The Trump administration’s plan for dealing with worsening water shortages along the Colorado River is encountering strong opposition from Arizona’s leaders, who are warning they could sue to challenge mandatory water cutbacks that they say would harm their state.

A legal fight could end up before the U.S. Supreme Court and further complicate efforts by seven Western states to agree on how to share water from the shrinking river and its reservoirs, which have fallen to record low levels.

Arizona laid out its objections in a sharply worded letter to the federal government last week. It said the Trump administration’s 10-year plan doesn’t adhere to the Colorado River Compact, the 1922 agreement that originally divided the water among the states.

“Arizona does not accept a framework that gives the federal government the discretion to select from a wide range of alternatives — including catastrophic cuts … every two years for the next decade,” Arizona Department of Water Resources Director Tom Buschatzke said in the letter.

The Trump administration’s plan, released July 31, allows for drastic water cuts if they’re deemed necessary, potentially requiring the three downstream states of Arizona, Nevada and California to cut up to 3 million acre-feet per year — as much as 40% of the combined allotments.

For the next two years, state officials say, the Trump administration has accepted an offer by the three states to reduce what they take from the river by about half that amount.

Lone Rock is reflected in a stream of water in a section of Lake Powell that previously was underwater at Lone Rock Beach in Big Water, Utah.

(Justin Sullivan / Getty Images)

But Buschatzke told the administration that Arizona does not agree to the federal approach for longer than two years and would not accept such large cuts. He said the state “reserves its rights” to demand the federal government comply with the 1922 agreement and the “Law of the River” — and also reserves the right to take its case to “an appropriate judicial forum.”

He did not say when or precisely under what circumstances the state might sue. But experts expect the brewing dispute to end up in court.

“Arizona’s back is against the wall,” said Kathryn Sorensen, director of research at Arizona State University’s Kyl Center for Water Policy. “What does Arizona have to lose?”

If the state were to sue and ultimately lose, it probably would not be in a worse situation than if it simply accepted large cuts under the federal plan. In this situation, she said, “you’ve got to fight.”

The Colorado River provides water for about 35 million people and 5 million acres of farmland, from the Rocky Mountains to northern Mexico.

The 1922 agreement overpromised what the river could provide. And in the last quarter-century, relentless drought intensified by climate change has sapped the river’s flow and left its reservoirs severely depleted.

Both Lake Mead and Lake Powell, the nation’s largest reservoirs, are at record low levels and continue to decline.

Arizona officials have pointed out that the amount of water flowing into Lake Mead soon could fall to a trigger point — a legal “trip wire” that would allow the state to demand cuts upriver and sue for a violation of the compact.

The century-old agreement requires the water released from upstream dams for Arizona, Nevada and California to average at least 7.5 million acre-feet over any decade, plus an allotment for Mexico. The water reaching the three downstream states could fall below that point this year.

Buschatzke noted in his letter that the Interior Department’s plan, outlined in a document called a final environmental impact statement, “does not even mention the Colorado River Compact.”

The Trump administration is soon expected to release another document called a record of decision formalizing its plan.

The federal government was required to set new rules for addressing shortages this year because the old rules are expiring. The three downstream states of California, Arizona and Nevada have deadlocked in negotiations in a tense dispute with the upstream states of Colorado, Wyoming, Utah and New Mexico.

The Trump administration’s plan is not requiring water cuts for the four upstream states over the next two years.

California is expected to cut its use of Colorado River water by about 12% through 2028, while Arizona will take about 31% less and Nevada 28% less.

Arizona faces especially large water cutbacks because the Central Arizona Project, the series of canals that run to the Phoenix and Tucson areas, isn’t nearly as old as other aqueducts, giving it low-priority water rights and putting it among the first in line for cuts.

Much or all of the water that the state gets via the CAP canals could be cut under the Trump administration plan.

“Arizona is on the razor’s edge,” said Brian Richter, a water researcher in New Mexico who studies the Colorado River. “Under what’s being proposed, they could lose the right to use the Central Arizona Project, and that’s going to be catastrophic for that state.”

Arizona is likely to prevail in a lawsuit because the 1922 agreement clearly says the three lower states are entitled to receive the agreed-upon minimum amount of water, Richter said.

However, he also said that the compact is woefully out of date and should be scrapped and rewritten to match the reality of the shrinking river.

“We keep trying to patch on a broken system,” he said. “It served its purpose for a long time, but it’s definitely time to revisit it.”

Richter has called for setting a cap on total water use and allocating water based on what the river can provide. About three-fourths of the water that’s taken out is used for agriculture, growing alfalfa for cattle as well as lettuce, broccoli and other crops.

Since 2020, the river flow has averaged 32% less than it did in the 20th century, according to federal data.

Scientists say climate change is driving the aridification of the Southwest. Research has shown that roughly half the decline in the river’s flow is because of higher temperatures, and for each 1.8 degrees Fahrenheit the region warms, the flow is likely to decrease about 9%.

This year, the Rocky Mountains had the least snow on record.

Southern California’s cities get nearly one-fourth of their water from the river. Managers of the Metropolitan Water District, which supplies nearly 19 million people, have said the federal plan represents a vital step, but they’re disappointed it contains no water reductions for the upstream states.

None of California’s water officials have indicated the state is considering suing.

Arizona appears to be issuing a “shot across the bow” to warn the Trump administration and other states it’s prepared to go to court, said Anne Castle, a senior fellow at the University of Colorado Law School Getches-Wilkinson Center.

However, Castle said any litigation won’t help resolve the fundamental problems: that demand for water continues to exceed the available supply, and that the century-old agreement dividing the river “just isn’t fair in the context of today’s hydrology.”