Nevada became the first state to sue over new Colorado River operating rules, filing suit in federal court Monday alongside the Colorado River Commission of Nevada and the Southern Nevada Water Authority.

The lawsuit filed against the U.S. Department of the Interior challenges the record of decision released Friday, which governs how Colorado River water is divided among the seven basin states. 

Under the decision, Nevada's 300,000-acre-foot allocation could be cut by up to 213,556 acre-feet, leaving Las Vegas and surrounding cities with less than 86,500 acre-feet a year. An acre-foot is the amount of water needed to cover an acre of land a foot deep, about 326,000 gallons, or enough to supply roughly two to three households for a year; state water officials say there's no way to meet Southern Nevada's basic needs with the volume of water Interior has proposed.

That figure is Nevada's worst-case exposure, applying only if Lower Basin shortages reach the framework's outer limit of 3.6 million acre-feet. Southern Nevada used just 198,000 acre-feet in 2025, according to the water authority.

Read more: Nevada to lose a third of its Colorado River water over the next two years. What happens next?

The four Upper Basin states — Colorado, Utah, New Mexico and Wyoming — face no mandatory cuts under the decision, only voluntary conservation goals. That's because of how the two halves of the river are legally structured. Lower Basin states hold fixed contractual entitlements to water released from Lake Mead, which gives the federal government authority to cut them administratively during a shortage.

Upper Basin states, by contrast, are only obligated under the 1922 Colorado River Compact not to deplete the river below a certain point, not to guarantee a set delivery downstream, so there's no fixed allocation for the government to cut. Nevada and the other Lower Basin states dispute that reading, arguing the compact's "equitable apportionment" language means Upper Basin states should share in the shortages too.  

"Under the proposed plan issued by the Department of the Interior, southern Nevada could lose more than 70 percent of its already meager Colorado River allocation while the Upper Basin states of Colorado, Utah, New Mexico, and Wyoming are not required to contribute a drop," Republican Gov. Joe Lombardo said in a statement announcing the suit.

John Entsminger, the water authority's general manager and Nevada's lead negotiator on the river, said the region has cut its Colorado River consumption by about 40 percent over the past 25 years even as it added more than 800,000 residents, but that "conservation has its limits." 

The suit asks the court to set aside the decision, arguing Interior failed to analyze the economic impact of steep cuts on Southern Nevada's $180 billion economy, despite being required to do so and despite Nevada raising the issue during the public comment period. No hearing date has been set.

Under an earlier federal framework released in July, Nevada was set to give up 50,000 acre-feet of Colorado River water a year over the next two years. That figure only applied to the near-term shortage-sharing agreement for 2027 and 2028, which the Lower Basin states — including Arizona and California — proposed in May and which Interior adopted. Beyond that, the framework shifts to allocating cuts by seniority instead, protecting the oldest water rights first. 

The cuts stem from a drought spanning more than a quarter century, worsened by climate change, that has pushed Lake Mead and Lake Powell to their lowest levels on record. 

Read more: Nevada water leaders say litigation is last resort despite big Colorado River cuts

At a water conservation summit in Las Vegas earlier this month, Entsminger said litigation remained a last resort for Nevada, but that the state would protect its legal interests if negotiations failed.