Before Hoover Dam could be built, engineers faced an obvious problem: the Colorado River was already occupying the exact space where they needed to construct it. The solution was almost as extraordinary as the dam itself. They would move the river.
Hoover Dam was built in Black Canyon during the early 1930s, a period when the project represented one of the most ambitious engineering efforts in the United States. The site was deep in the desert, more than 30 miles from the nearest town, with limited roads and little infrastructure. Before the dam itself could rise, workers had to create the roads, housing, power systems, communications, and construction facilities needed to support thousands of people. But the biggest obstacle was still flowing through the middle of the canyon.
To clear the construction site, engineers designed four enormous diversion tunnels through the canyon walls. Two were drilled through the Nevada side and two through the Arizona side. Each tunnel was excavated to approximately 56 feet in diameter and then lined with concrete, leaving a finished diameter of about 50 feet. Together, the tunnels stretched nearly three miles.
The work was brutally difficult. When construction began, there were no roads into the canyon, so workers and equipment initially had to be brought in by boat. Temperatures inside the tunnels reached approximately 140 degrees Fahrenheit during the summer. In winter, icy winds swept through the canyon. Workers were drilling, blasting, hauling rock, and lining the tunnels with concrete in an environment that offered little comfort and very little room for error.
Once the tunnels were complete, the Colorado River was diverted through them. Cofferdams were then built to isolate the section of the canyon where the dam would eventually stand. In November 1932, the river was successfully forced into the Arizona diversion tunnels, leaving the construction site exposed. For nearly two years, the Colorado River flowed through the tunnels while workers excavated the foundation and began building the dam.
The scale of the diversion is difficult to imagine. The tunnels were designed to carry more than 200,000 cubic feet of water per second, equivalent to more than 1.5 million gallons every second. The river was not stopped. It was redirected through a massive temporary underground route while one of the largest dams in the world was built beside it.
Eventually, the tunnels themselves became part of the permanent infrastructure of the dam. Some were plugged with enormous masses of concrete, while others were incorporated into the system used to carry water toward the powerhouses.
The next time you see Hoover Dam, it is easy to focus on the massive wall of concrete stretching across Black Canyon, but the story begins before the dam was ever poured.
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