WATER POLITICS

In laying out its approach to the crisis, the Trump administration has heightened a thorny legal and practical question of who should shoulder the burden of a shrinking Colorado River.

Its plan suggests that the answer would be the Sun Belt communities along the lower portions of the river basin — cities like Phoenix, Las Vegas and Los Angeles — and not the upstream Mountain West. The federal government’s main tool to control water supply on the Colorado is its ability to limit flows through the Hoover Dam, which straddles Arizona and Nevada and creates Lake Mead, and Glen Canyon Dam, which impounds Lake Powell along the Utah-Arizona border.

The question of how to most equitably balance the sacrifices made by Mountain States and Sun Belt communities has been one of the most difficult sticking points in years of fighting. It has prevented agreement in a rapidly growing region of the country. A solution needs to address not just changes to home landscaping or lawn-watering limits, but also the vast fields of water-guzzling alfalfa and soy, a growing data-center industry and the supply of much of the nation’s produce.

The Trump proposal alarmed Arizona officials on Friday.

“Phoenix has spent decades planning for a moment like this and is prepared to use less Colorado River water, but it is wrong to lay a disproportionate share of the burden on Arizona,” Mayor Kate Gallego of Phoenix said in a statement.

But the proposal was short on important specifics, including plans for water use over the next two years or detailed amounts of water it would allow to flow under varying river conditions.

And it appeared to leave room for the states to find their own way to share in the pain of increasing water scarcity, said Jennifer Pitt, the Colorado River program director for the National Audubon Society.

“Only time will tell if that is possible,” Ms. Pitt said.

So far, it has not. With decades-old rules governing water use set to expire at the end of this 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 what are collectively known as the upper-basin states — Colorado, Utah, Wyoming and New Mexico.

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.

Tom Buschatzke, Arizona’s chief negotiator on Colorado River issues, said the plan could have “draconian impacts” on that state and its economy, at the same time upper-basin states would not be required to use any less water.

“It’s unacceptable,” Mr. Buschatzke said.

Even without some details, the Interior Department’s plan 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.”

The Colorado River Is Shrinking. See What’s Using All the Water.

Though the plan, known as an environmental-impact statement, is final, more steps remain to govern how much water will actually flow to communities. The Interior Department’s Bureau of Reclamation, which oversees water resource management in the West, is expected to release more details in the coming days and weeks.

The bureau’s officials have also said they would scrap the plans if states can agree on a different solution.

Across the region, two decades of drought have reduced winter snowpack, which creates the majority of the river’s flow. At the same time, 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 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 said 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 the country and the United States are separately ongoing. About two dozen native tribes also have rights to the river’s water.

The rules laid out on 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.