Deep inside Hoover Dam, the concrete is still curing. Chemists confirmed decades after completion that the reactions binding it were still creeping onward, the mass still gaining strength — a structure not so much finished as slowly ripening in the Nevada heat. That fact is a fitting emblem for a project whose central engineering problem was not holding back a river but managing something almost comically mundane: heat.
The dam was authorised as the Boulder Canyon Project, though it was built in Black Canyon, a narrow slot of volcanic rock on the Colorado River between Nevada and Arizona — the first of many naming confusions. Its purpose was to tame a river notorious for swinging between trickle and catastrophe, to store water for the arid Southwest and to generate power to pay for it all. In 1931, in the depths of the Depression, the contract — the largest the US government had ever awarded — went to Six Companies, a consortium of western construction firms formed because no single firm could raise the bond. Their superintendent was Frank Crowe, a driven, revered and feared engineer whose nickname on site was "Hurry Up" Crowe, and who had spent years dreaming of building precisely this dam.
Thousands of desperate men converged on the site, camping in a squatters' settlement called Ragtown while the purpose-built Boulder City rose nearby. The first task was brutal: four diversion tunnels, each wide enough to swallow a house, blasted through the canyon walls to carry the river around the site. Men working inside them in summer faced temperatures well above 40°C, and in the early months heat prostration killed workers before the dam proper had even begun.
Dancing on the canyon walls
The most celebrated workers were the high scalers — men lowered on ropes down the sheer canyon faces to pry off loose rock with crowbars and set dynamite charges. Many were former sailors, circus performers and Apache and other Native American workers with a talent for the heights; between shifts of deadly work they swung, raced and performed stunts on their lines for the crowds gathering at the rim, to the fury of the safety men. Falling rock was a constant killer, and crews improvised their own hard hats by dipping cloth caps in tar and letting them bake stiff, before commercial hard hats — invented in 1919 by Edward W. Bullard — reached the site; the improvised headgear is still credited with saving several skulls. Officially, 96 workers died in industrial accidents on the project. The enduring legend that bodies lie entombed in the concrete is false: the pours were shallow, and a body left in a block would have created a fatal structural flaw. The record does hold one eerie coincidence — J. G. Tierney, a surveyor, drowned in the river on 20 December 1922, and his son Patrick, the last man to die on the project, fell from an intake tower on 20 December 1935, thirteen years to the day.
The refrigerator inside the dam
The dam itself is an arch-gravity wedge, 726 feet high — the tallest dam on Earth when built — containing roughly 3.3 million cubic yards of concrete, enough, in the Bureau of Reclamation's favourite image, to pave a two-lane road from San Francisco to New York. Concrete releases heat as it sets, and here lay the problem: poured as a single mass, the dam would have taken on the order of a century to cool, cracking uselessly as it shrank. The engineers' solution was twofold. First, the dam was built not as one pour but as a honeycomb of interlocking columns of blocks, each block a shallow lift of concrete, poured, cooled and grouted to its neighbours. Second, they refrigerated it: more than 580 miles of one-inch steel pipe were embedded in the pours, circulating cold water from an ammonia ice plant on site — a plant with the output of a small city's worth of refrigeration — pulling the heat out of the young concrete years ahead of nature's schedule.
The river was closed off in 1935, and Franklin Roosevelt dedicated the dam that September — two years ahead of schedule. Even the name became political: begun under Herbert Hoover and named for him in 1930 by his Interior Secretary, it was pointedly called Boulder Dam through the Roosevelt years until Congress restored the name Hoover Dam in 1947 — a switch that still trips up quiz players. Behind it, Lake Mead filled to become the largest reservoir in the United States by capacity, and the powerhouse turbines lit up Los Angeles and the new city of Las Vegas.
Frank Crowe took his bonus and built more dams. The men of Ragtown scattered into the workforce of the American West. And in Black Canyon the great wedge sits where they left it, holding back a lake, feeding the grid — and, quietly, still setting.
Quiz nuggets
- Hoover Dam was built in Black Canyon, not Boulder Canyon, on the Nevada–Arizona border, and finished two years ahead of schedule in 1935.
- Over 580 miles of embedded steel pipe circulated refrigerated water to cool the concrete, which would otherwise have taken about a century to cool.
- The official industrial death toll was 96; the story of bodies buried in the concrete is a myth.
- Surveyor J. G. Tierney and his son Patrick died on the project exactly 13 years apart, on 20 December 1922 and 1935.
- Renamed Boulder Dam in the Roosevelt era, it officially became Hoover Dam again by act of Congress in 1947.