Hoover Dam (originally authorized as Boulder Dam or the Boulder Canyon Project) is a concrete arch-gravity dam in Black Canyon on the Colorado River, on the Arizona–Nevada border about 30 miles southeast of Las Vegas. Built between 1931 and 1936 during the Great Depression, it stands 726.4 feet high, stretches 1,244 feet along its crest, is 660 feet thick at the base and 45 feet at the crest, and contains about 3.25 million cubic yards of concrete (roughly 4.4 million cubic yards including appurtenant works). It creates Lake Mead, one of the largest reservoirs in the United States by capacity (historically around 28–32 million acre-feet depending on sedimentation surveys and measurement basis).
Why It Was Built
In the early 20th century, the Colorado River was both a vital resource and a destructive force for the arid Southwest. Seasonal floods devastated farmland (most dramatically in 1905–1907, when the river broke through irrigation canals and created the Salton Sea in California’s Imperial Valley), while dry periods left water supplies unreliable. Growing cities such as Los Angeles needed dependable municipal water, and the region sought hydroelectric power to support development.
The U.S. Bureau of Reclamation (then the Reclamation Service) saw a large multipurpose dam as the solution: flood control, irrigation water storage and delivery (including via the later All-American Canal to the Imperial and Coachella Valleys), municipal supply, river regulation, and hydropower. Power sales were intended to make the project self-supporting by repaying construction costs. Early advocates included Arthur Powell Davis of the Reclamation Service. Herbert Hoover, as Secretary of Commerce, strongly supported the project and helped resolve interstate water disputes.
A major political obstacle was allocating the river’s water among the seven basin states (Wyoming, Colorado, Utah, New Mexico, Arizona, Nevada, and California). Upper Basin states feared Lower Basin (especially California) development would claim most of the water under prior-appropriation rules. In 1922, Hoover chaired negotiations that produced the Colorado River Compact. It divided the basin at Lee Ferry, Arizona, allocating 7.5 million acre-feet annually to each basin (with the Lower Basin allowed an additional 1 million acre-feet under certain conditions) and requiring the Upper Basin not to deplete flows at Lee Ferry below 75 million acre-feet over any 10-year period. Arizona initially refused to ratify; the Compact became effective after six states approved it.
Congress authorized the project via the Boulder Canyon Project Act, signed by President Calvin Coolidge on December 21, 1928. The Act ratified the Compact (with California limited to 4.4 million acre-feet of the Lower Basin share), authorized the dam and related works (including the All-American Canal), and apportioned Lower Basin water roughly as California 4.4 million acre-feet, Arizona 2.8 million, and Nevada 0.3 million. President Hoover proclaimed the Compact effective in 1929. A later 1944 treaty allocated 1.5 million acre-feet annually to Mexico.
How the Site Was Chosen
Surveys of potential dam sites along the lower Colorado began in the early 1900s. The 1922 Fall-Davis Report recommended a high dam “at or near Boulder Canyon.” Boulder Canyon sites offered strong granite foundations but presented problems: one was crossed by a geologic fault; others were too narrow for construction camps or spillways; accessibility was poorer; and depth to bedrock was greater in places.
Further investigations (including the Weymouth Report) compared Boulder and nearby Black Canyon (about 20 miles downstream). Black Canyon was selected primarily for practical advantages: greater accessibility (a railroad spur could more easily connect from Las Vegas), shallower depth to solid bedrock in key locations, a narrower gorge with steeper walls, lower construction cost for a given height, and somewhat greater reservoir capacity for the same dam height. Geologic conditions were judged adequate (though Boulder’s granite was sometimes rated superior for load-bearing). The project retained the “Boulder Canyon” name even after the site shift. An independent Colorado River Board later confirmed the upper Black Canyon site.
The design chosen was a massive concrete arch-gravity dam (convex upstream face), developed under Bureau engineer John L. Savage and others. It transfers water pressure into the canyon walls while relying on its own weight for stability—an ultra-conservative approach after the 1928 St. Francis Dam failure raised safety concerns.
