Dam Number Two. America’s Glen Canyon Dam Continent: North America
Finished in 1963. 710 feet tall. One quarter mile wide. 10 million tons of concrete vs. over a billion tons of water and silt in the reservoir. Built on sandstone. Location: About due south of Salt Lake City, Utah, just over the Arizona border on the Colorado River. The dam impounds Lake Powell, second largest reservoir in America and is part of the Colorado Basin Water Diversion Project which includes Lake Mead, America’s largest reservoir, also on the Colorado River, impounded by Hoover Dam (built in 1935), and situated downstream about 200 miles from Glen Canyon.
Colorado Basin provides water, electricity, and irrigation for the Southwest United States and Mexico. San Diego, Las Vegas, Los Angeles, Phoenix to name a few. Water from the Colorado Basin grows food for Americans all over the country and also food for export around the world.
Two worst case scenarios for any dam are they go to ‘dead pool’ for lack of water or they fail catastrophically from too much water and wash away everything downstream. Glen Canyon Dam has come within a hair’s breadth of both scenarios during its short life.
In 1983 a strong late-season snow pack, (some mountain passes getting 70 feet) followed closely by a warm spring, melted so much water down every tributary of the Colorado, and the river itself, that Lake Powell began rising rapidly up the face of the dam. Engineers opened the massive 175 ton, 52 foot tall, steel gates to spillway tunnels—41 feet in diameter, with three foot thick concrete liners—carved in the mountains on both sides of the dam to relieve pressure for just this scenario.
Due to a design flaw in the tunnels, water—tens of thousands of cubic feet per second—roaring down the tunnels at 100 miles per hour, began to cavitate inside the tunnels breaking progressively larger holes in the concrete liners, especially the left one, until the parts of the liners were eaten away. At that point the force of the water began excavating building size craters in the mountain. Water shooting out the base of the tunnels turned brown with pulverized stone as boulders the size of box trucks washed into the sluice.
That forced the engineers to gate back water flow to the spillway tunnels. In turn, that caused water to rise against the dam. The dangers were: a) if the water over topped the spillway gates, it would flow uncontrolled down the passage behind the gates into the spillways which would compromise them more. b) (stay with me on this one, it’s worth it) When the dam was built, the engineers punched a more or less straight tunnel through the mountains on each side of the structure’s foundation to divert the river while the dam was going up. Before they began filling the dam engineers plugged the upstream end of these tunnels with concrete. The concrete plugged ends were left at the bottom of the reservoir when it was filled. Meanwhile, the spillway tunnels, hundreds of feet above the plugged ends of the diversion tunnels, dove through the mountains on either side at a 55 degree (that’s steep) angle and connected to the open ends of what were formerly the diversion tunnels. So, the danger was, rushing water from an overtopped spillway might back up and compromise one or both of the concrete plugs, opening the tube at the bottom of the reservoir, causing the reservoir to drain uncontrollably. c) if the water overtopped the dam itself, it would become a 710 foot water fall pounding into sandstone at the base of the dam, potentially compromising the whole structure.
If Glen Canyon Dam were to fail, an unstoppable wall of water would shoot down the Colorado River to Lake Mead, which would overwhelm Hoover Dam. If that failed, the combined weight of both reservoirs would head south flattening every dam, not to mention farms, towns, cities, roads, bridges, sewer systems, and other infrastructure all the way to Mexico, leaving millions without water, electricity, or irrigation.
The way they saved the dam is a legend in Western America. With water just inches below the tops of the spillway gates, engineers sent workers out onto tops of the spillway gates with 4 x 8 sheets of plywood (!) that the workers butted sideways across the top of the gates then bolted down to the gates with angle iron. This made a 4 foot high extension above the gates. The water rose a few feet up the plywood but not all the way. The plywood held until they could fabricate 8 foot metal retaining walls. Between that and opening the spillways as much as they dared, Bureau of Reclamation workers saved the structure. Still, at its height in 1983, Lake Powell came within 7 feet of the top of the dam itself. Whew!
Another legend is how they repaired the spillways in just under a year. Which they had to because it was another big El Nino snow winter and the runoff was even greater but the repairs and modifications to the spillways worked ,and have continued to work, for the past forty years. (I’ll put a link to a Bureau of Reclamation documentary about it below. However you feel about Glen Canyon Dam, it makes you proud to watch it.)
The other worst case scenario for Lake Powell is going on now. During summer there’s a white bath tub ring extending more than 100 feet above the lake surface. The top is where the water used to be. After years of drought, the water’s way down there. It’s going on in Lake Mead downstream, too. We see that and say, “Whoa, that’s scary.” And it is, but even scarier is what we don’t see. Every day, since 1963 the Colorado River has been dumping, on average, the equivalent of 30,000 dump truck loads of silt into Lake Powell. Let me say that again. Every day, since 1963 the Colorado River has been dumping, on average, the equivalent of 30,000 dump truck loads of silt into Lake Powell. This is a silt load comparable to what the Yangtze is dumping into Three Gorges. So, in giant dams, while the water is going down, the bottom is coming up.
In 2021 Lake Powell went to 31 percent of capacity. There was concern it would go to dead pool but they were able to keep the lights on and pumps running. Still, concerns remain that it could happen in summer of 2022. To maintain reserves for the driest months, there are plans to proactively cut back water allotments for all the states receiving Colorado Basin water, and cutting 5 percent of the water to Mexico that the US has pledged to supply, (80,000 fewer acre feet). Less water means less agriculture and increased food prices.
We exist by the grace of nature that has the power to flatten us in multiple ways. It’s nothing personal. This being the case, even as drought makes dead pool the more likely scenario for stopping the dam, big weather events off the Pacific we’ve seen in recent years could bring another El Nino winter with massive snow and run-off creating a 1983 type flooding scenario. No one has that crystal ball to say which it will be. Either way, there’s growing sentiment that Glen Canyon Dam was a bad idea and that it should be removed. However it goes, the days of this dam are numbered and when it’s gone, food prices around the world will ripple upward as Colorado Basin water supply spirals down.
07.18.2019 For a While in 1983 Sheets of Plywood were all that kept the mighty Glen Canyon Dam from over flowing The Republic/Arizona Central by John D’Anna. Excellent article.
1993 edition Cadillac Desert: The American West and Its Disappearing Water, by Marc Reisner. 582 Pages. Far and away the best book ever on water issues in the American West. Penguin Books.
1975 The Monkey Wrench Gang by Edward Abbey. Abby’s most famous work centers around blowing up the Glen Canyon Dam.
Challenge At Glen Canyon Bureau of Reclamation documentary about repairing the dam spillways that failed in 1983. You might say the Bureau made themselves look good by saving something they shouldn’t have built in the first place. I wouldn’t argue. But still, it makes you proud to watch what those workers accomplished in less than a year.