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Lakes Powell and Mead, the nation’s largest reservoirs, are drying up, sending a Colorado River system that provides water and electricity for millions of people into a state of crisis with no end in sight.

Water levels in Lake Powell, along the Utah-Arizona border, fell to a record low on Saturday. Lake Mead, the larger of the two and farthest downstream on the Colorado River, dropped to its lowest recorded point a week earlier.

The combined reservoir system, an essential water source for cities and farms across Arizona, Nevada and Southern California, has not held so little water since 1957, before Lake Powell was filled. That was long before climate change brought three decades of drought, the worst dry spell for the Southwest in at least 1,200 years, scientists said.

Federal and state officials and communities across the region have taken emergency steps to try to head off catastrophe. They shifted water from a smaller reservoir to try to save Powell and have called for water conservation, though those requests are mostly voluntary so far.

The water shortage could have dire implications for growing Sun Belt cities such as Phoenix, for key industries including agriculture and data centers, and for hydroelectric dams that power millions of homes across the Southwest.

The shrinking reservoirs are the immediate result of record-low snowpack this winter, which was erased by record heat in the spring. Melted snowpack creates most of the Colorado’s flow during the dry summer months. But as the Southwestern climate has grown hotter and drier, the river’s flow has become smaller and less reliable.

“We expected to be in a very unpleasant place, and that’s where we find ourselves,” said Kathryn Sorensen, the director of research at the Kyl Center for Water Policy at Arizona State University. “Mother Nature is in charge of this system.”

At the same time, a booming population in the Southwest and intensive agriculture mean that the region is consuming far more water than the river can now provide.

Here is what to know about the crisis.

Who uses the reservoirs?

The Colorado River supplies water to 40 million people and 5.5 million acres of farmland across parts of seven states. About two dozen native tribes and two Mexican states also hold rights to the river’s water.

Lakes Powell and Mead are central to maintaining that water supply in the lower half of the Colorado basin. While the river’s upper basin — which stretches across Wyoming, Colorado, Utah, New Mexico — relies more heavily on seasonal stream and river flows for water, the two man-made lakes have served as a predictable supply for the more populous lower basin, which includes parts of Nevada, Arizona and California.

Agriculture is the biggest user of the Colorado River, with alfalfa and other sources of livestock feed accounting for nearly half of the water consumed. Municipal water systems use about a quarter.

And a growing share — about 10 percent of the river’s flow — is not consumed at all, but evaporates as temperatures climb across the Southwest.

How much has the water supply declined?

Dry and hot conditions across the Colorado River basin have cut the river’s flow by about 20 percent over the past two decades.

The current water use levels are unsustainable, too. While about 12 million acre-feet of water now flows down the Colorado in an average year, the century-old rules that govern the use of the river have allocated the rights to more than 16 million acre-feet. One acre-foot, or enough water to cover an acre a foot deep, is about 326,000 gallons, enough to supply two or three households for a year.

Diminishing supply, combined with decades of overuse, has caused water levels in Lakes Powell and Mead to decline steadily over the past 30 years. In both reservoirs, water storage has not been close to capacity since the late 1990s. Lake Mead is now about 27 percent full, while Lake Powell is about 23 percent full.

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The Glen Canyon Dam near Lake Powell in Arizona last month. Lakes Powell and Mead, the country’s largest reservoirs, have been recording new lows this summer. Credit…John Locher/Associated Press

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