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As climate change and prolonged drought continue to wreak havoc on the Colorado River, the federal government for the first time declared water shortages in Lake Mead, one of the river’s main reservoirs, on Monday.

The declaration triggers cuts to the water supply, which for now will mostly affect Arizona farmers. Starting next year, they will be deprived of most of the water they have relied on for decades. Much smaller discounts are mandatory for Nevada and Mexico on its southern border.

But larger disruptions, affecting far more of the 40 million people in the West who rely on the river for at least some of their water supplies, are likely in the coming years as a warming climate continues to reduce how much water flows into Colorado from rain. and melting snow.

“As this seemingly relentless decline in supply continues, the shortages we’re starting to see implemented will only increase,” said Jennifer Pitt, who runs the Colorado River program at the National Audubon Society. “Once we get on that train, it’s not clear where it stops.”

The Bureau of Reclamation, an agency of the Department of the Interior, declared the famine as it released its latest outlook for the river for the next 24 months. That forecast showed that by the end of this year, Lake Mead, the major reservoir near Las Vegas, will reach an elevation of 1,066 feet above sea level. It hadn’t seen a level this low since it began to fill up after the completion of the Hoover Dam in the 1930s. The capacity of the lake will be 34 percent.

“Today’s announcement highlights the challenges we face in the Colorado River basin and elsewhere in the West,” said Tanya Trujillo, deputy interior secretary for water and science.

Water levels in Lake Mead and in Lake Powell, the other major Colorado reservoir in Utah, have been falling for years, leaving a “ring of tubs” of clear white mineral deposits along the shoreline as demand outstrips supply.

The mandated cuts, called Tier 1 reductions, are part of a contingency plan approved in 2019 after lengthy negotiations between seven states that use Colorado River water (California, Nevada and Arizona in the lower basin, and New Mexico, Utah, Colorado). and Wyoming in the upper basin. American Indian tribes and Mexican officials were also involved in the planning.

The famine announced Monday only affects the lower basin states, but the Bureau of Reclamation may declare a similar famine for the upper basin perhaps as early as next year.

The famine declaration would reduce the Colorado River water by about 20 percent, or 512,000 acres-feet, provided by a system of canals and pumping stations called Arizona’s Central Arizona Project. (An acre foot is about 325,000 gallons, enough water for two or three households for a year.)

In anticipation of the cut, some farmers left their fields fallow or switched to less water-consuming crops. Others will pump more groundwater to compensate for the cuts, which raises additional questions about sustainability as groundwater resources are not unlimited.

“The river is an iconic resource,” said Kevin Moran of the Environmental Defense Fund. But we should also consider managing our groundwater.”

For Lake Mead and Colorado, the question is whether Tier 1 cuts will be enough to stop the decline in supply as climate change continues to affect the flow of the river. Additional layers, which will come into effect soon if the lake level continues to drop, will include increasingly drastic cuts, as with the projects released Monday. Even more discounts may need to be negotiated.

Ted Cooke, general manager of the Central Arizona Project, said this year has been one of the worst ever to flow into the Colorado River. “The big question is, what will happen in 2022?” said. After two decades of drought, “one thing we don’t have is resilience in the reservoirs, because they’re so low, it’s going to be based on the type of year we’ve had in a row this year.”

Sharon B. Megdal, director of the University of Arizona’s Center for Water Resources Research, said she thinks the declaration’s focus on the river’s dire state will lead to more efforts to use less water in the region. “I think we’re going to see some adaptation,” he said. “But I don’t know if we can do this much to avoid any further cuts.”

“We’re really only talking about 2025,” said Dr. Megdal. “If things continue to get worse, I think there will be some interventions to do more. We cannot let the river system fail.”

Lake Mead currently contains about 12 million acres of water, well below its capacity of around 30 million acres. The last time it was close to full was twenty years ago.

Since then, much of the Southwest has been mired in a drought that climate scientists say rivals some of the prolonged droughts of the past 2,000 years.

Even in recent years with occasional good snowfall in the Rocky Mountains, the amount of water flowing into the river has decreased. The researchers say warming is largely responsible, as the soils are so dry that it absorbs much of the melting snow like a sponge before it reaches the river.

Planning for the possibility of Colorado River water reduction began shortly after the drought first began. By 2007, states had developed guidelines to deal with the famines embodied by the 2019 agreement.

“Today’s announcement is an acknowledgment that hydrology is here that we hoped we would never see and was planned years ago,” said Camille Touton, Deputy Commissioner of Corrections.

“The river is in uncharted territory,” said Mr. Moran. “Climatologists have said quite well that 40 to 60 percent of the decline is due to a warming climate.”

Mr Moran said the new infrastructure bill, which has passed the Senate but faces a tougher path in the House, contains at least a few billion dollars that could help the region cope with this new reality. This includes money to improve so-called natural infrastructure, including forests, watersheds and underground aquifers, which can help increase supply or at least slow decline.

“Our water infrastructure is not just man-made reservoirs and treatment plants,” he said. “That’s the natural system.”