This story is part of the Grist series Parched, an in-depth look at how climate change-fueled drought is reshaping communities, economies, and ecosystems.

In theory, the federal government can unilaterally cut water deliveries from the Colorado River’s two main reservoirs, Lake Powell and Lake Mead, which release more than 2 trillion gallons of water to farms and cities across the Southwest each year. In reality, this has never happened: Previous cuts have always been negotiated between the federal government and the seven states that use the river. 

Late last week, however, the federal government sent its strongest signal yet that it is willing to single-handedly impose water cuts on the Colorado for the first time in history, as the U.S. West stares down the consequences of a climate-change-fueled megadrought that has parched the river.

The Department of the Interior, the federal agency that manages water in the Colorado River basin, announced on Friday that it would look into changing the rules for how it operates Lake Powell and Lake Mead, which are located in southern Utah and southern Nevada, respectively. This would pave the way for the department to impose sharp cuts on major water users in Arizona, Nevada, California, and Mexico, which receives water pursuant to a 1944 treaty.

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In effect, the letter is a formal warning to the river states, telling them that if they fail to make the major cuts necessary to prevent the reservoirs from bottoming out, the feds won’t hesitate to unilaterally cut their water deliveries to do so.

The Interior Department said in its Friday letter that it would conduct an environmental review before changing the rules to impose new cuts on the states. This will give states one more chance to come up with their own voluntary reductions before the government enacts its own. According to John Fleck, a professor of water policy at the University of New Mexico, the upshot of all this is that unprecedented water reductions are all but guaranteed next year.

“Whether those cuts are imposed by a government action, or voluntary action by the states, or the fact that the reservoirs are fucking empty, they will happen,” he told Grist.

The new review comes after months of tense negotiations between the federal government and the seven basin states: California, Colorado, Utah, Wyoming, New Mexico, Nevada, and Arizona. Earlier this year, as water levels in Lakes Powell and Mead fell to historic lows, officials at the Interior Department’s Bureau of Reclamation ordered states to reduce their water consumption. The Bureau wanted a total reduction of between 2 and 4 million acre-feet — roughly a third of all water usage on the river.

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The states have not even come close to meeting that goal. Major water users in California, which is the thirstiest of the seven states by far, agreed last month to cut water withdrawals by about 400,000 acre-feet, a decision that will have major implications for the agriculture-heavy Imperial Valley as well as the Los Angeles metro area. Arizona has reduced its Colorado usage over the past two years in compliance with pre-existing drought restrictions from the feds. The four states that comprise the river’s “upper basin” — Colorado, Utah, New Mexico, and Wyoming — have not announced any concrete steps to cut their water usage. 

Meanwhile, the outlook for the river’s two main reservoirs has continued to worsen. As runoff from melting snow in the northern Rocky Mountains works its way down through the Colorado River’s tributaries and into the river’s mainstem, the Bureau of Reclamation stores this water in Lake Powell, which sits on the border of Utah and Arizona. The Bureau then releases some of this water further down the river to Lake Mead in Nevada, and then further on to water users in the Southwest. 

The ongoing, two-decade drought has reduced overall precipitation and evaporated more Rockies snowmelt before it can reach the river, which has reduced inflow into both reservoirs. They now sit three-quarters empty, and the most recent federal projections show that they could each decline below a critical threshold in the next two years. In the worst scenarios, it’s possible that the reservoir dams might cease to generate hydropower, or that the water level in the reservoirs would fall lower than the pipes that release it from the dams. This would make it impossible for the Bureau to move water through the river system.

The Interior Department’s Friday announcement brought home the gravity of the situation, albeit in somewhat bureaucratic language.

“The Department currently lacks analyzed alternatives and measures that may be necessary to address such projected conditions,” wrote Tommy Beaudreau, the department’s deputy secretary. He added that the conditions “pose unacceptable risks” to the river system, and that a solution needs to be “expeditiously developed.”

The federal government technically has the authority to make changes to the amount of water it releases from the reservoirs without consulting the states, but it has never had to test that authority: the current shortage guidelines were the product of a yearslong negotiation process between the Interior Department and the states. The feds are now threatening to alter that agreement on their own, and the Interior Department’s announcement helps lay the groundwork for such an intervention. If the government does modify its guidelines, it could set a new threshold for when to stop releasing water from Lakes Powell and Mead, imposing deeper and earlier cuts than states have endured so far. The review process puts the feds on firmer legal footing in case a state water user sues over the new reductions.

The losers in such a scenario would be the lower basin states — California, Nevada, and Arizona — which rely on water that the government releases from Lake Mead, as well as Mexico, where decades of overuse caused the river delta to disappear during the twentieth century. The states use the bulk of this water for agriculture, but a significant share also flows to major cities. The upper basin states draw water from the river before it reaches the reservoir, so they would be insulated from changes to the reservoir rules.

The government’s review won’t conclude until next summer, but new rules could take effect immediately, which means painful new cuts may arrive in the Southwest as the region’s farmers are preparing for peak growing season.