An image showing a lengthwise view of a penstock pipe in the Hoover Dam powerhouse. Boulder Dam was officially named Hoover Dam in 1947.

An image of people standing atop Hoover Dam's spillway bridge. Note: Boulder Dam was officially renamed Hoover Dam in 1947.

The face of Hoover Dam as seen from the river. Part of the power plant's hydroelectric generators, as well as the canyon wall outlets and stoney gate are visible on the Nevada (left) side. The spillway tunnel is visible on the Arizona (right) side. During the years of lobbying leading up to the passage of legislation authorizing the dam in 1928, Hoover Dam was originally referred to "Boulder Dam" or as "Boulder Canyon Dam", even though the proposed site had shifted to Black Canyon. The Boulder Canyon Project Act of 1928 (BCPA) never mentions a proposed name or title for the dam. When Secretary Wilbur spoke at the ceremony starting the building of the railway between Las Vegas and the dam site on September 17, 1930, he named the dam "Hoover Dam", citing a tradition of naming dams after Presidents, though none had been so honored during their terms of office. After Hoover's election defeat in 1932 and the accession of the Roosevelt administration, Secretary Ickes ordered on May 13, 1933 that the dam be referred to as "Boulder Dam". In the following years, the name "Boulder Dam" failed to fully take hold, with many Americans using both names interchangeably and map makers divided as to which name should be printed. In 1947, a bill passed both Houses of Congress unanimously restoring the name to "Hoover Dam".