<P> With little guidance on water allocation from the Supreme Court, proponents of the dam feared endless litigation . A Colorado attorney proposed that the seven states which fell within the river's basin (California, Nevada, Arizona, Utah, New Mexico, Colorado and Wyoming) form an interstate compact, with the approval of Congress . Such compacts were authorized by Article I of the United States Constitution but had never been concluded among more than two states . In 1922, representatives of seven states met with then - Secretary of Commerce Herbert Hoover . Initial talks produced no result, but when the Supreme Court handed down the Wyoming v. Colorado decision undermining the claims of the upstream states, they became anxious to reach an agreement . The resulting Colorado River Compact was signed on November 24, 1922 . </P> <P> Legislation to authorize the dam was introduced repeatedly by two California Republicans, Representative Phil Swing and Senator Hiram Johnson, but representatives from other parts of the country considered the project as hugely expensive and one that would mostly benefit California . The 1927 Mississippi flood made Midwestern and Southern congressmen and senators more sympathetic toward the dam project . On March 12, 1928, the failure of the St. Francis Dam, constructed by the city of Los Angeles, caused a disastrous flood that killed up to 600 people . As that dam was a curved - gravity type, similar in design to the arch - gravity as was proposed for the Black Canyon dam, opponents claimed that the Black Canyon dam's safety could not be guaranteed . Congress authorized a board of engineers to review plans for the proposed dam . The Colorado River Board found the project feasible, but warned that should the dam fail, every downstream Colorado River community would be destroyed, and that the river might change course and empty into the Salton Sea . The Board cautioned: "To avoid such possibilities, the proposed dam should be constructed on conservative if not ultra-conservative lines ." </P> <P> On December 21, 1928 President Coolidge signed the bill authorizing the dam . The Boulder Canyon Project Act appropriated $165 million for the Hoover Dam along with the downstream Imperial Dam and All - American Canal, a replacement for Beatty's canal entirely on the U.S. side of the border . It also permitted the compact to go into effect when at least six of the seven states approved it . This occurred on March 6, 1929 with Utah's ratification; Arizona did not approve it until 1944 . </P> <P> Even before Congress approved the Boulder Canyon Project, the Bureau of Reclamation was considering what kind of dam should be used . Officials eventually decided on a massive concrete arch - gravity dam, the design of which was overseen by the Bureau's chief design engineer John L. Savage . The monolithic dam would be thick at the bottom and thin near the top, and would present a convex face towards the water above the dam . The curving arch of the dam would transmit the water's force into the abutments, in this case the rock walls of the canyon . The wedge - shaped dam would be 660 ft (200 m) thick at the bottom, narrowing to 45 ft (14 m) at the top, leaving room for a highway connecting Nevada and Arizona . </P>

Who authorized the construction of the boulder dam