 To protect the construction site from the Colorado River and to facilitate the river 's diversion , two cofferdams were constructed . Work on the upper cofferdam began in September 1932 , even though the river had not yet been diverted . The cofferdams were designed to protect against the possibility of the river flooding a site at which two thousand men might be at work , and their specifications were covered in the bid documents in nearly as much detail as the dam itself . The upper cofferdam was 96 ft ( 29 m ) high , and 750 feet ( 230 m ) thick at its base , as thicker than the dam itself . It contained 650 @,@ 000 cubic yards ( 500 @,@ 000 m3 ) of material . 
