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General Description| Plan| Development| Benefits

General Description

The American River Division of the Central Valley Project provides water for irrigation, municipal and industrial use, hydroelectric power, and recreation. Flood control is provided through a system of dams, canals, and powerplants. The division consists of the Folsom and Sly Park Units, both authorized in 1949, and the Auburn-Folsom South Unit, authorized in 1965.

The Folsom Unit consists of Folsom Dam, Lake, and Powerplant; Nimbus Dam and Powerplant; and Lake Natoma, all on the American River. The Sly Park Unit includes Sly Park Dam and Jenkinson Lake on Sly Park Creek, Camp Creek Diversion Dam on Camp Creek, and the Camino Conduit. All features of the Folsom and Sly Park Units are complete and in operation.

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Facility Descriptions - Folsom Unit

Folsom Dam

Originally authorized in 1944 as a 355,000 acre-feet flood control unit, Folsom Dam was reauthorized in 1949 as a 1,000,000 acre-feet multiple-purpose facility. The U.S. Army Corps of Engineers constructed Folsom Dam and transferred it to Reclamation for coordinated operation as an integral part of the Central Valley Project.

Construction of the dam began in October 1948 and was completed in May 1956. Water was first stored in February 1955.

Folsom Dam is a concrete gravity dam 340 feet high and 1,400 feet long. The main section is flanked by two earthfill wing dams. The right wing dam is 6,700 feet long and 145 high, and the left wing dam is 2,100 feet long and 145 feet high. In addition to the main section and wing dams, there is one auxiliary dam and eight smaller earthfill dikes.

Mormon Island Auxilary Dam

The Mormon Island Auxiliary Dam is in Blue Ravine. Along with eight other smaller dikes, it encloses low lying areas along the circumference of Folsom Lake The earthfill dikes range in height from 10 feet to 100 feet, and in length from 740 feet to 2,060 feet.

The auxillary dam is a rolled earthfill structure 4,820 feet long and 110 feet high. It has a volume of approximately 3,820,000 cubic yards. Its elevation is 480.5 ft. The earthfill dikes range in height from 10 feet to 100 feet, and range in length from 740 feet to 2,060 feet.

The combined length of the main dam, wing dams, auxiliary dam, and dikes is 26,730 feet, or over five miles. The total volume of materials in the dam, wing dams, auxiliary dam, and dikes is 13,970,000 cubic yards, including 1,050,000 cubic yards of concrete in the main section. The spillway in the concrete main section is divided into eight sections, each controlled by a 42- by 50-foot radial gate. The capacity of the spillway is 567,000 cubic feet per second.

The dam regulates flows of the American River for irrigation, power, flood control, municipal and industrial use, fish and wildlife, recreation, and other purposes.

Folsom Lake

Folsom Dam forms Folsom Lake. Folsom Lake has a capacity of 1,010,00 acre-feet with a surface area of 11,450 acres.

Folsom Lake is the most popular multi-use year round unit in the California State Park System. Recreation facilities at the 18,000-acre park,which is administered by the California Department of Parks and Recreation, include 50 miles of trails for hiking and horseback riding, picnicking, fishing, swimming, boating, water skiing, and camping.

Folsom Powerplant

Reclamation constructed and operates Folsom Powerplant at the foot of Folsom Dam on the north side of the river. Water from the dam is released through three 15-foot-diameter penstocks to three generating units. The plant has three generating units, each rated at just over 76,000 kilowatts, with a combined rating of 198,720 kilowatts. Water is supplied to the three 74,000 horsepower turbines that drive the generators through three 560-foot-long, 15.5-foot-diameter penstocks that run through the right abutment of the main dam.

Work on the Folsom Powerplant began in early June 1951 with construction of the tailrace channel access road. Excavation for the tailrace channel began on June 27, 1951, with excavations for the powerhouse beginning on July 7. Work under the powerplant contract began on May 28, 1952, with excavations on the first of three power penstock tunnels. Each of the tunnels was driven 30 to 40 feet from the upstream end, and the work was then advanced from the downstream end to meet with the upstream headings.

