How the Hoover Dam was built: diverting the Colorado, the high scalers, cooling 3.25 million cubic yards of concrete, and five years in Black Canyon

Architecture · 4 September 2026

The Hoover Dam was built in Black Canyon on the Colorado River, on the Nevada–Arizona border 30 miles from Las Vegas, between April 1931 and March 1936, two years ahead of schedule, by the Six Companies consortium under Frank Crowe for the Bureau of Reclamation, to stop the river's floods, store its water for the farms and cities of the Southwest and generate power; at 726 feet (221 m) it was the tallest dam in the world, and its 3.25 million cubic yards of concrete were the most ever placed in one structure. It was built by diverting the river through four tunnels blasted in the canyon walls, stripping the cliffs with men on ropes, and pouring the concrete in interlocking columns of small blocks laced with cooling pipes, so that heat which would otherwise have taken 125 years to escape was drawn out in two. About 21,000 men worked on it over five years in the Depression, 5,000 at a time; 96 died on the site by the official count. Below: why and where, each stage, the concrete, the design, the power and the lake, the workers, the numbers and the myths, with the era's other feats in the Golden Gate Bridge and Art Deco architecture.

Why and where

The Colorado drains a quarter of a million square miles of the American West and runs through desert to the Gulf of California, flooding in spring with the Rocky Mountain snowmelt and shrinking to a trickle by autumn; in 1905 it burst its banks into the Imperial Valley of California and ran for two years into the Salton Sink, making the Salton Sea, and the farmers of the valley, the growing cities of Los Angeles and the seven states of the basin wanted it controlled. The Colorado River Compact of 1922, negotiated by Commerce Secretary Herbert Hoover, divided the water between the states, and the Boulder Canyon Project Act of 1928 authorised a dam of 165 million dollars, the largest federal project ever, to be repaid by the sale of its electricity to Los Angeles and the cities of Southern California. The Bureau of Reclamation's engineers, led by Elwood Mead, after whom the lake is named, chose Black Canyon over Boulder Canyon upstream because its rock was sounder and the canyon narrower, but the name Boulder Dam stuck; the dam was named for Hoover at the start of construction in 1930, renamed Boulder Dam by the Roosevelt administration in 1933, and renamed Hoover Dam by Congress in 1947. Hoover Dam.

Step 1: moving the river, 1931–1932

The contract went in March 1931 to Six Companies Inc., a consortium of western builders including Bechtel, Kaiser, Morrison-Knudsen and Utah Construction, whose superintendent Frank Crowe had built dams for the Bureau for twenty years and wanted this one more than anything; the bid was 48.9 million dollars, and the contract's penalty of 3,000 dollars a day for lateness made speed the whole plan. First the river had to be taken out of the canyon. Four diversion tunnels, two on each side, 56 feet in diameter and about 4,000 feet long, were drilled and blasted through the canyon walls with "jumbo" drilling rigs, trucks carrying 30 drills on a frame that advanced a whole tunnel face at once, in air full of dust and carbon monoxide from the trucks that killed men and led to a strike in August 1931; the tunnels were lined with three feet of concrete and finished in November 1932, and on 13 November the river was turned into them by dumping rock from a trestle until the flow gave up and went through the Arizona tunnels. Cofferdams of earth and rock were built upstream and downstream, 98 feet high, and the canyon floor between them was pumped out and dug down 40 feet to bedrock, in June 1933.

Step 2: the high scalers

The dam's arch had to key into solid rock, and the canyon walls, loose and weathered, had to be stripped: the high scalers, many of them former sailors and circus men, some Apache and Yaqui, were lowered down the cliffs on ropes with jackhammers and dynamite to pry off the loose rock, swinging out to avoid what they dropped, and dropping it into the canyon 800 feet below. They were the aristocrats of the site, paid 5.60 dollars a day against the labourers' 4, and the most photographed; their trick of catching a falling man by swinging out on a rope was real and done at least once. The hard hats they made by dipping cloth caps in tar were adopted by the company as the first standard hard hats on an American site. The canyon was clean by 1933 and the excavation for the dam and the powerhouse, 1.5 million cubic yards of rock, was done.

Step 3: the concrete, 1933–1935

A dam of solid concrete poured in one mass would have taken 125 years to cool, and cracked as it did, because the setting of cement gives off heat, so the engineers divided it into columns: 230 vertical columns of interlocking blocks, from 25 to 60 feet square and five feet high, each poured in turn, with a network of one-inch steel pipe laid in every lift, 582 miles of it in all, through which river water and then ice water from a refrigeration plant that could make 1,000 tons of ice a day was pumped to draw the heat out; the blocks shrank as they cooled, and the gaps between the columns were grouted with cement to make the dam one piece. The first bucket was poured on 6 June 1933 and the last on 29 May 1935, from two concrete plants in the canyon, carried in 8-cubic-yard buckets on an overhead cableway system of five lines that crossed the canyon, at a rate of a bucket a minute at the peak; 3.25 million cubic yards in the dam, 4.36 million with the powerhouse and works, enough for a two-lane road from San Francisco to New York. The concrete is still curing and gaining strength, and core samples show it stronger than when it was placed.

