Hoover Dam is a concrete dam across the Colorado River in Black Canyon, on the border between Arizona and Nevada, southeast of Las Vegas. It is about 221 meters (726 feet) tall, measured from the rock it stands on to the road along its top. Built between 1931 and 1936, it holds back Lake Mead, and its power plant makes electricity for three states.

Size

The US Bureau of Reclamation, the federal agency that built and runs the dam, gives its height as 726.4 feet (221 meters). That is measured from the foundation rock to the roadway on the crest, the top of the dam. Towers and ornaments on the crest rise another 40 feet (12 meters). Part of that height is out of sight: to reach solid rock, workers dug as deep as 139 feet (42 meters) below the river's low-water level.

Along its crest, the dam is 1,244 feet (379 meters) long and 45 feet (14 meters) thick. At its base it is 660 feet (201 meters) thick, almost 15 times as thick as at the top. It is an arch-gravity dam: it holds back the water partly by its great weight and partly by its curved, arch shape. It weighs more than 6.6 million US tons (about 6 million metric tons).

Size comparisons

The dam is about 29 meters (96 feet) taller than the Gateway Arch in St. Louis, which is 192 meters (630 feet) tall.
It is about two-thirds the height of the Eiffel Tower, which is 330 meters (1,083 feet) tall.
Its height is almost exactly the length of two American football fields, and its crest is about three and a half fields long.
The Titanic was 269 meters (883 feet) long. Stood on end beside the dam, it would reach about 48 meters (156 feet) above the crest.

How the dam was built

The dam was built as about 215 concrete blocks, arranged in columns. The blocks ranged from about 18 meters (60 feet) square at the upstream face to about 7.6 meters (25 feet) square at the downstream face. Concrete gives off heat as it sets. The Bureau's engineers calculated that a dam poured in one piece would have taken 125 years to cool, and would have cracked as it cooled. Instead, workers laid more than 582 miles (937 kilometers) of steel pipe inside the blocks and pumped ice water through it.

The first concrete was placed on June 6, 1933, and the last on May 29, 1935. In all, 21,000 men worked on the project, as many as 5,218 of them on a single day in June 1934. When it was finished, Hoover Dam was the tallest dam in the world. The Bureau of Reclamation also calls it the first structure built by people to contain a greater mass of masonry than the Great Pyramid of Giza.

What the dam does

The dam holds back Lake Mead. When full, the lake stretches about 110 miles (177 kilometers) up the Colorado River toward the Grand Canyon and holds about 36 cubic kilometers of water. That is enough to store the river's entire average flow for two years.

Water from the lake turns 17 main turbines in a power plant at the base of the dam. The plant generates about 4 billion kilowatt-hours of electricity a year for Nevada, Arizona and California, enough for 1.3 million people. A kilowatt-hour is the energy delivered by 1,000 watts of power in one hour.

Sources

Hoover Dam Frequently Asked Questions and Answers: The Dam. US Bureau of Reclamation, 2015.
Hoover Dam. Projects and Facilities database, US Bureau of Reclamation.
What is the biggest dam in the world?. US Bureau of Reclamation, 2015.
Concrete. The Story of Hoover Dam, US Bureau of Reclamation, 2015.
Hoover Dam Frequently Asked Questions and Answers: Lake Mead. US Bureau of Reclamation, 2015.
Hoover Dam Frequently Asked Questions and Answers: Hydropower at Hoover Dam. US Bureau of Reclamation, 2018.
Frequently Asked Questions. Gateway Arch National Park, National Park Service.
From 300 to 330 meters: the story of the Tower's height. Société d'Exploitation de la Tour Eiffel, 2022.
2026 NFL Rulebook. National Football League, 2026.
RMS Titanic facts. Royal Museums Greenwich.

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