Risks to the Glen Canyon Dam
[citation needed] The Colorado and San Juan rivers deposit large volumes of silt into Lake Powell, slowly decreasing its capacity.
[1] The dam, anchored in unstable[citation needed] Navajo sandstone (sometimes said to be "solidified sand dunes"),[2] nearly failed in 1983 as the result of a flood on the upper Colorado River that led to extended use of its tunnel spillways.
[1][4] The dam's useful lifespan has been estimated by some to be 85 to 100 years, and was described as "America's most regretted environmental mistake" by David Brower, then-head of the Sierra Club.
When the reservoir filled, the dam began to deliver a steady, regulated flow of water downstream and generate a cheap, plentiful supply of electricity.
In 1983, major floods nearly destroyed the two tunnel spillways that could have led to the dam's collapse, but disaster was averted by a close margin.
[1] It will take still longer for the sediment to accumulate to the point where it could clog the outlet works, which are the lowest openings in the upstream face of the dam.
When Lake Powell levels drop, sediments deposited in its upper reaches are carried into the receding water by the Colorado and its tributaries.
One large flash flood caused by heavy rainfall could move all or most of these sediments into Lake Powell, creating a sudden loss of storage capacity.
[9] In May 1983, three years after Lake Powell was first filled, an unusually long-lasting winter due to the 1982–83 El Niño event produced increased snowfall over the entire 108,335-square-mile (280,590 km2) multi-state Colorado River basin.
[12] The Bureau of Reclamation states that the highest inflow the Glen Canyon Dam can withstand is 697,000 cubic feet (19,700 m3) per second, almost seven times the 1983 total.
[14] While this made the spillways more economical to construct, they had less capacity in part because engineers must maintain at least 30 percent clearance between the water level and the tunnel ceiling.
[15] in addition to the tunnel spillways, the dam has a set of river outlet works designed to release 15,000 cubic feet (420 m3) per second.
Reclamation Bureau officials met in late June and agreed that the maximum water level the dam could handle was 3,708 feet (1,130 m).
Water exiting the spillways contained noticeable debris, including sandstone, signaling severe erosion taking place within the tunnels.
The noise was so noticeable that a worker in the employee dining room, located near the power plant, reported that it "sounded like the barrages that he had experienced in Vietnam".
The Bureau of Reclamation was concerned that the water would eventually erode the diversion plug altogether, creating a connection to the reservoir floor.
[19] On July 15, 1983, Lake Powell reached its peak level, 3,708.34 feet (1,130.30 m) about 8 inches (200 mm) from where engineers thought they'd lose control.
Eventually, Hoover Dam was also forced to open its gates; its discharge peaked at 40,000 cubic feet (1,100 m3) per second and still caused downstream flooding.
It was feared that this would produce a flood greater than the 1983 one, and as the spillway repairs continued, water was constantly released through the dam power plant and outlet works, a rate of roughly 45,000 cubic feet (1,300 m3) per second.
When the snowpack began to melt in spring of 1984, water levels reached several inches below the top of the flashboards in late June.
