To divert the river's flow around the construction site, four diversion tunnels were bored through the canyon walls, two on the Nevada side and two on the Arizona side. These tunnels were dug to a diameter of 56 feet. Tunneling began at the lower portals of the tunnels on the Nevada side of the Black Canyon in May 1931. Shortly after, work began on two similar tunnels on the Arizona side of the canyon. In March 1932, work began on lining the tunnels with concrete. First the base or invert was poured. Gantry cranes, running on rails through the entire length of each tunnel were used to place the concrete. The sidewalls were poured next. Movable sections of steel forms were used for the sidewalls. Finally, using pneumatic guns, the overhead sections were filled in. The concrete lining, which is 3 feet thick, reduced the finished tunnel to a diameter of 50 ft. The four tunnels had combined length of nearly 16,000 feet, just over three miles.
Following the completion of the dam, the entrances to the two outer diversion tunnels were sealed at the upstream end and near the mid point of the tunnels with concrete. The downstream halves of the two outer tunnels are now the main sections of the spillway tunnels. The spillways can be seen directly above the outer diversion tunnels. They drop sharply from their entrance point and merge directly into the old diversion tunnels.
The two inner diversion tunnels also were sealed. In each of the inner tunnels, a concrete plug was placed at roughly the mid-point of the tunnel’s length and a second plug was placed around ¾ of the distance from the upstream opening of the tunnel.
The section between the two concrete plugs is used as part of the tunnel that water travels from the outermost intake towers to the generators. The two innermost intake towers have separate tunnels.The large spillway tunnels have only been used three times in the history of the dam; the first was during the second half of 1941 for testing. The second was for about six weeks during the summer of 1983, when record precipitation and snow-melt in the Colorado River basin drained into Lake Mead and the third in 1999, again with heavy precipitation that filled Lake Mead.
Read more and learn about How the Hoover Dam was Built.