Arizona has moved to join Nevada's legal challenge to the federal government's plan to impose deep, mandatory cuts on the Lower Basin states' share of Colorado River water, turning a single-state lawsuit into a two-state front against Washington's management of the shrinking river. The Arizona Department of Water Resources and Governor Katie Hobbs announced on Friday that the state had filed a motion to intervene in the lawsuit Nevada brought against the US Department of the Interior in August, challenging the final operating rules that will govern the river in 2027 and 2028.
The announcement adds the state facing the deepest near-term cuts to the fight over how federally mandated reductions are divided among the seven basin states. "We are drawing a line in the sand," Hobbs said in a statement, adding that Arizona would keep working alongside Nevada and California to carry out the three states' own two-year water agreement while the case moves through the courts.
Nevada's governor, Joe Lombardo, welcomed Arizona's decision, calling it "an important step in challenging a federal plan that unfairly burdens the Lower Basin and fails to require the Upper Basin to make meaningful reductions." "Nevada will not accept a framework that puts our water supply and the future of our communities at risk," he said in a statement.
What the federal plan actually demands
In August, the US Department of the Interior and the Bureau of Reclamation finalised new operating rules for the Colorado River covering 2027 and 2028. Under the framework, the three Lower Basin states - Arizona, California and Nevada - must collectively reduce their water use by 1.25 million acre-feet in each of those two years, a cut more than twice the size of the largest reductions required under the current guidelines, which expire at the end of this year.
The burden is divided unevenly. Arizona would be required to give up 760,000 acre-feet per year, California about 440,000 acre-feet, and Nevada 50,000 acre-feet. An acre-foot is equal to 325,851 gallons - enough water to cover an acre of land a foot deep, or roughly enough to supply two to three households for a year. At the plan's outer limits, the reductions bite far harder: Nevada's 300,000 acre-foot allocation could be cut by as much as 213,556 acre-feet, around 71 per cent, leaving Southern Nevada with less than 86,500 acre-feet a year. The Las Vegas valley used about 198,000 acre-feet in 2025. Arizona officials have warned that under the framework's worst-case scenarios, Lower Basin cuts could ultimately reach up to 3 million acre-feet a year.
The Upper Basin states - Colorado, New Mexico, Utah and Wyoming - would not be subject to mandatory water reductions under the plan. That exemption is the central grievance driving both states' legal action.
Why the Upper Basin exemption is the heart of the dispute
Arizona and Nevada argue that the federal framework ignores the basic legal structure of the river's governance - what water lawyers call the "law of the river" - by placing the entire burden of shortage on the Lower Basin while asking nothing mandatory of the Upper Basin. Tom Buschatzke, director of the Arizona Department of Water Resources and the state's lead Colorado River negotiator, said the framework "ignores the basic elements of the law of the river" and warned it could "devastate Arizona's economy." Arizona officials have also warned that the cuts could effectively dry up the Central Arizona Project, the canal system that delivers Colorado River water to nearly 6 million people across central and southern Arizona.
Nevada's complaint makes a parallel case about fairness and process. The state, along with the Colorado River Commission of Nevada and the Southern Nevada Water Authority, filed its suit on 24 August, becoming the first state to challenge the Interior Department's final plan, which had been released three days earlier. The lawsuit contends the rules threaten the water security of more than 2.3 million residents of the Las Vegas valley - a region that has already cut its Colorado River consumption by more than 40 per cent since 2002, even as its population grew by roughly 875,000 people, according to the regional water authority.
The Upper Basin states, for their part, have called for negotiations to resume and for mediation to be brought in, signalling that they see a negotiated settlement rather than a court-imposed one as the path forward. The standoff sets up a prolonged legal and political contest over how a river that can no longer deliver what the law promises should be shared.
A river in its 25th year of megadrought
The lawsuit lands after years of failed negotiations among the seven states that depend on the Colorado River, set against a hydrological crisis that has steadily worsened. The basin is in the grip of a 25-year megadrought, and the winter of 2025 to 2026 delivered the lowest observed snowpack on record for the region that feeds the river's headwaters.
The consequences are visible in the system's great reservoirs. As of July 2026, Lake Mead stood at just 28 per cent of capacity and Lake Powell at 24 per cent, with the two reservoirs' combined storage at its lowest level since Lake Powell began filling in the 1960s. The entire Colorado River system was only about 33 per cent full that month. The low water levels leave a chalk-white "bathtub ring" of mineral deposits on the canyon walls around Lake Mead - a stark visual marker of how far the waterline has fallen.
The river's decline also exposes a century-old miscalculation. When the 1922 Colorado River Compact divided the river's flow between the upper and lower basins, negotiators estimated annual flow at 15 million acre-feet. Later analysis found the true average was closer to 12.5 million. Since 2020, the river's average annual flow has dropped to about 10.2 million acre-feet - meaning the legal promises made on paper now far exceed what the river can physically deliver. Federal officials designed the new cuts in part to keep Lakes Powell and Mead from falling to "dead pool," the level at which water drops below the dams' outlet works and can no longer be released downstream at all.
The Lower Basin's own two-year deal
Even as they sue, Arizona, Nevada and California are trying to implement a negotiated conservation plan of their own. In May, water leaders from the three Lower Basin states sent the Bureau of Reclamation a proposal to use 3.2 million acre-feet less Colorado River water across 2027 and 2028, with an additional 700,000 acre-feet of conservation on top, describing it as a way to "stabilise the system in the face of deteriorating hydrology." The plan was contingent on the bureau providing funding to compensate some Lower Basin water users for the reductions.
