 Before the dam could be built , the Colorado River needed to be diverted away from the construction site . To accomplish this , four diversion tunnels were driven through the canyon walls , two on the Nevada side and two on the Arizona side . These tunnels were 56 ft ( 17 m ) in diameter . Their combined length was nearly 16 @,@ 000 ft , or more than 3 miles ( 5 km ) . The contract required these tunnels to be completed by October 1 , 1933 , with a $ 3 @,@ 000 @-@ per @-@ day fine to be assessed for any delay . To meet the deadline , Six Companies had to complete work by early 1933 , since only in late fall and winter was the water level in the river low enough to safely divert . 