Construction
Six Companies, Inc. (a joint venture of major contractors including Utah Construction, Bechtel, Kaiser, Morrison-Knudsen, and others) won the contract in March 1931 with a bid of about $48.9 million—the largest U.S. government construction contract to that point. Work began in the spring of 1931 under Bureau construction engineer Walker Young and Six Companies superintendent Frank Crowe.
Key challenges included extreme desert heat, isolation, and the need to divert the entire river. Workers blasted four massive diversion tunnels (two on each side of the river, roughly 56 feet in diameter and totaling several miles in length) through the canyon walls. The river was diverted in November 1932. Cofferdams protected the site; the riverbed was excavated to solid rock; and canyon walls were scaled of loose rock. Concrete placement began June 6, 1933, and the last dam concrete was poured May 29, 1935. Cooling pipes embedded in the concrete controlled the heat of hydration so the massive structure would not crack. The dam began impounding water in February 1935, forming Lake Mead. Power generation started in late 1936. The project finished more than two years ahead of the seven-year contract schedule and was dedicated by President Franklin D. Roosevelt on September 30, 1935.
Roughly 21,000 people worked on the project at various times (peak employment over 5,000). Conditions were harsh; official figures record around 96–112 worker deaths from construction accidents (the exact total varies by counting method and whether indirect causes are included). Boulder City was built as a planned community for workers and their families. The dam was initially called Hoover Dam by the Hoover administration, renamed Boulder Dam under Roosevelt, and officially restored as Hoover Dam by Congress in 1947.
Hydropower revenues repaid the federal construction costs decades later. The powerplant (nameplate capacity roughly 2,080 MW after later uprates, with 17 main turbines) has historically supplied electricity to utilities in Arizona, Nevada, and California.
Current Struggles with Low Water
Lake Mead and the broader Colorado River system have been under severe stress from a multi-decade megadrought (intensified by climate change, higher temperatures, and increased evaporation), combined with chronic overuse relative to long-term average flows. The original Compact allocations assumed higher average river flows (around 16–17+ million acre-feet) than the observed long-term average (closer to 14–15 million acre-feet, and significantly lower in recent decades).
As of late August 2026, Lake Mead stood near record-low elevations for the modern period—approximately 1,038.87 feet, about 26% full (storage roughly 6.9 million acre-feet on a ~26 million acre-feet capacity basis used in some daily reporting). This is well below the full-pool elevation of about 1,220 feet and marks new lows relative to recent historical records (surpassing previous lows from 2022 in some metrics). Levels have continued a multi-year decline, with further drops expected until the next major snowmelt runoff.
Low water reduces hydropower generation because lower “head” (water pressure/height above the turbines) decreases output. Capacity falls sharply at lower elevations; generation has already been substantially reduced from historical averages, with further declines projected. Minimum power pool is around 950 feet (with some older turbines limited higher); dead pool (where water can no longer pass the dam by gravity) is about 895 feet. The reservoir remains above these critical thresholds but with a narrowing buffer.
Shortage declarations under existing guidelines have already triggered cuts in Lower Basin deliveries. Federal operating guidelines for 2027–2028 and the post-2026 framework emphasize protecting critical elevations at both Lake Mead and upstream Lake Powell (itself also near or at record lows). Measures include reduced releases from Powell to protect its infrastructure and power generation, mandatory and voluntary reductions in Lower Basin consumptive use (on the order of 1.25 million acre-feet annually plus additional conservation in early years of the new guidelines), and efforts to balance the system. Arizona faces particularly large percentage cuts under priority-based shortage sharing. Power production at Hoover is expected to remain constrained, raising costs for replacement energy and affecting ratepayers.
These challenges highlight the tension between the river’s engineered development (which enabled vast agricultural and urban growth in the Southwest) and the physical limits of a variable, arid basin under climate pressure. Ongoing negotiations among the basin states, tribes, Mexico, and the federal government focus on reducing overall use to match available supply while protecting critical infrastructure and ecosystems.
Hoover Dam remains an engineering landmark, a National Historic Landmark, and a major tourist destination, while continuing to serve its original purposes of flood control, water storage, and power generation—albeit under increasingly constrained hydrologic conditions.