Trinity Dam

Trinity Dam
Country	United States
Location	Whiskeytown-Shasta-Trinity National Recreation Area,Trinity County,California
Coordinates	40°48′04″N122°45′48″W
Construction began	1957(1957)
Opening date	1962(1962)
Owner(s)	U.S. Bureau of Reclamation
Dam and spillways
Type of dam	Earthfill
Impounds	Trinity River
Height	538 ft (164 m)
Length	2,450 ft (750 m)
Spillway type	Morning glory
Spillway capacity	22,400 cu ft/s (630 m3/s)
Reservoir
Creates	Trinity Lake
Total capacity	2,447,650acre⋅ft(3,019,130dam3)
Inactive capacity	400,000 acre⋅ft (490,000 dam3)
Catchment area	692 sq mi (1,790 km2)
Surface area	17,722 acres (7,172 ha)
Power Station
Hydraulic head	426 ft (130 m)
Turbines	2xFrancis
Installed capacity	140MW
Annual generation	424,192,000kWh(2001–2012)

Trinity Damis an earthfill dam on theTrinity Riverlocated about 7 miles (11 km) northeast ofWeaverville,Californiain theUnited States.The dam was completed in the early 1960s as part of the federalCentral Valley Projectto provide irrigation water to the aridSan Joaquin Valley.

Standing 538 ft (164 m) high, Trinity Dam formsTrinity Lake– California's third largest reservoir, with a capacity of more than 2,400,000acre-feet(3,000,000dam³). The dam includes a hydroelectric plant, and also providesflood controlto the Trinity and Klamath river basins.

Below the dam isLewiston Lake,formed by a second dam, which diverts water through a 10.7 mile tunnel to the Sacramento Valley.

Background

In response to theGreat Depressionand drought conditions in California during the early 20th century, the United States Congress passed the 1935Rivers and Harbors Act,which authorized the Central Valley Project (CVP) – a system of dams and canals to provide a stable supply of irrigation water to California'sCentral Valley.Among the project works was a 1942 proposal to divert water from the Trinity River in northwestern California to augment water supplies in the CVP service area, known as the Trinity River Division. However, the state dropped the Trinity River project from the CVP in 1945.

Six years later, however, theU.S. Bureau of Reclamation(USBR), which was responsible for the construction and operations of most CVP facilities, revived the division, which comprised a system of four dams and two tunnels to capture and store the flow of the Trinity and transport it to theSacramento River,generating a net surplus of hydroelectric power along the way. Trinity Dam was to be the main storage feature of the division, providing a stable flow to theLewiston Dam,the diversion point for Trinity River waters into the Central Valley.

Construction

Construction at Trinity Dam began on June 4, 1956 with preliminary excavation and the river was diverted around the dam site on July 8, 1957. Work was bogged down in late 1957 when the river flooded and took out thecofferdamthat protected the construction site. However, workers cleared out the dam site by the summer of 1958, and the hiatus also allowed them time to line the diversion tunnel with concrete.

Rock placement began on the main structure as early as 1957, but it was not until 1959 when an 11,000 ft (3,400 m) conveyor belt was incorporated, allowing much faster transport of fill to the dam site. The spillway tunnel was completed in October 1959 and the embankment was topped out in late 1960. The dam's power plant, however, was not finished until December 1963. Trinity stood as the highest earthfill dam in the world until eclipsed by theOroville Dam,also in California, in 1968.The latter remains the tallest dam in the United States.

As the reservoir filled, it submerged the pioneer towns of Trinity Center, Stringtown and Minersville as well as parts of a historic Northern California stagecoach route.The reservoir was originally named Clair Engle Lake afterClair Engle,a United States senator and major proponent of the CVP. This name was unpopular among local residents, many of whom had watched their homes inundated by the rising waters. By 1995, it had been renamed Trinity Lake.

Specifications

Overview of Trinity Dam complex showing the spillway to the left and powerhouse in right center

Trinity Dam is an earth and rock-filled dam composed mainly of river gravel and local rock, rising 538 ft (164 m) from its foundations and 440 ft (130 m) above the riverbed. The dam's crest is 2,450 ft (750 m) long and 2,395 ft (730 m) above sea level.High water releases are controlled by three different sets of gates. Theoutlet workshave a capacity of 24,000 cu ft/s (680 m³/s), and the service spillway, a morning-glory (bell-mouth) design, can take 22,400 cu ft/s (630 m³/s) of water. The dam's auxiliary spillway has a capacity of 2,250 cu ft/s (64 m³/s).

Trinity Lake can hold up to 2,447,650acre-feet(3,019,130dam³) at full pool, and encompasses up to 17,722 acres (7,172 ha) of water.Trinity Powerplant, located at the base of the dam, has twoFrancis turbineswith a capacity of 140megawatts(MW) combined, uprated from an original capacity of 100 MW. The plant generates an average of 358.97 millionkilowatt hours(kWh) and has a lowcapacity factor,which means it operates mainly on apeakingbasis.

Salmon Hatchery

Trinity Dam forms an impassable barrier for migratingsalmonin the Trinity River, but along with other division facilities has also had adverse impacts on salmon habitat in the lower reaches of the river. Within ten years of the dam's completion, more than three-quarters of Trinity River water was being diverted to the Central Valley each year, and the little flow left in the river was warm, shallow and less suitable for spawning salmon. In addition, the dam blocks the Trinity River's annual floods and sediment loads, leading to degradation in the gravel bars that are vital for salmon habitat.

In response to declining salmon populations, the USBR built the Trinity River Fish Hatchery. The hatchery is operated by theCalifornia Department of Fish and Gameand has a capacity of 40 million eggs.In 1992, Congress passed the Central Valley Project Improvement Act, which established a minimum annual release of 340,000acre-feet(420,000dam³) into the lower Trinity River.

Also, below the Trinity Dam, is theLewiston Dam (California)in which that water is sent to other water customers that live throughout the State.This is important because this requires the help of two dams just to move water from the reservoir to the customers that require water for either Agriculture or Urban uses.

Recreation

With 145 miles (233 km) of shoreline,Trinity Lake is a popular summer destination for Northern California residents. Since 1965, the lake has been part of the Trinity Unit of theWhiskeytown-Shasta-Trinity National Recreation Area.The lake has been developed with boat ramps and marinas, most of which are concentrated in the Stuart Fork arm in the southwest and around the resort town ofTrinity Centernear the north end of the reservoir.

See also

• List of dams and reservoirs in California
• List of largest reservoirs of California
• List of power stations in California
• List of the tallest dams in the United States

External links

• Bathymetric map and navigation safety on Trinity Lake
• California Department of Water Resources: Trinity Lake Reservoir Up-to-Date Data
• Trinity River Restoration Program
• USBR Central Valley Operations OfficeArchived2004-02-09 at theWayback Machine