Buschatzke described the proposal as "a really proactive proposal in the lower basin that protects that Colorado River system total," telling reporters it would create "robust protection" for Lake Powell and Lake Mead over the next two years. The federal government adopted the three states' near-term split - 760,000 acre-feet a year from Arizona, 440,000 from California and 50,000 from Nevada - into its own framework for 2027 and 2028. But the states object to the wider federal rules layered around that split, particularly the absence of Upper Basin obligations and the prospect of far deeper cuts under the plan's 10-year framework in later years.
Hobbs said Arizona would continue working with Nevada and California to carry out their two-year agreement while the lawsuit proceeds. Buschatzke said intervening was necessary both to protect the state's legal rights to the river and to keep a negotiated outcome with California and Nevada on the table - suggesting Arizona sees the courtroom and the negotiating table as parallel, not mutually exclusive, tracks.
What happens next
The immediate question is procedural: whether the court grants Arizona's motion to intervene and how quickly the case advances. With the new operating rules set to take effect for the 2027 water year, both states have an incentive to seek clarity before the cuts begin to bite - though complex water litigation has historically moved slowly, and a final ruling could easily arrive after the 2027-2028 window the rules cover.
Beyond the courtroom, the deeper fight is about what happens after 2028. The current guidelines expire at the end of this year, the interim federal framework runs for a decade with the prospect of deeper cuts in its later years, and all seven basin states still need a durable agreement for the river's long-term management. Buschatzke has said he expects the states to keep meeting to discuss "whether or not we could find a seven-state path forward after 2028," while Upper Basin officials have pressed for resumed talks with a mediator.
For the tens of millions of people and the vast farmlands that depend on the Colorado, the stakes are concrete. Arizona's warning that the framework could hollow out the Central Arizona Project puts nearly 6 million residents' supplies in the frame; Nevada's figures show a Las Vegas valley that has already conserved aggressively now facing the prospect of losing the majority of its remaining allocation in the worst case. Whether the resolution comes from a judge, a mediator or a new round of state negotiations, the river's arithmetic - a 10-million-acre-foot river divided by 15-million-acre-foot promises - will not change.
FAQs
What does Arizona's motion to intervene actually change in the lawsuit?
Intervention would make Arizona a formal party to Nevada's case rather than a bystander, giving the state the right to present its own evidence, file briefs and appeal an unfavourable ruling. That matters because Arizona faces by far the largest absolute cuts - 760,000 acre-feet a year - so its participation lets the court weigh the framework's economic harms at their most severe. It does not guarantee a ruling before the 2027 operating year begins; federal water cases often take years, meaning the states may be litigating the legality of rules already in force.
Why is Arizona being asked to cut so much more water than Nevada?
The split reflects the river's priority system, not population size. When the Central Arizona Project canal was authorised in 1968, Arizona accepted junior priority status for much of that water in exchange for federal funding to build it - meaning CAP supplies are among the first cut in a shortage, behind California's senior agricultural rights and Nevada's municipal allocation. Arizona's 2.8 million acre-foot apportionment is large, but a significant share of it is junior water. Nevada's small 300,000 acre-foot allocation, by contrast, is almost entirely municipal supply for Las Vegas, which carries stronger legal protection.
What is "dead pool," and how close are the reservoirs to it?
Dead pool is the water level at which a reservoir drops below its dam's outlet works, making it physically impossible to release water downstream - for Lake Powell, that would cut off deliveries to the entire Lower Basin. As of July 2026 the reservoirs were nowhere near that point in absolute terms, but their combined storage was the lowest since Lake Powell began filling in the 1960s, at 28 and 24 per cent of capacity for Mead and Powell respectively. The federal cuts are explicitly designed as a buffer: by reducing demand now, managers hope to keep enough water banked in both lakes to stay well above dead pool even if the megadrought deepens.
What do the Upper Basin states say in their defence?
Upper Basin officials argue that mandatory cuts on their states would ignore how the river actually works: in dry years there is simply less natural flow reaching Lake Powell, and the 1922 Compact measures their delivery obligation over rolling ten-year averages rather than as a fixed annual guarantee. They point to conservation measures their farmers and cities have already undertaken and contend that the Lower Basin - which uses nearly all of its far larger apportionment every year - should absorb the shortage created by overallocation. Their preferred route is a return to negotiations with a neutral mediator rather than a court decision.
Will taps run dry or water bills rise in Phoenix and Las Vegas?
No major utility is warning of taps running dry in the near term; municipal supplies generally hold higher priority than the agricultural and junior water that would absorb the deepest cuts. The more likely near-term effects are financial and regulatory: water agencies may need to buy or develop costlier replacement supplies, and some of those costs eventually reach customer bills, while cities tighten conservation rules such as limits on outdoor watering and ornamental turf. Southern Nevada is already among the most water-efficient urban regions in the United States after cutting consumption more than 40 per cent since 2002, which is precisely why its officials argue further mandatory cuts punish past conservation rather than reward it.
🤖 This article was rewritten by Feed and Figures' editorial AI from a report originally published by The Guardian. Facts and quotes are preserved from the original; the rewrite focuses on clarity and structure. For the unedited original, see the source link below.
