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, when Lake Powell was still being 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 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.

What will happen if they fall even more?

Both reservoirs are contained by hydroelectric dams — the Hoover Dam at Lake Mead, and Glen Canyon Dam at Lake Powell. Water levels at Lake Powell are dropping close to the point where Glen Canyon Dam will not be able to generate electricity. Current forecasts by the Bureau of Reclamation, the federal agency that oversees the dams, suggest that Lake Powell’s water levels could cross that threshold as soon as winter, depending on how much precipitation falls during the wetter fall and winter. About 6 million people across the Southwest rely on electricity from the dams. Dwindling water levels have already diminished power output.

And if Lake Powell’s level drops below the point needed for hydropower, it could reach a condition known as “dead pool,” when water can no longer flow through the dam at all, cutting off the water supply for the lower basin states. It is unclear when that might occur.

What is being done to shore up the reservoirs?

The Reclamation Bureau this spring took emergency measures to prevent Lake Powell from falling too low to produce hydropower. It began moving water to Powell from an upstream reservoir on the Wyoming-Colorado border, known as Flaming Gorge. And it limited releases from Lake Powell, which flow through the Grand Canyon into Lake Mead.

Communities across the West have put drought plans into action and imposed restrictions on water use. At the same time, investments in water efficiency means large cities like Phoenix are consuming as much water as they did when they were far less populous.

Reclamation officials and the seven Colorado River basin states are continuing negotiations over a long term solution to the water crisis, but negotiations have not been fruitful.

A federal plan to guide water use for the next decade released last month included drastic cuts for the states that depend on Lakes Powell and Mead — which Arizona officials called “unacceptable” and said threatened the growing state’s economy. Reclamation officials are expected to release more detailed plans for the next two years soon.