Hoover Dam Diversion Tunnels
Reviewed and updated by AZ Leisure Staff Writers on .
Before Hoover Dam could rise from Black Canyon, engineers first had to move the Colorado River out of the construction site. Crews excavated four enormous diversion tunnels through the canyon walls—two on the Arizona side and two on the Nevada side—so the river could bypass the future dam. The tunnels were drilled and blasted through solid rock, lined with concrete and completed in time to divert the Colorado River through the Arizona tunnels on November 14, 1932.
- Diversion Tunnel Quick Facts
- Why The River Had To Be Diverted
- Drilling & Blasting The Tunnels
- Working Conditions In Black Canyon
- Concrete-Lining The Tunnels
- Diverting The Colorado River
- What Happened To The Tunnels?
- Diversion Tunnels FAQ
Hoover Dam Diversion Tunnel Quick Facts
- Number of tunnels: Four
- Location: Two on the Arizona side and two on the Nevada side of Black Canyon
- Excavated diameter: 56 feet
- Finished diameter: 50 feet after concrete lining
- Combined length: 15,946 feet, or about 3 miles
- Concrete lining: 3 feet thick
- Concrete used for lining: About 300,000 cubic yards
- Maximum combined capacity: More than 200,000 cubic feet of water per second
- Tunneling began: May 1931
- Colorado River first diverted: November 14, 1932
Why The Colorado River Had To Be Diverted
Hoover Dam could not be constructed in the active Colorado River channel. Before excavation and concrete work could begin at the dam site, engineers needed a way to temporarily carry the river around the construction area while keeping the canyon floor accessible to workers and equipment.
The solution was to drive four enormous tunnels through the walls of Black Canyon. Once the tunnels were ready, the river could be blocked by cofferdams and redirected through the tunnels, leaving the central construction area sufficiently dry for excavation and construction of the dam and powerplant.
Drilling & Blasting The Hoover Dam Diversion Tunnels
Tunneling began at the lower portals of the Nevada tunnels in May 1931, followed shortly by work on the Arizona side. Pneumatic drills bored holes into the rock for explosives, while large compressor plants supplied compressed air through a network of pipes to the work areas.
To speed excavation, crews developed massive drilling jumbos built on 10-ton trucks. Each jumbo could position approximately 24 to 30 drills against the tunnel face so many blast holes could be drilled at once. Eight drilling jumbos were eventually used.
After the holes were drilled, crews loaded them with dynamite. The Bureau of Reclamation reports that approximately one ton of dynamite was used for every 14 feet of tunnel excavated. After each blast, miners inspected the area before crews removed broken rock and loaded it into trucks for disposal. More than 1.5 million cubic yards of material were ultimately excavated from the diversion tunnels.
Dangerous Working Conditions In Black Canyon
Building the diversion tunnels placed workers in one of the harshest environments on the Hoover Dam project. Initially there were no roads into Black Canyon, so workers and equipment had to be transported by boat until roads and catwalks could be constructed.
Heat inside the tunnel workings reached about 140 degrees during the summer of 1931, according to the Bureau of Reclamation. Winter brought the opposite extreme, with icy winds and temperatures dropping to freezing. Workers also faced blasting hazards, falling rock, heavy equipment, dust and exhaust inside the tunnels.
Flooding was another danger. In September 1932, rising water forced workers to abandon the site temporarily as floodwaters swept through Black Canyon.
Concrete-Lining The Diversion Tunnels
Concrete lining began in March 1932. Crews first poured the tunnel floors, or inverts, then used rail-mounted gantry cranes and movable steel forms to construct the sidewalls. Pneumatic concrete equipment was used to complete the overhead portions.
The finished concrete lining was approximately three feet thick. This reduced each tunnel from its original 56-foot excavated diameter to a finished diameter of about 50 feet. Altogether, approximately 300,000 cubic yards of concrete were used to line the diversion tunnels.
Diverting The Colorado River Through The Tunnels
By November 1932 the diversion tunnels were ready. A temporary barrier across the Arizona tunnel entrances was breached with explosives, and earth and rock were dumped from a trestle into the Colorado River channel to force the river toward the tunnel openings.
On November 14, 1932, the Colorado River was first diverted through the two Arizona tunnels. With the river routed around the central dam site, crews could move ahead with excavation, cofferdams and construction of Hoover Dam in the former river channel.
What Happened To The Hoover Dam Diversion Tunnels?
The diversion tunnels continued to play an important role after their original job was complete. Portions of the tunnels were permanently plugged with massive sections of concrete as construction progressed and Lake Mead began forming behind Hoover Dam.
Today, the two inner diversion tunnels contain large steel penstocks that connect the reservoir intake system with the powerplant and canyon-wall outlet works. The downstream portions of the two outer diversion tunnels became part of Hoover Dam's spillway system.
The structures originally built simply to move the Colorado River around the construction site therefore became permanent components of Hoover Dam's water-control system.
From River Diversion To Building Hoover Dam
Diverting the Colorado River was only the beginning of the Hoover Dam construction effort. Once the river was routed through the tunnels, workers could excavate the foundation, prepare the canyon walls and begin placing the enormous concrete structure between Arizona and Nevada.
Continue with Building Hoover Dam, learn about the specialized Hoover Dam High Scalers, or see original construction scenes in the Hoover Dam Historic Photo Gallery.
Hoover Dam Diversion Tunnels FAQ
Why were diversion tunnels needed to build Hoover Dam?
The Colorado River had to be moved away from the future dam site so workers could excavate and build Hoover Dam in the river channel. Four tunnels were constructed through Black Canyon to carry the river around the construction area.
How many Hoover Dam diversion tunnels were built?
Four diversion tunnels were built, with two excavated through the Arizona wall of Black Canyon and two through the Nevada wall.
How large were the Hoover Dam diversion tunnels?
The tunnels were excavated to a diameter of approximately 56 feet. After receiving a three-foot-thick concrete lining, the finished diameter was about 50 feet.
How long are the Hoover Dam diversion tunnels?
The four tunnels have a combined length of 15,946 feet, or approximately three miles.
When were the Hoover Dam diversion tunnels built?
Tunneling began in May 1931. The tunnels were ready to divert the Colorado River by November 1932.
When was the Colorado River diverted for Hoover Dam construction?
The Colorado River was first diverted through the two Arizona diversion tunnels on November 14, 1932.
How much water could the Hoover Dam diversion tunnels carry?
Together, the tunnels were capable of carrying more than 200,000 cubic feet of water per second, or more than 1.5 million gallons per second.
How thick was the concrete lining in the diversion tunnels?
The tunnel lining was approximately three feet thick and required about 300,000 cubic yards of concrete.
What happened to the diversion tunnels after Hoover Dam was completed?
Parts of the tunnels were permanently plugged. The inner tunnels now contain large penstocks connected to the powerplant and outlet works, while downstream portions of the outer tunnels became part of the spillway system.
How hot was it while workers built the diversion tunnels?
The Bureau of Reclamation reports that temperatures in the tunnel workings reached approximately 140 degrees during the summer of 1931.
Historic photographs courtesy of the Boulder City/Hoover Dam Museum.
Related Hoover Dam Interest
Hoover Dam
Building Hoover Dam
Hoover Dam High Scalers
Hoover Dam Historic Photos
Hoover Dam Map
Hoover Dam Men
Hoover Dam Pictures
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