dam. The dam was built in the 1930s as a w ...

. The dam provides flood control,

groundwater recharge Groundwater recharge or deep drainage or deep percolation is a hydrologic process, where water moves downward from surface water to groundwater. Recharge is the primary method through which water enters an aquifer. This process usually occurs in ...

flows and hydroelectricity for the heavily populated

San Gabriel Valley The San Gabriel Valley ( es, Valle de San Gabriel) is one of the principal valleys of Southern California, lying immediately to the east of the eastern city limits of the city of Los Angeles, and occupying the vast majority of the eastern part ...

in the

Greater Los Angeles Greater Los Angeles is the second-largest metropolitan region in the United States with a population of 18.5 million in 2021, encompassing five counties in Southern California extending from Ventura County in the west to San Bernardino Coun ...

metropolitan area.

History

In the 1920s, the Los Angeles County Flood Control District proposed to impound the San Gabriel River just below the confluence of the East and West forks with a high, long concrete

arch dam An arch dam is a concrete dam that is curved upstream in plan. The arch dam is designed so that the force of the water against it, known as hydrostatic pressure, presses against the arch, causing the arch to straighten slightly and strengthen ...

to capture floods and provide water conservation. To be named the San Gabriel Forks Dam, the project was canceled by the State Engineer after having convened an inquiry to investigate problems which were occurring at San Gabriel Dam site, including a landslide that destroyed a large portion of the construction site, in early November 1929. The inquiry panel concluded and issued a report to the State Engineer stating that the proposed dam "cannot be constructed without creating a menace to life and property." Subsequently, the design of the San Gabriel River flood control project was changed from one large dam to two smaller dams: San Gabriel No. 1 (San Gabriel Dam), about below the original Forks site, and San Gabriel No. 2 (

Cogswell Dam Cogswell Dam is a rockfill dam on the West Fork of the San Gabriel River in Los Angeles County, California. It is located in the San Gabriel Mountains, northeast of Mount Wilson, and within the Angeles National Forest. It forms Cogswell Reser ...

), on the West Fork about above the confluence. The cost of San Gabriel No. 1 was greatly understated, creating a scandal; in context with the

Great Depression The Great Depression (19291939) was an economic shock that impacted most countries across the world. It was a period of economic depression that became evident after a major fall in stock prices in the United States. The economic contagio ...

, the Los Angeles ''Times'' observed that "it is not now necessary to gamble $26 million on a roubled and wasteful agency. The

bond measure A municipal bond, commonly known as a muni, is a bond issued by state or local governments, or entities they create such as authorities and special districts. In the United States, interest income received by holders of municipal bonds is often, ...

necessary for financing of the project was defeated by a narrow margin of 52 to 48 percent. As a result, the city turned to the federal

Public Works Administration The Public Works Administration (PWA), part of the New Deal of 1933, was a large-scale public works construction agency in the United States headed by Secretary of the Interior Harold L. Ickes. It was created by the National Industrial Recove ...

for money to complete the project. The San Gabriel No. 1 dam was to be made of rock-fill, which provides greater seismic stability. Construction of the high San Gabriel Dam at this site began in 1932 and was completed in 1939. In early 1938, before the dam was finished, Southern California was hit by record floods. The heaviest rains fell in the San Gabriel Mountains, swelling the San Gabriel River; on the night of March 2–3, 1938 a flood of poured out of the mountains and into San Gabriel Reservoir. San Gabriel Dam was able to knock about off the peak of the flood. Further downstream, Morris Reservoir was able to absorb roughly , reducing the flood to less than half of what it would have been if not for the dams.

Water management

Water stored behind San Gabriel Dam is an important source for

during the dry season of April through October. Water from San Gabriel, Cogswell and

s is released gradually through the dry months to

spreading ground A spreading ground is a water conservation facility that retains surface water long enough for it to percolate into the soil. Spreading grounds must be located where underlying soils are permeable and connected to a target aquifer. Locating them ...

s at San Gabriel Canyon (Azusa) and Peck Basin (near

Arcadia Arcadia may refer to: Places Australia * Arcadia, New South Wales, a suburb of Sydney * Arcadia, Queensland * Arcadia, Victoria Greece * Arcadia (region), a region in the central Peloponnese * Arcadia (regional unit), a modern administrative un ...

