Editing Hoover Dam (section)

=== River diversion ===
[[File:Hoover-summary-map.jpg|thumb|Overview of dam mechanisms; diversion tunnels shown]]
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 {{convert|abbr=on|56|ft|m}} in diameter.<ref name="usbrfaq" /> Their combined length was nearly 16,000&nbsp;ft, or more than {{convert|3|mi|km|0}}.<ref name="hovtun" /> 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.{{sfn|Stevens|1988|p=84}}

Tunneling 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 invert, was poured. [[Gantry crane]]s, 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 overheads were filled in. The concrete lining is {{convert|3|ft|m|0}} thick, reducing the finished tunnel diameter to {{convert|abbr=on|50|ft}}.<ref name="hovtun" /> 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.{{sfn|Hiltzik|2010|pp=305–306}}

Following the completion of the dam, the entrances to the two outer diversion tunnels were sealed at the opening and halfway through the tunnels with large concrete plugs. The downstream halves of the tunnels following the inner plugs are now the main bodies of the spillway tunnels.<ref name="hovtun" /> The inner diversion tunnels were plugged at approximately one-third of their length, beyond which they now carry steel pipes connecting the intake towers to the power plant and outlet works.<ref name="usbrfaq" /> The inner tunnels' outlets are equipped with gates that can be closed to drain the tunnels for maintenance.<ref name="usbrfaq" />