Concrete work at Folsom Powerplant began on October 24, 1952, with placement of concrete in the gravity training wall between the powerplant site and spillway stilling basin. First placement of concrete in the powerhouse was on April 3, 1953. Work from the downstream headings began on May 25, 1953, with the last of the three tunnels being holed through on September 14, 1953. The tunnels were cleaned and readied for placement of the penstock pipes by October 9. Installation of the three, 15.5-foot diameter penstock tubes that supply water to the turbines at the Folsom Powerplant began October 5, 1953.

Nimbus Dam and Lake Natoma

Nimbus Dam is on the American River in Sacramento County, California, 7 miles downstream from Folsom Dam. It reregulates the releases for power made through the Folsom Powerplant.

Nimbus Dam is a concrete gravity dam 1,093 feet long and 87 feet high. Eighteen radial gates, each 40-feet by 24-feet, control the flows. The total volume of material used in the dam is 121,100 cubic yards. Reclamation operates the dam.

Nimbus Dam and Powerplant was completed and accepted by the Government in July 1955.

Nimbus Dam forms Lake Natoma, with a capacity of 8,760 acre-feet and a surface area of 540 acres.

Nimbus Powerplant

Nimbus Powerplant, constructed and operated by Reclamation, is located on the right abutment of Nimbus Dam, on the north side of the river. Each of its two generators has a capacity of 7,763 kilowatts, with a combined output of 3,500 kilowatts. Water is supplied to two 9,400 horsepower turbines that drive the generators through six 46.5-foot-long by 13.75-foot by 15.95-foot penstocks. It started operating in 1955.

The powerplant consists of two vertical shaft, synchronous, 3-phase generators with a combined rating of 13.5 MW. The generators were manufactured by the Elliot Company under Specification Number DC-3696. The generators are rated at 6.75 MVA at 0.90 power factor, 150 rpm, 4.16 kV, 60 hertz. The turbines are Kaplan type rated at 41.5-foot head.

Reclamation operates this powerplant.

Nimbus Fish Hatchery

Construction of Folsom and Nimbus Dams blocked access to natural spawning grounds of salmon and steelhead trout. To compensate for the loss of these spawning areas, Reclamation constructed a fish hatchery about a quarter of a mile downstream from Nimbus Dam. The hatchery is on the left bank of the river, about 0.3 mile below the dam.

It is operated by the State of California with Reclamation funds. Work began on April 20, 1955, with excavations for holding pond No.1. Lining of the holding ponds began on May 5, with erection of the office, processing, and hatchery buildings beginning on July 28. All machinery, processing equipment and refrigeration systems were installed and tested by October 7. All work was finished and accepted on October 17, 1955.

The Nimbus Fish Hatchery has a capacity of 30,000,000 eggs. Water for the hatchery is supplied through a 1,415 foot long, 42-inch diameter concrete pipe that runs from the left abutment of Nimbus Dam.

Lake Natoma forms an attractive recreation area. Facilities administered by the California Department of Parks and Recreation provide boating, picnicking, swimming, fishing, and camping.

Facility Descriptions - Sly Park Unit

Reclamation constructed the Sly Park Unit, including Sly Park Dam and Jenkinson Lake, Camp Creek Diversion Dam and the Camino Conduit, and Camino and Camp Creek Tunnels. THis provides municipal and industrial water for the nearby community of Placerville, and irrgation water for the El Dorado Irrigation District. Camp Creek Diversion Dam diverts some flow from Camp Creek to Jenkinson Lake via Camp Creek Tunnel. The Camino Tunnel and Conduit delivers water from Jenkinson Lake to the El Dorado Irrigation District for Irrigation and municpal use.