), where it percolates into the local groundwater basin. Dam operations are coordinated by the San Gabriel River Water Committee (Committee of Nine), established in 1889 to represent water-rights holders on the San Gabriel River and with rights to of river water; and the San Gabriel Valley Protective Association, which has rights to all water flows above 97,700 acre-feet as well as all storage space in San Gabriel, Cogswell and Morris Dams. The large reservoir, known as San Gabriel Reservoir No. 1, is nearly long when full. The reservoir stores of water when full, creating one of Southern California's largest instream reservoirs. This is 17% less than the original capacity of when the dam was first built, because

sedimentation Sedimentation is the deposition of sediments. It takes place when particles in suspension settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to the ...

has reduced the water volume. Interest in sediment removal began in the 1980s, after several major wildfires in the San Gabriel Mountains and subsequent flooding caused millions of cubic yards of sediment to wash into the reservoir. The reservoir was dewatered in 2004 to allow for sediment removal, but work was delayed after heavy rains during the winter of 2004–2005 refilled the reservoir. Disposal of the removed sediment has been a contentious issue. The 2005 sediment removal project stored about of sediment in a nearby canyon, but this is now prohibited due to environmental and safety issues. Another option would have been to truck the sediment to local beaches for replenishment, but this has also been banned by the

California Coastal Commission The California Coastal Commission (CCC) is a state agency within the California Natural Resources Agency with quasi-judicial control of land and public access along the state's coastline. Its mission as defined in the California Coastal Act is " ...

. Currently, most of the sediment is sold for building material or ends up as landfill, but these options are highly limited. Flood control releases are coordinated in conjunction with the other mountain dams, Morris and Cogswell; and with Santa Fe and

Whittier Narrows The Whittier Narrows is a narrows or water gap in the San Gabriel Valley of Los Angeles County, California, United States, between the Puente Hills to the east and the Montebello Hills to the west. The gap is located at the southern boundary of th ...

Dams on the lower San Gabriel River.

Power plant

The dam supports two small hydroelectric plants producing a maximum of 4.95 MW and owned by the Department of Public Works. Water from the reservoir can be bypassed through a tunnel called the Azusa Conduit to another power plant downstream of Morris Dam. The power plant is located on the south side of the San Gabriel River directly below the mouth of San Gabriel Canyon. The original power plant was first built in 1898, by the San Gabriel Electric Company, and during its first few decades of operation powered electric railway lines in Los Angeles. At the time, this 2000

kilowatt The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James Wa ...

(KW) plant drew water directly from the San Gabriel River. In 1917, the plant was sold to

Southern California Edison Southern California Edison (or SCE Corp), the largest subsidiary of Edison International, is the primary electricity supply company for much of Southern California. It provides 15 million people with electricity across a service territory of app ...

, before being incorporated into the Pasadena municipal electric system. The completion of San Gabriel Dam in 1939 ensured a steadier water supply for the power plant, and in the late 1940s a new 3000 KW facility was built to replace the old one.

Trivia

The dam appears in the 2001 action/war film Behind Enemy Lines and is set in southern

Bosnia Bosnia and Herzegovina ( sh, / , ), abbreviated BiH () or B&H, sometimes called Bosnia–Herzegovina and often known informally as Bosnia, is a country at the crossroads of south and southeast Europe, located in the Balkans. Bosnia and He ...

References

{{LA County dams Los Angeles County Department of Public Works dams Rock-filled dams San Gabriel River (California) San Gabriel Mountains Angeles National Forest Dams completed in 1939 1939 establishments in California

History

Water management

Power plant

Trivia

See also

References