Glen Canyon Dam Confonts Potential Loss Of Generating Capacity, Damage To River Outlet Works As Drought Continues

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Meanwhile, the physical infrastructure that enables Colorado River water management is on the verge of its own real and potentially catastrophic crisis — and yet Reclamation has barely acknowledged this, with the exception of an oblique reference in an unposted technical memorandum from 2024. The falling reservoir levels reveal another, deeper set of problems inside Glen Canyon Dam, which holds back the Colorado and Lake Powell. The 710-foot-tall dam was designed for a Goldilocks world in which water levels would never be too high or too low, despite the well-known fact that the Colorado is by far the most variable river in North America, prone to prodigious floods and extended droughts. But the Bureau, bursting with Cold War confidence — or hubris — chose to downplay the threat. In the record-breaking El Niño winter of 1983, the Bureau almost lost the dam to overtopping, due to both its mismanagement and its design, because the dam lacks sufficient spillway capacity for big floods. Only sheets of plywood installed across its top and cooler temperatures that slowed the melting of that year’s snowpack saved Glen Canyon Dam.

Today, the dam is threatened not by too much water but too little. In March 2023, the water level of Lake Powell dropped to within 30 feet of the minimum required for power generation, known as “minimum power pool.” At 3,490 feet above sea level, minimum power pool is 20 feet above the generators’ actual intakes, or penstocks, but the dam’s eight turbines must be shut down at minimum power pool to avoid cavitation — when air is sucked down like a whirlpool into the penstocks, forming explosive bubbles which can cause massive failure inside the dam. Even more worrisome is what would happen next. At minimum power pool, the penstocks would have to be closed, and the only remaining way to pass water through the dam is the river outlet works, or ROWs: two intakes in the rear face of the dam leading to four 96-inch-diameter steel pipes with a combined maximum discharge capacity of 15,000 cubic feet per second. However, the ROWs, also known as bypass tubes, have a serious design flaw: They are unsafe to use for extended intervals, and start to erode when the reservoir is low.

In 2023, when the ROWs were used to conduct a high-flow release into the Grand Canyon at low-reservoir levels, there was, in fact, damaging cavitation, and the Bureau has warned that there would likely be more in the event of their extended use. In practice, safe releases downstream may only be a fraction of their claimed capacity — and if the tubes begin to experience cavitation, flows may need to be cut off entirely. Such a scenario would compromise the dam’s legal downstream delivery requirements, or, to put it bluntly, its ability to deliver enough water to the 25 million people downstream who rely on it — as well as the billions of dollars’ worth of agriculture involved. This means that Lake Powell — and with it, the entire Colorado River system — is perilously close to operational failure.

If reservoir levels drop to the ROWs’ elevation of 3,370 feet above sea level, Lake Powell would reach “dead pool,” where water would pass through the dam only when the river’s flow exceeded the amount of water lost to evaporation from the reservoir. No other intakes nor spillways exist below the ROWs. There is no “drain plug.” Yet there is more dam — 240 feet more before the bottom of the reservoir, effectively the old riverbed. This not-insignificant impoundment — about 1.7 million acre-feet of water — would be trapped, stagnant and heating in the sun, prone to algal blooms and deadly anoxia. The lake would rise and fall wildly, as much as 100 feet in a season, because of the martini-glass shape of Lake Powell’s vertical cross section.

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https://www.hcn.org/articles/the-coming-failure-of-glen-canyon-dam/