On September 30, 1935, Franklin Roosevelt stood on a platform in Black Canyon and dedicated Boulder Dam, the structure later renamed Hoover Dam, in front of a crowd of some 10,000 people in a desert canyon where, five years earlier, there had been nothing but a river and a railway built to reach the site. It was the tallest dam. It had been finished two years ahead of schedule and several million dollars under budget.
The dam exists because the Colorado River is unusable without it. In flood it carried silt and destroyed the Imperial Valley; in drought the valley had no water at all. The Boulder Canyon Project, authorised in 1928, was to build a dam high enough to store two years of the river's flow, control the floods, irrigate the valleys downstream, and generate power that would pay for the whole thing over fifty years. Six states had to agree on how the water was divided before Congress would fund it.
Building it meant first taking the water out of the canyon. Four diversion tunnels were driven through the rock, the largest 56 feet across, and the river was sent through them while the site dried out. Then the canyon walls were scaled of loose rock, a cofferdam was built, and concrete was poured in blocks no larger than 50 feet square and 5 feet deep, with pipes carrying river water through the mass to carry off the heat of setting. Poured in one piece, the concrete would have taken 125 years to cool and would have cracked.
The work was dangerous. Men worked on the canyon walls on ropes, in summer heat above 40 degrees and in a narrow gorge where rock fell. Ninety-six workers died in accidents and illness during the construction, and the figure usually quoted includes deaths from pneumonia caused by the exhaust fumes in the tunnels. The men had struck in 1931 over conditions and were dispersed; the contractor had brought in strikebreakers and then rehired most of them.
The results matched the plan. Lake Mead filled behind it, the Imperial and Coachella valleys got reliable water, and the power generated at the dam, sold under long contracts, has repaid the cost of the project and still subsidises electricity in three states. Los Angeles, Phoenix and Las Vegas were supplied from it, and Las Vegas would not exist in its present form without the water and the power. It is the largest concrete structure in the United States.