Communities and farms across Arizona, Nevada and California face potentially drastic cuts in the water they can use from the Colorado River over the next decade under a plan the Trump administration published on Friday.

The proposal could ignite a complex legal battle that could play out for years. The Colorado River basin supplies water to some 40 million people, including in Phoenix, Las Vegas and Los Angeles, as well as rich agricultural land. And it faces some of the most dire drought conditions on record.

For years, seven states have fought over how to divide up the water. With decades-old rules governing water use set to expire at the end of the year, the Trump administration stepped in and said it would decide.

The plan, released by the Interior Department, does not force mandatory cuts to water use on communities up the river in Colorado, Utah, Wyoming and New Mexico, collectively known as the upper basin states. That has been a stubborn sticking point for the seven Colorado River basin states, for years preventing agreement on a plan to divvy up a dwindling source of water for a growing region of the United States.

The booming Phoenix area could find its water supply severely reduced in the driest years under the plan, because Arizona holds some of the lowest-priority rights in a river system where many water rights were determined a century and a half ago. Arizona officials have signaled they are preparing to fight for their water in court, a step that could send Colorado River conflicts on an unpredictable path to the U.S. Supreme Court.

More details on the federal plan, including guidelines laying out water use cuts over the next two years, had not yet been released as of Friday morning. The Interior Department’s plan nonetheless provided states more clarity on what could be coming in the years ahead, said Sarah Porter, director of the Kyl Center for Water Policy at Arizona State University.

“It is an agency statement of what it believes it’s empowered to do,” Ms. Porter said. “It’s not necessarily the last word.”

Across the region, two decades of drought have reduced winter snowpack, which creates the majority of the river’s flow, and warming temperatures caused by climate change have increased water losses from evaporation.

The basin faced an October deadline to have a new water use plan in place. Its implementation fell to the Interior Department’s Bureau of Reclamation, after years of negotiations among the river basin states failed.

Interior Secretary Doug Burgum said the plan released on Friday “provides the flexibility to respond to changing hydrologic conditions.” He added that the Interior Department had “a responsibility to ensure the Colorado River system remains reliable and resilient for the millions of Americans, communities and industries that depend on it.”

The Colorado not only supplies drinking water to tens of millions of people but also provides irrigation to 5.5 million acres of farms. Mexico also receives a share of the Colorado’s water, and negotiations between that country and the United States are separately ongoing.

The rules laid out Friday call for a wide range of potential water releases — including reductions by more than half in the driest years — from Lake Powell, the nation’s second-largest reservoir and the dividing point between the upper and lower basins. Arizona, California and Nevada, the lower basin states, depend heavily on steady releases from the reservoir, and from Lake Mead farther downstream, for municipal water and farm irrigation.

The smallest reservoir releases under the plan would be low enough to potentially lead to legal action by the lower basin states citing the Colorado River Compact, an interstate agreement made in 1922 that is central to river operations. The threat of such a lawsuit has loomed over discussions among states as the river’s flow has dwindled.

Officials from those states had already offered to make significant water use cuts in the coming years. The Bureau of Reclamation’s plan goes even further in requiring potential reductions over the next 10 years.

The plan includes opportunities for the bureau to reconsider water use rules every two years over the next decade, and for states to renegotiate with one another. That stipulation was designed to create opportunity for consensus, but state officials have warned it may raise tensions across the river basin.

Water supply estimates and forecasts have only grown more grim as basin states and Reclamation officials have searched for a long-term solution over the past nine months. Water storage in Lakes Powell and Mead has plummeted to its lowest level since 1957, when Glen Canyon Dam, which impounds Lake Powell, was still under construction, according to an analysis published this month.

By March, Lake Powell is forecast to reach a critical low. Its surface is expected to fall too low to generate hydropower at Glen Canyon Dam, according to a federal analysis released this month.