How Boulder Canyon Dam Ended Up in Black Canyon as Hoover Dam

Investigations of potential dam sites along the lower Colorado River in Boulder and Black Canyons were initially carried out by the Hydrology Branch of the U.S. Geological Survey in 1901–02. These were further evaluated by the newly-formed Reclamation Service from 1914 until the dam’s eventual construction in 1931–35. The seminal event triggering these studies was the accidental flooding of the Imperial Valley in 1905–07, which inundated more than 500 square miles of irrigable lands. It was assumed that a great embankment dam would be constructed near the head of Boulder Canyon, where the gorge was narrowest and comprised of granite. At this location the average depths of channel sands and gravels was about 65 feet. The site’s location thereafter caused the federal effort to be part of the Boulder Canyon Project. In 1919 California representatives in the House and Senate began introducing bills to approve what would eventually become the largest line item appropriation in American history, up to that time. In 1922–24 Reclamation received funds to explore alternative dam sites in Black Canyon, 18 to 23 miles downstream of the Boulder Canyon site. Dams of similar height built 20 miles downstream would increase the reservoir storage by about 40%, but the average depth to bedrock in the river channel was 120 feet. In 1924 Reclamation recommended a concrete gravity arch dam 740 ft high in Black Canyon. During each session of Congress in the 1920s the Boulder Canyon Act was introduced but failed to win congressional approval for nine years. Along the way, it became a more multi-faceted project, providing irrigation to an enlarged area, increased flood storage, constructing an All-American Canal along the international border, and the novel proposal for Reclamation to construct and operate the world’s largest hydroelectric facility, using the proceeds to repay to cost of the project. In 1927 a board of advisors were appointed to make detailed surveys of the Lower Colorado River and answer five questions that had been raised by critics of the act. These refinements and concessions eventually led to congressional approval in May 1928, but with a special proviso resulting from the recent failure of the St. Francis Dam built by the city of Los Angeles, which had killed ~430 people. Congress appointed a Colorado River Board (CRB) comprised of eminent engineers and geologists to make a detailed evaluation of Reclamation’s designs and report their findings within six months. The CRB’s recommendations were accepted and the Boulder Canyon Act was approved by Congress and President Calvin Coolidge in December 1928.