Hoover Dam is a 221-metre (726 ft) concrete arch-gravity dam in Black Canyon on the Colorado River, straddling the border between Nevada and Arizona about 48 km (30 mi) southeast of Las Vegas. Six Companies, Inc. built it between 1931 and 1936, and the federal government accepted the finished dam on 1 March 1936, more than two years ahead of schedule. It holds back Lake Mead, the largest reservoir in the United States by capacity at about 34.8 km³ (28.2 million acre-feet) when full, and its power plant is rated at roughly 2,080 MW. The dam tamed a flood-prone desert river, and Lake Mead's water now serves around 18 million people in Arizona, Nevada and California and irrigates more than 400,000 hectares of farmland.
1902–1928: From an Edison Survey to the Boulder Canyon Project Act
The dam's story starts with a river that kept escaping control. In the late 1890s the land speculator William Beatty cut the Alamo Canal to carry Colorado River water into a desert he named the Imperial Valley. The canal dipped through Mexico and was hard to maintain, and after a catastrophic breach the river poured into the Salton Sea; the Southern Pacific Railroad spent $3 million in 1906–07 closing the gap. In 1902 the Edison Electric Company of Los Angeles surveyed the lower river for a 12-metre rock dam, but power could then be transmitted only about 130 km, and Edison let its land options lapse, including one on the future dam site.
The Reclamation Service, led by Arthur Powell Davis, took up the idea. Davis once proposed collapsing the walls of Boulder Canyon with dynamite and building a dam from the rubble; the plan was dropped in 1922. That year the Fall-Davis report argued that a river crossing several states and entering Mexico was a federal matter. Engineers found Boulder Canyon unsuitable, with a geologic fault at one site and no room for a camp or spillway at others, and moved the project to Black Canyon, where a railway could reach the rim from Las Vegas. The name "Boulder Canyon Project" stuck anyway.
Water had to be divided before concrete could be poured. Delph Carpenter, a Colorado lawyer, proposed an interstate compact, and on 24 November 1922 the seven basin states signed the Colorado River Compact after talks with Commerce Secretary Herbert Hoover. Congressman Phil Swing and Senator Hiram Johnson of California pushed authorising bills for years. Two disasters changed minds: the 1927 Mississippi flood, and the collapse of Los Angeles's St. Francis Dam on 12 March 1928, which killed up to 600 people; reviewers then demanded "conservative if not ultra-conservative" design. President Calvin Coolidge signed the Boulder Canyon Project Act on 21 December 1928, appropriating $165 million for the dam, Imperial Dam and the All-American Canal.
1931–1933: Six Companies, Ragtown and Four Diversion Tunnels
The Bureau of Reclamation's chief design engineer, John L. Savage, oversaw a wedge-shaped dam 200 m thick at the base and 14 m at the crest. Bid documents went on sale on 10 January 1931 for five dollars a copy. The winning bid of $48,890,955 came from Six Companies, Inc., a joint venture of the Utah Construction Company, Morrison-Knudsen, Pacific Bridge, Henry J. Kaiser and W. A. Bechtel, MacDonald & Kahn and the J.F. Shea Company. Morrison-Knudsen's Frank Crowe, the country's leading dam builder, ran the job as general superintendent.
The Great Depression supplied the labour. Las Vegas, a town of about 5,000, saw between 10,000 and 20,000 jobless people arrive. Families squatted at McKeeversville and at a riverside camp officially called Williamsville but known as "Ragtown", because President Hoover ordered work to begin in March 1931, before the company town of Boulder City was ready. That summer the daytime high averaged 48.8 °C, and 16 workers and riverbank residents died of heat prostration between 25 June and 26 July. When Six Companies cut tunnel workers' pay on 7 August 1931, the men struck; Crowe laid off almost everyone on 9 August, rehiring began on 13 August, and the strike ended two days later. Employment peaked at 5,251 in July 1934. The contract barred Chinese labour, and the company never employed more than 30 Black workers, who were kept in a segregated crew.
First the Colorado had to be pushed out of its bed. Crews drove four tunnels, two through each canyon wall, each 17 m in diameter and together nearly 5 km long, then lined them with a metre of concrete. On 13 November 1932 a temporary cofferdam was blasted and rubble dumped into the channel, sending the Colorado into the two Arizona tunnels. Upstream and downstream cofferdams were built, the riverbed was drained, and "high scalers" dangled on ropes to jackhammer loose rock from the walls. Some dipped cloth hats in tar for protection, and Six Companies went on to order thousands of the "hard boiled hats" that became the modern hard hat.
