In the first salvo of what could become a prolonged court battle over Colorado River water, the state of Nevada has sued the Trump administration over its plan to impose major water cutbacks on Las Vegas.

The state filed the lawsuit on Monday, just three days after the Trump administration released the plan, which requires Nevada, Arizona and California to cut water use by about 20% for the next two years. The plan allows for much larger mandatory cuts over the next decade if necessary to prevent the river’s drought-depleted reservoirs from reaching critically low levels.

Announcing the lawsuit, Nevada Gov. Joe Lombardo said that under the plan issued by the Interior Department, “southern Nevada could lose more than 70% of its already meager Colorado River allocation,” while four other states — Colorado, Utah, New Mexico and Wyoming — “are not required to contribute a drop.”

“This isn’t about political posturing,” said Lombardo, a Republican. “This is a matter of survival for a community that represents about two-thirds of our state’s citizens and the lion’s share of its economy.”

The Colorado River’s reservoirs have dropped dramatically over the last 26 years during a historic drought that research shows is being intensified by global warming. The largest reservoirs, Lake Mead and Lake Powell, have fallen to their lowest levels on record.

Arizona’s leaders are also threatening to challenge the mandatory water cutbacks.

A legal fight could end up before the U.S. Supreme Court and further complicate efforts among the states to negotiate a solution to the worsening crisis.

Nevada filed the lawsuit in federal district court against the Trump administration’s 10-year plan. State leaders said that under the plan, Nevada’s annual allotment of 300,000 acre-feet could be slashed to as little as 86,500 acre-feet, which they said would be devastating for Las Vegas and surrounding cities.

“That’s simply not a reduction that a major metropolitan area can adjust to,” said John Entsminger, Nevada’s lead negotiator and general manager of the Southern Nevada Water Authority. “It would have catastrophic effects on our ability to provide a safe, reliable water supply.”

The Las Vegas area gets about 90% of its water from the the river. It has already dramatically reduced its use of Colorado River water by about 40% over the last 25 years, including by aggressively promoting the removal of thirsty grass and outlawing front-yard lawns for new subdivisions.

“We have clearly demonstrated our ability to adapt and do more with less,” Entsminger said. “However, conservation has its limits, and there is just no way to meet even the basic needs of this community with the volume of water Interior has proposed.”

The Colorado River provides water for about 35 million people in cities from Denver to San Diego, as well as 30 Native tribes and 5 million acres of farmland. About three-fourths of the water that’s taken from the river flows to farms via canals, producing alfalfa for cattle as well as lettuce, broccoli and other crops.

This year, the Rocky Mountains had the least snow on record, and the lack of snowmelt is sinking reservoir levels lower.

Lake Mead, near Las Vegas, is now 27% full. Lake Powell, on the Utah-Arizona border, has declined to just 22% of capacity.

The federal government was required to set new rules for addressing shortages this year because the old rules are expiring.

Over the last three years, state negotiators repeatedly held talks to try to reach a seven-state water-sharing agreement, but the three downstream states, California, Arizona and Nevada, deadlocked with the four upstream states.

As part of its plan, the Trump administration accepted an offer by California, Arizona and Nevada to reduce the water they take from the river by 3.2 million acre-feet between now and the end of 2028.

California will cut its Colorado River water use by about 12%, while Arizona will take about 31% less and Nevada 28% less.

But in its rules for the next two years, the Trump administration is not requiring water cuts for the four other states — Colorado, Utah, New Mexico and Wyoming.

Andrea Travnicek, the Interior Department’s assistant secretary for water and science, said last week this is because the federal government has different “responsibilities and authorities” in the upstream states.

In previous agreements, however, all seven states have contributed, Entsminger said, and the federal plan represents a sharp break with that history of consensus-building.

“Everybody that uses water from the Colorado River has a stake in using that water more efficiently, so that we can all get by with less,” Entsminger said. “A plan that focuses on a minority of states ultimately has no chance of success.”

Lombardo said “the solution needs to involve everybody” and until that happens, “we are prepared to fight for as long as it takes.”

Even as the state goes to court, Entsminger stressed that Nevada is open to negotiating.

He noted that many lawsuits end in a negotiated settlement rather than a judge’s decision.

In its complaint, Nevada argues that the Interior Department failed to account for the severe damage that drastic water cuts would inflict on southern Nevada’s $180 billion economy.

Arizona’s leaders have raised similar concerns, saying the most draconian cuts under the plan would severely harm their state.

Tom Buschatzke, Arizona’s lead negotiator, said last week that the state is “reserving our right to potentially take legal action.”

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 reductions. Much or all of the water that Arizona gets via this canal could be cut off under the Trump administration plan.

Brenda Burman, general manager of the Central Arizona Project, called the federal plan “flawed” and said “every state that relies on the river should be part of the solution.”

Her agency said in a written statement that after 2028, the federal plan would allow for “disproportionate, unlawful, and intolerable cuts on Arizona.”

The U.S. Supreme Court last got involved in a settling a long-running dispute over Colorado River water in a landmark 1963 decision in the case Arizona v. California.

This case is also likely to end up before the Supreme Court, and Nevada makes strong claims, though the outcome is hard to predict, said Rhett Larson, a water law professor at Arizona State University.

“The litigation over the Colorado River isn’t just about who gets how much water, but about what the U.S. Constitution says about who does or should have the power to make that decision,” Larson said in an email.

The case will probably take years, he said, and if a court blocks the plan, states could have to figure out what to do in the meantime.

“Litigation is far from an ideal outcome, and carries a lot of costs, risks, and uncertainties that everyone wants to avoid,” Larson said. “But it has its benefits. When parties are not communicating very well ... it’s time for litigation.”

California uses more Colorado River water than any other state.

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

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