When its construction was completed in 1936, Hoover Dam was the tallest dam in the world, rising over 700 feet from base to top. Its location in Black Canyon, on the Colorado River between Nevada and Arizona, was a forbidding, desolate, dangerous place, devoid of the basic necessities to sustain the army of construction workers that would be needed. There were no computers to assist in the design and engineering work. Everything was drafted by hand and calculated using slide rules.

It was, in fact, one of the largest, most expensive, and most complex public works projects ever attempted. And yet, it was completed two years ahead of schedule. How in the world did such a massive edifice ever get built?

Step 1: Prepare the site

Credit: Mapmakers in employ of U.S. Department of Interior / Public domain

A dam the size of Hoover Dam requires thousands of tons of material and equipment, and at the time, there were no roads leading to the site. So the first step was to build a railroad spur from the then-quiet town of Las Vegas, some 30 miles away, along with communication and electric power lines.

Another important requirement is a place for the construction workers to live. As part of the U.S. government’s contract to build the dam, the construction contractor, a joint venture called Six Companies, Inc., was to build a model city nearby, on the Nevada side of the site. The city, called Boulder City, was completed as the dam construction was ramping up in late 1931.

Once all that’s in place, you can start pouring concrete, right?

Step 2: Divert the water

Credit: Bureau of Reclamation / Public domain

Not so fast. Unless you’re a beaver, before you dam up a river, you have to do something with the water. It has to be diverted away from the construction site.  With a river the size of the Colorado, this is no easy task.

It was accomplished by boring four huge tunnels through the canyon walls, two on each side of the river; each tunnel measured 56 feet in diameter and had to be lined with concrete. The tunnels came with a high human price: Temperatures in the tunnels could reach 140°, and many workers died of heat prostration in the process. With the tunnels safely rerouting the river, smaller dams called cofferdams were built, one upstream and one downstream of the dam site.

Now can you start pouring concrete?

Step 3: Excavation

Credit: National Archives and Records Administration / Public domain

Nope. You can’t just plop a dam on the riverbed—it would soon be undermined by water washing away the mud and silt underneath. You have to excavate the riverbed until you reach solid bedrock. In the case of Hoover dam, the canyon walls also had to be excavated of old, weathered (and therefore weaker) rock to reach the virgin bedrock. This was important because the dam’s design, called an “arch gravity” type, transferred the force of the dammed-up water to the sides.

Step 4: Start building!

Credit: Bureau of Reclamation / Public domain

Now you can get on with the concrete work. And so they did. Enough concrete was used in Hoover Dam to build a two-lane road across the entire United States. The base of the structure measures 660 feet from the upstream side to the downstream side. The top is 45 feet across. Until recently, you could drive across it on U.S. Route 93 before a bridge was built across the river just downstream of the dam.

In all, over 3 million cubic yards of concrete was used in the construction of the dam. Thousands of workers were employed in the project, reaching a peak of over 5,000 men on the payroll in mid-1934. About 100 workers died during the construction. Boulder Dam, as it was called before its name was changed by Congress in 1947, was formally dedicated in September 1935, with a speech given by President Franklin Roosevelt in front of 10,000 spectators.

As the dam structure was being completed, the Colorado River was allowed to flow up to the dam, slowly filling what would become Lake Mead. By the time the separate powerhouse structure was completed in 1936, there was enough water in Lake Mead to start generating electricity. At the time the last generator went online in 1939, Hoover Dam was the largest hydroelectric generation facility in the world.

Step 5: Make it pretty

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Hoover Dam was not designed for aesthetics. In response to criticism of its plain look, architect Gordon B. Kaufmann was commissioned to lend some style to match the grandeur of the project. He added Art Deco touches, bas-relief sculptures, and statues, as well as decorations inspired by the Navajo and Pueblo tribes that once inhabited the area.

Hoover Dam has since been eclipsed in both size and power output by other dams around the world, but given the challenges of the locale and the limited technology available at the time, the dam stands as a remarkable achievement whose benefits — electric power, flood control, and irrigation water — are still felt throughout the American Southwest.