BOULDER, Colo. — Low water in the Colorado River’s largest reservoir triggered the first-ever federal declaration of a shortage on Monday, a bleak marker of the effects of climate change in the drought-stricken American West and the imperiled future of a critical water source for 40 million people in seven states.

Water in Lake Mead, the mammoth reservoir created by the Hoover Dam that supplies the lower Colorado basin, is projected to be 1,065.85 feet above sea level on Jan. 1, nearly 10 feet below a threshold that requires Arizona, Nevada and Mexico to reduce their consumption in 2022. On Monday, it was just under 1,068 feet, or about 35 percent full, according to the U.S. Bureau of Reclamation, which manages the water that states and Mexico have rights to use.

“We are seeing the effects of climate change in the Colorado River basin through extended drought, extreme temperatures, expansive wildfires, and in some places, flooding and landslides,” Tanya Trujillo, the Interior Department’s assistant secretary for water and science, told reporters Monday. “And now is the time to take action to respond to them.”

The declaration of a so-called Tier 1 shortage was expected on a river whose flows have been overallocated for a century and rapidly declining since 2000, and it was staved off by agreements under which states cut their water use. But the announcement still came as a blow at the end of a summer of extraordinary heat in the West, as well as a harbinger for further cuts that many in the region view as inevitable.

“This drought is like a boa constrictor. It just keeps getting tighter every year,” said Tom Davis, president of the Agribusiness and Water Council of Arizona, a state where some farmers will feel the cuts. Davis said he still has hope for drought-ending precipitation, but he is alarmed.

“It is a sobering thing to realize we are at Tier 1 already.” Davis said. “A few years ago, no one was thinking this would happen. It’s damn sure got our attention.”

The river starts in the Rocky Mountains of Colorado and snakes Southwest for 1,450 miles, its waters dammed and diverted along the way to irrigate fields and lawns and deliver water to industry, cities and taps. States and Mexico divvy up its waters under a complex system first agreed to in 1922, but the flow has rarely been sufficient. Allocations were made following a period of unusually heavy precipitation, meaning “we’ve been living beyond our means in terms of water delivery compared to water available,” said Sharon Megdal, a University of Arizona water expert.

A 22-year drought — the region’s most severe in more than a millennium — and climate change have made that fundamental problem worse. The alpine snowpack that feeds the river has been diminishing and was melting earlier this year. Parched soil soaks up much of it before it even enters rivers and streams. Extreme heat evaporates water in Lake Mead and other reservoirs more quickly and causes evaporation from plants.

Brad Udall, a senior water and research scientist at Colorado State University, said about half the decline in the river’s average annual flow — which has fallen 20 percent compared with the past century — is attributable to rising temperatures and half to declines in precipitation. He and other scientists say “drought” is no longer the appropriate word to describe the climate in the West. Instead, they say, it is aridification — a long-term, more permanent desiccation of a region.

“It’s as if a switch got flipped in 2000, and we now have a completely different river than we had in the 20th century,” he said. “The really big years occur half as often, and the low-flow years occur twice as often.”

The Arizona Game and Fish Department is on pace to haul nearly 3 million gallons of water throughout the state as drought continues to threaten wildlife. (Erin Patrick O'Connor/The Washington Post)

The Tier 1 shortage will hit hardest in Arizona, which agreed decades ago to “junior rights” to the river in exchange for federal funding for an aqueduct that delivers water to Phoenix, Tucson and other central parts of the state. Users of that pipeline must reduce use by 512,000 acre-feet of water, one of which is equivalent to about one foot of water spread over a football field. Arizona says that represents about 30 percent of the pipeline’s supply, 18 percent of the state’s Colorado River water, and 8 percent of the state’s total water use.

Nevada’s share of the Colorado River water will drop by about 7 percent, and Mexico’s by about 5 percent, officials said. California, which also depends on water in Lake Mead, loses no water under a Tier 1 shortage.

Arizona officials said Monday that they had planned for the cuts, and they emphasized that taps will not run dry and that most Arizonans will not feel any effect. The reductions mostly do not affect cities or tribes and instead fall primarily on central Arizona farmers.

The state says it has banked water in aquifers, implemented conservation measures, and facilitated water-sharing among users.

But mitigation efforts won’t make up for all the reductions. Those affected in Arizona “will physically not have the water, and they will have to figure out how to deal with the ramifications of that outcome,” Tom Buschatzke, director of the Arizona Department of Water Resources, said Monday.

Stefanie Smallhouse, a rancher who heads the Arizona Farm Bureau, said Arizona farmers who rely on water from the aqueduct, known as the Central Arizona Project, may have to fallow up to 40 percent of their acreage. Farmers in that region grow the alfalfa that cows eat and produce cotton seed for much of the country, she said.

“The fact that we’ll have farmers fallowing cotton production is going to reverberate into other areas of the United States which rely upon our production of cotton seed,” Smallhouse said.

Smallhouse said farmers in the area hope the $1 trillion infrastructure package recently passed by the U.S. Senate, which includes funding for new climate resilience initiatives, will help in preparing for “a drier future.”

“We have to be able to store the water when it comes, we have to be able to move the water efficiently, and we have to be able to practice conservation when we get it on the field,” she said. “That infrastructure package is extremely important to these farmers in Arizona and really anyone that’s growing food or fiber in the Colorado River basin.”

Davis, the agribusiness leader, said some farmers will pump groundwater to make up for losses. But that is a finite source.

Nevada’s allocation from the Colorado River will drop to 279,000 acre-feet next year. But conservation efforts over the past two decades, including paying residents $3 per square foot to remove grass from their homes and businesses, have reduced the state’s water usage below that level even as the population has grown, said John J. Entsminger, general manager of the Southern Nevada Water Authority.

Last year, Nevada consumed about 256,000 acre-feet of water, down from about 325,000 in 2002, he said.

“We’ve now taken out enough turf in the Las Vegas valley to lay an 18-inch wide piece of sod around the circumference of the Earth at the equator,” Entsminger said. “That’s how we’ve driven down our water usage.”

The shortage declaration is unlikely to be the last, experts say. According to some projections, Lake Mead could reach a more severe Tier 2 shortage within two years, and Tier 3 not long after that. At 950 feet above sea level, the dam’s turbines could no longer turn. At 895 feet, its waters could no longer leave the reservoir — a low point referred to as “deadpool.”

“This is a sobering moment in that it’s real, it’s here, it’s not a possibility,” said Megdal, director of the University of Arizona’s Water Resources Research Center. Arizonans are not panicking, Megdal added, “but if we have very poor precipitation and runoff conditions, we could be looking at Tier 2 sooner than anybody ever imagined.”

The shortage should serve as a call for intensified conservation efforts and reductions of planet-warming emissions. Human activities have already raised global average temperatures more than 1 degree Celsius — 1.8 degrees Fahrenheit — above preindustrial levels. In many Western states, the increase is close to 2 degrees Celsius — a threshold the United Nations associates with catastrophic warming.

“The Colorado River is ground zero for climate change in the U.S.,” said Kevin Moran, senior director of the Environmental Defense Fund’s Colorado River program. “We have to shift our thinking from managing supply and demand in the context of temporary drought to managing the reality of a permanently more arid climate.”