Sly Park Dam and Jenkinson Lake

Work on the Sly Park Unit began November 20, 1952, with preparations for `holing in` the Camp Creek Diversion Tunnel to carry water from Camp Creek to Sly Park. Excavations on the tunnel itself were underway by the end of 1952, and the tunnel was `holed through` on April 19, 1953. Work on the tunnel was completed by the end of October. Work at the Sly Park site began with clearing operations in early May 1953. Reclamation completed the project and transferred it to the El Dorado irrigation District for operation and maintenance in mid-1955.

Sly Park Dam, on Sly Park Creek, is a zoned earthfill structure, 190 feet high, with a crest length of 760 feet. It has an auxiliary earthfill dam, 130 feet high, with a crest length of 600 feet. The total volume of the main dam and dike is 1,130,000 cubic yards. The outlet works consist of concrete conduits through the base of the main dam that are controlled by two, 2.25-foot square high pressure gates. The capacity of the outlet works is 250 cubic feet per second. The spillway is an uncontrolled, concrete lined chute in the left abutment of the auxiliary dike, with a capacity of 6,700 cubic feet per second.

Excavations for the foundation began on June 25, 1953, with work on the right abutment. Excavations for the outlet works began on July 3, with work in the spillway area beginning on July 16. Concrete operations began on August 25, with concrete placement in the outlet works beginning in early September. Embankment placing operations started on October 28, 1953, and were completed in November 1954. In early 1955, some cracking and movement was noted in the crest, but the situation corrected itself, and the embankment stabilized without repairs.

Jenkinson Lake, formed by Sly Park Dam, has a storage capacity of 41,000 acre-feet with a surface area of 650 acres. Municipal and industrial water is furnished to the city of Placerville and nearby small communities, and irrigation water is furnished to El Dorado Irrigation District.

Camino Conduit

The Camino Conduit, with a capacity of 125 cubic feet per second, extends 38,016 feet west from Sly Park Dam to the community of Camino, California, to deliver supplemental water to El Dorado Irrigation District for irrigation and municipal purposes. The conduit starts with a diameter of 4 feet and is reduced to 3 feet. It is constructed of precast concrete and welded plate steel. Excavations for the Camino Conduit Tunnel began on December 11, 1953. The Camino Tunnel is 2,289 feet long with a diameter of seven feet. It is a steel reinforced, concrete lined structure with a capacity of 125 cubic feet per second.

The Sly Park Recreation Area, operated by El Dorado Irrigation District in cooperation with Reclamation, offers camping, boating, swimming, picnicking, and fishing.

Camp Creek Diversion Dam

Camp Creek Diversion Dam is a concrete overflow weir, 20 feet high and 119 feet long. The total volume of concrete in the dam is 2,000 cubic yards, and the diversion capacity is 500 cubic feet per second. Camp Creek Dam diverts a portion of the flow of Camp Creek through the Camp Creek Tunnel, a 2,845 foot long, concrete lined tunnel with a diameter of 7 feet and a capacity of 500 cubic feet per second, into the upper part of Jenkinson Lake.

Work on the dam began February 25, 1953, and was completed on November 29, 1953.

Distribution System

Construction has been completed on the water treatment facilities and a distribution system for irrigation and municipal purposes in western El Dorado County. These facilities were built by Reclamation and will be operated, maintained, and paid for by the El Dorado Irrigation District.

The distribution system consists of three pipelines, the El Dorado Main, Pleasant Oak Main, and El Dorado Main No. 2. These pipelines extend a total of 46.3 miles, from the vicinity of Sly Park Dam to the community of Placerville, California.

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The first investigations into water development on the American River were included in the Alexander Report, conducted by Corps in the 1870s and presented to Congress by President Grant in 1874. In the 1930s, the State Water Plan called for the construction of a 355,000 acre-feet (acre-feet) reservoir near the town of Folsom. The Flood Control Act of 1944 authorized construction of Folsom Dam as proposed by the State of California Water Plan. The final authorization came in the American River Division Authorization Act of October 14, 1949.

