Hoover 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. It was constructed between 1931 and 1936, and was dedicated on September 30, 1935, by President Franklin Roosevelt. Its construction was the result of a massive effort involving thousands of workers, and cost over one hundred lives.
Since about 1900, the Black Canyon and nearby Boulder Canyon had been investigated for their potential to support a dam that would control floods, provide irrigation water and produce hydroelectric power. In 1928, Congress authorized the project. The winning bid to build the dam was submitted by a consortium called Six Companies, Inc., which began construction on the dam in early 1931. Such a large concrete structure had never been built before, and some of the techniques were unproven. The torrid summer weather and the lack of facilities near the site also presented difficulties. Nevertheless, Six Companies turned over the dam to the federal government on March 1, 1936, more than two years ahead of schedule.
Hoover Dam impounds Lake Mead, and is located near Boulder City, Nevada, a municipality originally constructed for workers on the construction project, about 25 mi (40 km) southeast of Las Vegas, Nevada. The dam’s generators provide power for public and private utilities in Nevada, Arizona, and California. Hoover Dam is a major tourist attraction, and U.S. 93, a heavily travelled road, runs along its crest. In late 2010, through traffic is scheduled to be rerouted onto a bypass presently under construction, and the dam roadway will be restricted to use by visitors.
Excavation for the powerhouse was carried out simultaneously with the excavation for the dam foundation and abutments. Excavation for the U-shaped structure located at the downstream toe of the dam 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.
The agreement whereby the government accepted the dam from Six Companies included a provision allowing the Bureau of Reclamation to use Six Companies-owned machinery in the canyon for seven months to allow for the installation of additional powerhouse equipment. In the last 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. 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.The changes in water use caused by Hoover Dam’s construction had an impact on the Colorado River Delta. The construction of the dam has been credited as causing the decline of this estuarine ecosystem. For six years, after the construction of the dam and while Lake Mead filled, virtually no water reached the mouth of the river. The delta’s estuary, which once had a freshwater-saltwater mixing zone stretching 40 miles (64 km) south of the river’s mouth, was turned into an inverse estuary where the level of salinity was actually higher closer to the river’s mouth.
The Colorado River had experienced natural flooding before the construction of the Hoover Dam. The dam eliminated the natural flooding, which imperiled many species adapted to the flooding, including both plants and animals. The construction of the dam decimated the populations of native fish in the river downstream from the dam. Four species of fish native to the Colorado River, the Bonytail chub, Colorado pikeminnow, Humpback chub, and Razorback sucker, are currently listed as endangered.