1933–1936: Concrete Columns, Art Deco and Roosevelt's Dedication
The first concrete went in on 6 June 1933. Engineers calculated that a single monolithic pour would take 125 years to cool and would crack as it did, so the dam was built as interlocking columns in blocks up to 15 m square and 1.5 m high. More than 937 km of 25 mm steel pipe carried river water and then refrigerated water through the blocks to draw out the heat of curing, after which the pipes and joints were grouted. Crowe patented the 18-tonne buckets that swung 6 m³ of concrete at a time along aerial cableways. Pouring ended on 29 May 1935 after 2.48 million m³ in the dam alone, plus 850,000 m³ in the power plant and other works. Despite the legend, nobody is buried in the concrete: each bucket raised a form by only about 25 mm.
The look of the dam changed late. The Bureau's first facade, with a Gothic balustrade and eagles, was judged too plain, and the Los Angeles architect Gordon B. Kaufmann restyled the whole complex in Art Deco, adding the intake-tower clocks set to Nevada and Arizona time. The Denver artist Allen Tupper True designed floors and walls from Navajo and Pueblo motifs, and the sculptor Oskar J. W. Hansen made the 9-metre bronze Winged Figures of the Republic, set above a terrazzo star map fixing the sky at the moment of dedication.
That dedication came on 30 September 1935, when President Franklin D. Roosevelt spoke to 10,000 people in 39 °C heat without mentioning Hoover, who had not been invited. The official death list for the project counts 112 people from 1921 to 1935, 96 of them at the site during construction; 42 further deaths recorded as pneumonia, which workers said hid carbon-monoxide poisoning in the tunnels, are not included.
1933–1947: Boulder Dam or Hoover Dam?
Interior Secretary Ray Wilbur named the structure "Hoover Dam" on 17 September 1930, at the ceremony starting the railway from Las Vegas. After Hoover lost the 1932 election, Wilbur's successor Harold Ickes ordered on 13 May 1933 that it be called Boulder Dam, and the 1935 commemorative three-cent stamp carried that name. Ickes said "Boulder Dam" five times in thirty seconds at the dedication. Mapmakers split and the public used both names until 1947, when Congress unanimously restored "Hoover Dam".
The dam was producing power by then. Lake Mead began filling on 1 February 1935, and the first three Allis-Chalmers Francis generators, all on the Nevada side, started in the second half of 1936. By September 1939 the plant was the largest hydroelectric facility in the world. The seventeenth and last main generator went into service in 1961, bringing capacity to 1,345 MW. Downstream the cost showed up in Mexico: for six years while the reservoir filled almost no water reached the Colorado River Delta, and four native fish, among them the razorback sucker and humpback chub, are now listed as endangered.
1983–2010: Spillway Damage, Uprating and the Bypass Bridge
Hoover's two spillways, each with four 30-metre steel drum gates weighing 2,300 tonnes apiece, have run only twice: in a 1941 test and during the 1983 flood. Both times cavitation tore into the tunnel linings, and after 1983 aerators were fitted. An uprating programme from 1986 to 1993 lifted the plant's rating to about 2,080 MW. In 1987 the Bureau of Reclamation took over the powerhouse from the Los Angeles Department of Water and Power and Southern California Edison, and a 2011 law extended power contracts to 2067, setting aside 5% for tribes, cooperatives and other new buyers.
The crest also carried U.S. Route 93 until security concerns after 11 September 2001 led to restrictions on trucks and buses. The four-lane Hoover Dam Bypass opened on 19 October 2010, carrying traffic over the Mike O'Callaghan–Pat Tillman Memorial Bridge 460 m downstream. Visitors still cross the crest by car on the Nevada approach, and nearly a million people a year take the Bureau's tours.
2000–2026: Drought, Wide-Head Turbines and Lake Mead's White Ring
Lake Mead has not been full since 1983. A drought that set in around 2000, together with heavy demand on the river, cut both storage and output. By 2014 the plant had been down-rated by 23% to 1,592 MW and ran mainly at peak hours. The lake fell to 326.6 m (1,071.6 ft) above sea level on 1 July 2016, and slipped below that on 10 June 2021. The original turbines could not run below 320 m (1,050 ft), so five wide-head turbines designed for lower flows were installed to push the minimum power pool down to about 290 m (950 ft).
By May 2022 the reservoir held only 26.6% of capacity, low enough to expose the ruins of St. Thomas, Nevada, a town abandoned to the rising lake in 1938. Conservation deals raised the level by about 5 m over the following two years, but by August 2026 the lake had fallen to its lowest level since first filling: 317.1 m (1,040.4 ft) on 7 August 2026 and 316.6 m (1,038.8 ft) on 31 August 2026, below the 320 m level at which the original turbines would have stopped. A white mineral band on the walls marks the old waterline.
The dam's fate is now tied to the whole river system: what reaches Black Canyon depends on releases from Glen Canyon Dam and Lake Powell upstream, and on how the seven compact states and Mexico share a shrinking flow. Hoover was listed on the National Register of Historic Places in 1981 and became a National Historic Landmark in 1985. To follow the water from the Rocky Mountains to the sea, read our guide to the Colorado River, and see what decades of damming did at the river's mouth in our piece on the Colorado River Delta.