The water is saved, for now.
Seven Western states have agreed on a plan to manage the Colorado River amid a 19-year drought, voluntarily cutting their water use to prevent the federal government from imposing a mandatory squeeze on the supply.
State water officials signed the deal on Tuesday after years of negotiations, forestalling what would have been the first federally enforced restrictions on the river’s lower basin. But any victory may be short-lived. Climate change promises to make the American West increasingly hot and dry, putting further pressure on the Colorado and the 40 million people who depend on its water.
“We all recognize we’re looking at a drier future,” said Tom Buschatzke, director of the Arizona Department of Water Resources.
The water in Lake Mead, the vast reservoir formed by the Hoover Dam that supplies the lower basin, has dropped to levels not seen since the 1960s. Lake Mead and Lake Powell, another reservoir on the river, are essential sources of water for Southern California and Arizona, and sit at less than 40 percent full.
By the beginning of March, the water level in Lake Mead had dropped to 1,088 feet above sea level. At 1,075 feet, under guidelines agreed to in 2007, the federal government would declare a shortage on the lower Colorado River, and mandatory water restrictions would go into effect.
Without sacrifices by the states — Colorado, New Mexico, Utah and Wyoming in the upper basin, and Arizona, California and Nevada in the lower basin — the reservoir could reach the trigger point next year, though recent heavy snowfall in the mountains that feed the river may help for a time.
Brenda W. Burman, commissioner of the Bureau of Reclamation, the federal agency that oversees the river, pressured states and their water agencies to make a deal. Without an agreement, she said, she would “take action to protect the river,” without specifying what that action would be.
In a conference call with reporters on Tuesday, she said she was “pleased to see their hard work is done.” The seven states and participating water districts sent a drought contingency plan to Congress, seeking legislation “for immediate implementation,” according to a statement from the bureau.
“It’s a hard-to-put-together puzzle, all about sharing some burdens,” said Sharon B. Megdal, director of the Water Resources Research Center at the University of Arizona. The plan builds on water conservation efforts that have, for example, kept Southern California water use relatively flat for decades despite a population boom.
The Imperial Irrigation District, California’s largest user of water from the river, threatened to derail the process when it demanded $200 million from the federal government to help restore the Salton Sea, the state’s largest lake. But other state water districts said they would cover Imperial’s share of water cuts without requiring that flows to the Salton Sea be reduced.
The Salton Sea is drying up in part because of measures taken by California farmers to use less water, which result in less runoff flowing into the lake. As it dries, fine dust from the lake bed blows into the air, which has been linked to childhood asthma and other illnesses.
Robert D. Schettler, a spokesman for the Imperial District, called the decision to move forward without its participation “unfortunate,” adding that a pact without the Imperial District “may mean getting it done, but not getting it right.”
The federal government regulates the water, but the states own the rights to it, said Jennifer Pitt, an expert on river issues with the Audubon Society. “So there’s a tension there,” she said. “The federal government’s consistent approach is to use that authority as a stick, but not ever go so far as to have to claim it.”
The river is important to the people who use its water, but also to “all of nature that depends on the river in the arid landscape of the Southwest,” Ms. Pitt said.
Another big risk is that Lake Mead could eventually drop below 950 feet, when water could no longer turn the dam’s turbines, or even 895 feet, when the lake would reach “deadpool” status and no water could flow out. That, said Patricia Aaron, a spokeswoman for the Bureau of Reclamation, need never happen. “That’s what the drought contingency planning on the river is about,” she said.
Brad Udall, a senior scientist at Colorado State University and an expert on water supplies in the West, told a congressional panel last month that the lower basin uses about 10.2 million acre-feet of water from the river each year, while upstream flows provide just nine million. (An acre-foot is the volume of one foot of water over one acre, about 325,000 gallons.)
Beyond that drain, climate change is bringing on a long-term crisis. “The Colorado River, and the entire Southwest, has shifted to a new hotter and drier climate, and, equally important, will continue to shift to a hotter and drier climate for several decades after we stop emitting greenhouse gases,” he said in his testimony.
In an interview, Mr. Udall said the influence of climate change was already apparent in the West. “Climate change is not some distant process,” he said. “It’s here, it’s now, it’s in our faces. It’s creating messes we have to deal with.”
Jonathan T. Overpeck, a climate scientist at the University of Michigan, said that politicians and policymakers needed to factor climate change into their plans. Lack of river water will lead people to pump more groundwater, which was deposited in the ice ages. “We’re using this fossil groundwater in unsustainable ways,” he said.
In a warming world, Dr. Overpeck said, less water in rivers and lakes is inevitable, whatever relief a wet season might bring. But for the most part, Western political leaders “don’t want to talk about it,” he said. “It is the disaster that’s over the horizon, if we don’t talk about it.”
For more news on climate and the environment, follow @NYTClimate on Twitter.