The design and the power

• Arch-gravity. The dam is both an arch, convex upstream so that the water pushes it into the canyon walls, and a gravity dam, heavy enough at 6.6 million tons to stand by its weight; it is 660 feet thick at the base and 45 feet at the crest, 1,244 feet long, and 726.4 feet high from foundation to crest. The engineers John Savage and the Bureau's design office computed it, and the Bureau's model tests in the 1920s confirmed it.
• Intake towers. Four towers of 395 feet on the upstream face take water from the lake into 30-foot penstocks that run through the rock to the powerhouse; the old diversion tunnels were plugged and partly reused for the spillways and penstocks.
• Spillways. Two, one on each side, 650 feet long, with drum gates, feeding 50-foot tunnels into the old diversion tunnels; used only in 1941 and 1983, when they were damaged by cavitation and rebuilt with air slots.
• Powerhouse. A U-shaped building at the foot of the dam, 1,500 feet long, with 17 turbines today of about 2,080 megawatts, the first delivered in October 1936 and the last in 1961; the power goes to Nevada, Arizona and Southern California, Los Angeles having paid for it in advance.
• Lake Mead. Behind the dam, 115 miles long, the largest reservoir in the United States at 28.9 million acre-feet when full, which it last was in 1983; drought since 2000 has taken it as low as 27 per cent, with a "bathtub ring" of white on the rocks, and its future is the future of the Southwest.
• Art Deco. The Bureau's utilitarian drawings were given to the Los Angeles architect Gordon Kaufmann, who simplified the crest, the towers and the powerhouse into streamlined Art Deco forms, and Allen Tupper True designed the terrazzo floors with Native American motifs; Oskar Hansen made the bronze "Winged Figures of the Republic" and the star map on the dedication plaza, set so a future astronomer could date the dam.

The workers and Boulder City

Men came from all over the Depression country to the Nevada desert, where the summer of 1931 reached 120 degrees and the first workers lived in "Ragtown" by the river with their families, dying of heat; the government built Boulder City, a model town of 5,000 with no alcohol and no gambling, run by the Bureau, and Six Companies built the dormitories and the mess hall that served 6,000 meals a day. The workforce averaged 3,500 and peaked at 5,251 in 1934, in three shifts around the clock, seven days a week, at wages from 4 to 5.60 dollars a day, with Black workers, a few dozen, admitted only after protests in 1932 and no Chinese. Ninety-six men died on the site by the official count, from falls, rock, drowning, heat and the trucks, and more of heat and carbon monoxide that the company recorded as pneumonia; the first, J.G. Tierney, a surveyor, drowned on 20 December 1922, and the last, his son Patrick, fell from an intake tower on 20 December 1935, thirteen years to the day. The dam was dedicated by President Roosevelt on 30 September 1935 before 10,000 people, and the last concrete was placed in March 1936; Crowe's bonus was 350,000 dollars.

The numbers

• Height. 726.4 feet (221.4 m); the tallest dam in the world until 1957 and the tallest concrete dam in the United States after Oroville by height of any type.
• Length and thickness. 1,244 feet (379 m) along the crest; 660 feet (201 m) thick at the base, 45 feet (14 m) at the top, where the road ran until the bypass bridge of 2010.
• Concrete. 3.25 million cubic yards (2.48 million m³) in the dam; 4.36 million in all; 5 million barrels of cement.
• Weight. 6.6 million tons.
• Time. 5 years, from April 1931 to March 1936, against a contract of 7.
• Cost. 49 million dollars to Six Companies, 165 million for the project; repaid with interest by 1987.
• Workers and deaths. 21,000 over the project, 5,251 at peak; 96 official deaths, about 112 counting the pre-construction surveys.
• Power. About 4 billion kilowatt-hours a year, 2,080 MW of capacity, serving 1.3 million people.
• Visitors. About 7 million a year on the crest road and a million on the tours of the powerhouse and the tunnels; the bypass bridge of 2010, 900 feet above the river, gives the view of the whole face.

Versions and theories

• Men are buried in the concrete. No. The concrete was poured in 5-foot lifts with crews standing on it, and a body would have made a flaw the engineers could not accept; the story comes from the deaths in the tunnels and the belief that a fall into the forms was fatal, and every worker who died is on the record. The dam has no bodies in it.
• The first and last deaths were father and son. True, thirteen years apart to the day, by the Bureau's own record.
• The dam will take a century to cool. It would have; the cooling pipes drew the heat out in less than two years, and the concrete is now at the temperature of the rock.
• Hoover named it after himself. His Interior Secretary Ray Wilbur named it at the 1930 groundbreaking, with Hoover's consent; Roosevelt's Interior Secretary Harold Ickes reversed it out of spite in 1933, and Congress restored it in 1947, when Hoover was 72.
• The dam is cracking or leaking. It seeps, as all dams do, through drains built for the purpose; it has been monitored since 1935 and has not moved beyond the tiny deflections designed for.
• The turbines were built for the war. Half of them came online during the Second World War, and their power ran the aluminium and magnesium plants and the aircraft factories of Southern California; the dam was guarded by the army after 1939 against sabotage, and the plan to bomb it with a German plane from a Mexican airfield, reported in 1939, was real and stopped by the FBI.

Open the sections: buildings by style, ancient temples of the world, all articles.