Hoover Dam, often called Boulder Dam, is located on the Colorado River at the southern tip of Nevada and the Arizona border. In the early 1900s, after many failed efforts to control the lower Colorado River with levees and irrigation canals, the U.S. Bureau of Reclamation decided the best solution for the arid Southwest would be to dam the river and create a year-round water supply. Surveyors investigated seventy sites along the entire river's course and settled on Nevada's Boulder and Black Canyons, both offering a potential reservoir of more than thirty million acre-feet. Surveys revealed the superiority of Black Canyon for storing the Colorado River's entire flow for up to two years, though in 1922 the “Fall-Davis Report” recommended to the U.S. Senate construction of the All-American Canal in California's Imperial Valley and a high dam at or near Boulder Canyon.

All seven Colorado River states—Wyoming, Colorado, Utah, New Mexico, Arizona, Nevada, and California—could see the advantages of a dam in the river's lower basin (below Lee's Ferry near the Utah/Arizona boundary), but were concerned for their own interests. Mining engineer Herbert Hoover, later to be United States President, was a strong proponent of the project and served as presiding officer of the Colorado River Commission, which painstakingly worked out the details of states' water rights, apportioning water to two groups: the upper and lower basin states, which would divide the water rights between themselves at a later date. With that settled, in 1928, Congress passed and President Calvin Coolidge signed the Swing-Johnson bill, better known as the Boulder Canyon Project Act.

In 1931, Secretary of the Interior Ray Lyman Wilbur announced that a 726.4-foot high dam would be built in Black Canyon rather than Boulder Canyon due to a fault at the upper end of Boulder Canyon and greater storage capacity in Las Vegas Wash. It would be named Hoover Dam. (But when Hoover lost the presidency to Franklin D. Roosevelt in 1932 because people blamed him for the Great Depression, the dam was officially renamed Boulder Dam in 1933. Congress restored the name Hoover Dam in 1947.) A labor contract was awarded to Six Companies of San Francisco composed of Utah Construction of Ogden, Utah; Pacific Bridge of Portland, Oregon; Henry J. Kaiser & W.A. Bechtel of Oakland, California; MacDonald & Kahn of Los Angeles; Morrison-Knudsen of Boise, Idaho; and J.F. Shea of Portland, Oregon—all major Western contracting firms. The bid was $48,890,995.50, the largest labor contract let by the U.S. Government up to that time.

During the Great Depression, many workers and their families arrived long before work on the dam began in hopes of finding jobs. They lived in jerry-rigged dwellings in makeshift tent camps next to the river. Because the area was totally desolate, with recordings of 110 to 120 degrees in the canyon (later peaking at 140 in the airless portals of the diversion tunnels), much preparation was necessary, including rushing Boulder City, seven miles southwest of the dam site, to completion in fifteen months to provide suitable living conditions for workers. Railroad lines were built, as were highways, a construction camp, a gravel-screening plant, concrete-mixing plants, air compressor plants, a plate steel fabricating plant, and much more. The plan of attack was to drill four diversion tunnels through the canyon walls during the low-water season of 1932-33, divert the river through the tunnels, build earthen cofferdams above and below the dam site to block the river, de-water and excavate the site, and build the dam and power plant. The two outer tunnels would be outlets for huge spillways; the inner tunnels would convey water from intake towers in the reservoir to the power plant.

Many ingenious techniques and devices were developed on this project, such as using dishpan reflectors to light the diversion tunnels, the hard-hat (cloth hats dipped into tar and hardened into a tough shell), the motor-driven jumbo drill equipped with thirty 144-pound rock drills boring into the rock face at the same time to speed up drilling and blasting, and the on-site fabrication of steel pipe for the penstock system because standard railroad cars could not carry the pipe, which exceeded 44,000 tons in gross weight, from eastern fabricating mills.

Six Companies built an aggregate producing and batch plant to supply 4.5 million yards of concrete and gravel. Workers poured concrete into a honeycomb of tiered blocks that ice-cold water running through embedded pipes cooled, solving the problem of intense heat generated by the chemical reactions solidifying concrete. The crest height of the dam was reached in March 1935, two years ahead of schedule, with 3,250,335 cubic yards or 6.6 million tons of concrete poured into the dam structure. Contrary to oft-repeated legend, no one is buried in the concrete, though ninety-six men were officially listed as having died on the project.

In twenty-one months, 5,000 men had built a structure greater in volume than the largest pyramid in Egypt, which, according to Herodotus, required 100,000 men working twenty years to build. The American Society of Civil Engineers has named Hoover Dam one of the seven modern civil engineering wonders of the United States. It is listed as a National Historic Landmark.