Arizona joins Nevada lawsuit over Colorado River plan, warns of steep water cuts

Arizona has filed a motion to intervene in Nevada’s lawsuit challenging the federal government’s new Colorado River operating framework, escalating a dispute that could shape how water is allocated across the drought-strained basin for years to come. State officials say the plan could force deep cuts to Arizona’s share of the river and leave key parts of the state vulnerable to supply disruptions.

The filing was announced Friday, October 16, 2026, by the Arizona Department of Water Resources and state leaders. It seeks to join Nevada in contesting the U.S. Department of the Interior’s recent decision governing post-2026 operations of the Colorado River, a system that supplies water to millions of people in Arizona, California, Nevada and other western states.

Arizona’s move comes after months of increasingly fraught negotiations over rules that are set to replace the current operating guidelines, which expire at the end of 2026. The state has warned repeatedly that without a durable agreement, the next phase of river management could bring significant shortages to the Lower Basin, where Arizona depends heavily on Colorado River deliveries routed through the Central Arizona Project.

In its motion, Arizona argues that the federal plan does not adequately protect the state’s legal rights to Colorado River water. State officials said the new framework could require substantial reductions in deliveries and fails to require the kind of upstream contributions to Lake Mead that Arizona believes are necessary to stabilize the system. The legal filing also places Arizona alongside Nevada in arguing that the federal rules are inconsistent with governing law and long-standing river agreements.

A deeper fight over post-2026 river rules

The Colorado River has long been divided among seven U.S. states and Mexico under a complex set of compacts, court rulings and federal operating guidelines. But the basin has been under intense strain for years as drought and overuse have reduced the storage levels at Lake Powell and Lake Mead, the two largest reservoirs on the system. Federal officials and basin states have spent years negotiating a successor to the current framework, with some proposals warning of major cuts if the states fail to agree.

Arizona has already been bracing for substantial reductions. State water officials have said the 2026 operating year includes a shortage condition that would reduce Arizona’s Colorado River entitlement, and prior state filings warned that several federal alternatives under review would lead to severe shortages for Arizona users. The new lawsuit adds another layer to a dispute that could determine how much water reaches farms, cities and tribal water users across the state.

The state has not said when the court will act on its motion, and no final resolution has been announced. For now, Arizona is positioning itself to defend its water interests in a legal fight that officials say could affect the future of supplies for Phoenix, Tucson and other communities that rely on the river.