Hoover Hydroelectric Dam, once known as Boulder Dam, is a concrete arch-gravity dam in the Black Canyon of the Colorado River, on the border between the US states of Arizona and Nevada.

Excavation for the powerhouse was carried out simultaneously with the excavation for the dam foundation and abutments. A U-shaped structure located at the downstream toe of the dam, its excavation was completed in late 1933 with the first concrete placed in November 1933. Filling of Lake Mead began February 1, 1935 even before the last of the concrete was poured that May. The powerhouse was one of the projects uncompleted at the time of the formal dedication on September 30, 1935—a crew of 500 men remained after the dedication to finish it and other structures. To make the roof of the powerhouse proof against bombs, it was constructed of layers of concrete, rock, and steel with a total thickness of about 3.5 feet (1.1 m), topped with layers of sand and tar.

In the latter half of 1936, water levels in Lake Mead were high enough to permit power generation, and the first three generators, all on the Nevada side, began operating. In March 1937, one more Nevada generator went online and the first Arizona generator by August. By September 1939, four more generators were operating and the dam's power plant became the largest hydroelectricity facility in the world. The final generator was not placed in service until 1961, bringing the maximum generating capacity to 1345 megawatts at the time. Original plans called for 16 large generators, eight on each side of the river, but two smaller generators were installed instead of one large one on the Arizona side for a total of 17. The smaller generators were used to serve smaller municipalities at a time when the output of each generator was dedicated to a single municipality, before the dam's total power output was placed on the grid and made arbitrarily distributable. The present contracts for the sale of electricity expire in 2017.

Before water from Lake Mead reaches the turbines, it enters the intake towers and enters four gradually narrowing penstocks which funnel the water down towards the powerhouse. The intakes provide a maximum hydraulic head (water pressure) of 590 ft (180 m) as the water reaches a speed of about 85 mph (140 km/h). The entire flow of the Colorado River passes through the turbines. The spillways and outlet works (jet-flow gates) are rarely used.[2] The jet-flow gates, located in concrete structures 180 feet (55 m) above the river and at river level, may be used to divert water around the dam in emergency or flood conditions, but have never done so, and in practice are only used to drain water from the penstocks for maintenance.[87] Following an uprating project from 1986 to 1993, the total gross power rating for the plant, including two 2.4 megawatt electric generators that power Hoover Dam's own operations is a maximum capacity of 2080 megawatts. The annual generation of Hoover Dam varies. The maximum net generation was 10.348 TWh in 1984, and the minimum since 1940 was 2.648 TWh in 1956. The average has been about 4.2 TWh/year.

Control of water was the primary concern in the building of the dam. Power generation has allowed the dam project to be self sustaining: proceeds from the sale of power repaid the 50-year construction loan, and those revenues also finance the multi-million dollar yearly maintenance budget. Power is generated in step with and only with the release of water in response to downstream water demands. Lake Mead and downstream releases from the dam provide water for both municipal and irrigation uses. Water released from the Hoover Dam eventually reaches the All-American Canal for the irrigation of over 1,000,000 acres (400,000 ha) of land. Water from the lake serves 8 million people in Arizona, Nevada and California.

Power from the dam's powerhouse was originally sold pursuant to a fifty-year contract which terminated in 1987. When the contracts ended, the Bureau of Reclamation assumed control of the powerplant from the Los Angeles Department of Water and Power and Southern California Edison Co. The contracts were renegotiated and implemented for a 30-year period, and will expire in 2017. The Bureau of Reclamation reports that the energy generated is allocated as follows:

Area Percentage
Metropolitan Water District of Southern California 28.53%
State of Nevada 23.37%
State of Arizona 18.95%
Los Angeles, California 15.42%
Southern California Edison Company 5.54%
Boulder City, Nevada 1.77%
Glendale, California 1.59%
Pasadena, California 1.36%
Anaheim, California 1.15%
Riverside, California 0.86%
Vernon, California 0.62%
Burbank, California 0.59%
Azusa, California 0.11%
Colton, California 0.09%
Banning, California 0.04%

Hoover Hydroelectric Dam
Official name Hoover Dam
Locale Clark County, Nevada / Mohave County, Arizona, USA
Construction began 1931
Opening date 1936
Construction cost $49 million
Owner(s) United States Government
Dam and spillways
Type of dam Concrete gravity-arch
Height 726.4 ft (221.4 m)
Length 1,244 ft (379 m)
Crest width 45 ft (14 m)
Base width 660 ft (200 m)
Volume 3,250,000 cu yd (2,480,000 m3)
Impounds Colorado River
Type of spillway 2 x controlled drum-gate
Spillway capacity 400,000 cu ft/s (11,000 m3/s)
Creates Lake Mead
Capacity 35.2 km3 (28,500,000 acre·ft)
Catchment area 167,800 sq mi (435,000 km2)
Surface area 247 sq mi (640 km2)
Max. water depth 590 ft (180 m)
Power station
Operator(s) U.S. Bureau of Reclamation
Hydraulic head 590 ft (180 m) (Max)
Turbines 17 main, Francis-type
Installed capacity 2080 MW
Annual generation 4.2 billion KWh