The first task at the project site was to build two cofferdams just upstream from the dam site to protect the construction site from flooding. The construction of the upper cofferdam started in September 1932, before the river was diverted. A temporary horseshoe-shaped dike protected the cofferdam on the Nevada side of the river. After the Arizona tunnels were completed, and the river diverted, the work was completed much faster.
Once the cofferdams were in place and the construction site dewatered, excavation for the dam foundation began. For the dam to rest on solid rock, it was necessary to remove all the riverbed's accumulated erosion soils and other loose materials until sound bedrock was reached. Due to the dam’s arch and gravity design, the side-walls of the canyon would bear the force of the impounded lake. The load of the water in the storage reservoir is resisted by both the gravity/mass of the dam, and by the arch of the dam pressing against and into the side walls of Black Canyon. The two vertical foundations for each of the arch walls (the Nevada side and Arizona side) had to be founded on sound "virgin" rock; free of the cracks and the weathering that surface rock of the canyon walls had from thousands of years of weathering and exposure.
Therefore, it was necessary to remove loose rock from the canyon walls before construction began on the dam itself. Work on the foundation excavations required removing approximately 1,500,000 cubic yards of material and was completed in June 1933.The men who removed this rock were called "high-scalers." Suspended from the top of the canyon with ropes, these individuals climbed down the canyon walls and removed the loose rock with jackhammers and dynamite.
Read more and learn about Herbert Hoover: Secretary of Commerce, Conservationist and President.