 <unk> began at the lower portals of the Nevada tunnels in May 1931 . Shortly afterward , work began on two similar tunnels in the Arizona canyon wall . In March 1932 , work began on lining the tunnels with concrete . First the base , or <unk> , was poured . <unk> cranes , running on rails through the entire length of each tunnel were used to place the concrete . The <unk> were poured next . <unk> sections of steel forms were used for the <unk> . Finally , using <unk> guns , the <unk> were filled in . The concrete lining is 3 feet ( 1 m ) thick , reducing the finished tunnel diameter to 50 ft ( 15 m ) . The river was diverted into the two Arizona tunnels on November 13 , 1932 ; the Nevada tunnels were kept in reserve for high water . This was done by exploding a temporary cofferdam protecting the Arizona tunnels while at the same time dumping rubble into the river until its natural course was blocked . 
