Journalists reporting on the state and future of the Colorado River increasingly use the phrase “dead Pool. “That sounds ominous. And it is.
dead Pool occurs when water in a reservoir falls so low that it cannot flow downstream of the dam. The biggest concerns are Lake Powellbehind Glen Canyon Dam on the Utah-Arizona border, and Lake Mead, behind Boulder Canyon Dam on the Nevada-Arizona border. These two reservoirs, the largest in the United States, provide drinking water, irrigation and hydroelectricity to millions of people in Nevada, Arizona and California.
Some media reports erroneously define dead pond as the point at which a dam no longer has enough water to generate hydroelectricity. The most accurate term for this situation is minimum power pool elevation.
Like a 22 year drought in the Colorado River Basin persists, reaching the minimum elevation of the power pool is the first problem. Both Lakes Powell and Mead have turbines at the base of their dams, well below the surface of the reservoirs. The water flows through valves in the water intake towers in the reservoirs and is piped through the turbines, turning them to generate electricity.
This system relies on what hydrologists call Hydraulic head – the amount of liquid pressure above a given point. The higher the water level above the Lake Powell and Lake Mead turbines, the more hydraulic head they have and the more power they generate.
When the level in a reservoir approaches the minimum elevation of the power pool, the turbines lose their ability to generate electricity as they begin to suck in air with water and must be shut down before being damaged. A reservoir that reaches this point usually has enough water before it falls into the dead pool and the water stops flowing from the dam.
The US Bureau of Reclamation recently announced unprecedented changes in its regulation of the water of Lake Mead and Lake Powell. First, the bureau will retain 480,000 acre-feet of water in Lake Powell that was meant to flow along the Colorado River through the Grand Canyon and into Lake Mead for use by California, Nevada and Arizona. An acre-foot is approximately 325,000 gallons.
Second, the office will free up an additional 500,000 acre-feet of Blazing Gorge Dam at the Wyoming-Utah border. Water from Flaming Gorge flows into the Green River and eventually into Lake Powell. Lake Powell’s water level was 3,522 feet on April 30, 2022 – just 32 feet above the Electric Pool’s minimum elevation of 3,490 feet. The dead pool is 120 feet lower, at 3,370 feet.
The office acted suddenly because the levels of both lakes fell much faster than expected. Last year, Lake Mead dropped 22 feet; Lake Powell, 40 feet.
Extreme drought and climate change partly explain this rapid decline. Another factor is that Glen and Boulder Canyons are V-shaped, like martini glasses – wide at the rim and narrow at the bottom. As lake levels drop, each foot of elevation holds less water.
For now, finding enough water to continue generating electricity is the goal. But unless California, Nevada, and Arizona drastically reduce the amount of water they use, a dead pool in Lake Powell and Lake Mead cannot be ruled out.