Folsom and Sly Park Units

The Folsom and Sly Park Units were authorized in 1949 as part of the American River Division. Folsom Dam was originally authorized in the Flood Control Act of 1944 as a 355,000 acre-feet flood control unit. The reauthorization under the American River Division Authorization Act of 1949 changed the dam to a 1,000,000 acre-feet multipurpose facility with a 162,000 kilowatt powerplant. The act also authorized construction of Nimbus Dam and Lake Natoma as a regulating reservoir for the Folsom Powerplant.

The authorization for construction of Nimbus Dam included a 13,500 kilowatt powerplant and a fish hatchery. The Sly Park Unit was authorized under the same act as the Folsom Unit to provide irrigation and municipal water to the El Dorado Irrigation District and the communities of Placerville and Camino.

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The uses of water from the American River Division vary greatly, from flood control to recreation and fisheries enhancement.

Flood Control

First and foremost on the list of water use priorities is flood control. The primary flood control feature of the American River Division is Folsom Dam and Reservoir. Many times since it was placed in service, the dam has demonstrated its ability to harness and control potentially devastating floods of the American River.

Since its completion, no unit in California has controlled flooding more effectively than the Folsom Unit. Folsom and Nimbus Dams prevented $20,000,000 in damage in the 1955 floods. In 1963 and 1964, Folsom and Nimbus tamed 6-day flows of 630,000 acre-feet and 990,000 acre-feet, respectively, preventing an estimated total of $90,000,000 in damage. But the toughest test came in the winter of 1986. During the 6 day period beginning February 14, 1986, Folsom and Nimbus held in check inflows of greater than 1,140,000 acre-feet, well above the design limit of 978,000 acre-feet for a 6 day flood. By 1994, the Folsom/Nimbus combination had prevented an estimated total of more than $4,830,000,000 in flood damage.

Water Supply

Folsom Dam provides 500,000 acre-feet of water for irrigation and municipal and industrial uses. Downstream from Folsom, Nimbus Dam and Lake Natoma combine to regulate the often fluctuating releases from Folsom, maintaining a consistent flow down river and providing a reliable water supply for the Folsom South Canal and Nimbus Fish Hatchery.

Sly Park Dam and Jenkinson Lake are operated independently from other units of the Central Valley Project. Water from Jenkinson Lake and Camp Creek is supplied to the cities of Placerville and Camino for municipal and industrial uses and to the El Dorado Irrigation District for agricultural use. Recreational facilities at Jenkinson Lake include facilities for boating, swimming, and camping, and are operated by the El Dorado Irrigation District in cooperation with Reclamation. The primary agricultural products grown with water supplied by the Sly Park Unit are apples and pasture hay, with the rest of the water divided among other orchard crops and nursery plants.

In 1991, the total number of acres in the American River Division irrigated with project water was more than 7,000 acres, with a total crop value of more than $12,000,000. Municipal and industrial uses far exceed other uses of water supplied by units of the American River Division. In 1991, over 51,000 acre-feet of water was supplied to nonagricultural users. Of that, over 39,000 acre-feet was supplied by the Folsom Unit.

Fisheries Enhancement and Water Quality

Folsom Dam also plays an important role in fisheries enhancement and water quality improvement. Due to recent changes in operation of Shasta Dam to enhance the salmon run on the Sacramento River, water releases from Folsom have been used to fulfill water delivery obligations and downstream water quality standards that would normally be met by releases from Shasta.


Power production by the Folsom and Nimbus Powerplants in 1991 was over 264,000,000 kilowatts. Power generated at the Folsom and Nimbus Powerplants is marketed by the Western Area Power Administration.


As a recreational facility, Folsom is second to none, having the highest number of visitor days per year of any recreation area operated by the California Department of Parks and Recreation.

Lake Natoma is a popular recreation area with fishing, boating, and camping facilities provided by the California Department of Parks and Recreation.

Recreational Opportunities


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Last updated: Jan 31, 2012