 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 . 
 To protect the construction site from the Colorado River and to facilitate the river <a href='null_genetive' title='1'><mark>'s</mark></a>diversion , two cofferdams <a href='uninflect' title='2'><mark>were</mark></a> constructed . Work on the upper cofferdam began in September 1932 , even though the river <a href='been_done' title='3'><mark>had</mark></a> not yet been diverted . The cofferdams <a href='uninflect' title='4'><mark>were</mark></a> designed to protect against the possibility of the river flooding a site at which two thousand men might be at work , and their specifications <a href='uninflect' title='5'><mark>were</mark></a> 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 . 
To protect da construction site from tdaColorado River and to facilitate the river  diversion , 2 cofferdams was constructed . Work on the upper cofferdam began in September 1932 , even doe thdaiver done not yet been diverted . The cofferdams was designed to protect against thedassibility of the river flooding a site at which two thousand men mite be at work , and their specifications was covered in the bid documents in nearly as much detail as the da itself . The upper cofferdam was 96 ft ( 29 m ) high , and 750 feet ( 230 m ) thick at its base , as thicker den the ddaitself . It contained 650 @,@ 000 cubic yards ( 500 @,@ 000 m3 ) of material .