Hoover Dam

A massive concrete arch-gravity dam built across the Colorado River during the Great Depression.

The dam is an arch-gravity dam: it curves into the water like an arch, and its enormous weight also helps hold back the river. It is 221 metres tall. Behind it lies Lake Mead, a huge reservoir (an artificial lake for storing water). The dam controls floods, stores water for farms and cities, and makes hydroelectric power, electricity from falling water, for Nevada, Arizona and California.

How it was built

First, workers dug tunnels through the canyon walls to send the river around the building site. Then they poured about 2.5 million cubic metres of concrete. Engineers worked out that if the dam were poured all at once, the concrete would take about 125 years to cool and would crack. Instead, they built it as columns of separate concrete blocks, some as large as 15 metres square, with steel pipes inside them. Cold river water, and then ice-cold water from a refrigeration plant, flowed through the pipes to cool the concrete quickly.

The people who built it

About 21,000 people worked on the dam in total, with an average of 3,500 a day and more than 5,000 at the busiest time in 1934. A new town, Boulder City, was built nearby to house them. The work was hot and dangerous: 112 deaths were reported in connection with the project. The concrete pouring finished in May 1935, President Franklin D. Roosevelt dedicated the dam on 30 September 1935, and all the work was completed in March 1936, more than two years ahead of schedule.

One dam, two names

The dam is named after Herbert Hoover, who, before becoming US president, had met with seven western states in 1922 to help plan how to share the river's water. Congress used his name for the dam while it was being built. But from 1933 to 1947 its official name was Boulder Dam, because the project had first been planned for nearby Boulder Canyon. In 1947 Congress brought back the name Hoover Dam. The dam is also known for its stylish Art Deco design. Next to it stand two tall bronze statues called the Winged Figures of the Republic, above a floor set with a map of the stars.

On the globe

The model shows the huge curved concrete wall plugging the Colorado River canyon on the Nevada–Arizona border, with the reservoir behind it. Zoom in to see how the dam curves against the water.