The Arizona Department of Water Resources announced Friday that the state would join the case. Nevada filed its lawsuit in August, challenging the process used to develop the river’s new operating guidelines.

The federal plan, finalized in August and effective Oct. 1, requires Arizona, California and Nevada to reduce their water use. It does not impose mandatory cuts on Colorado, Utah, Wyoming or New Mexico, the four Upper Basin states.

Arizona officials have argued that Upper Basin states should also be required to conserve water. Those states say they already use less when river flows decline and that their legal obligations differ from those of the downstream states.

Negotiations among the seven states have yet to produce a long-term agreement. This week, Arizona and five other states submitted a proposal to the U.S. Bureau of Reclamation to reduce their Colorado River allocations. California submitted a separate plan that would eliminate Phoenix’s share of the river’s water; about 40% of the city’s water supply comes from the Colorado.

Under the proposed sharing agreement, Arizona would take on 760,000 acre-feet in annual reductions, about 27% of its full Colorado River allocation. The federal framework’s cuts to Arizona deliveries are scheduled to start Jan. 1, 2027, with deeper reductions possible in later years.

The stakes extend beyond Phoenix. The Central Arizona Project carries Colorado River water through a 336-mile canal network to communities, farms and tribal water users in central and southern Arizona, including the Phoenix and Tucson areas.

Arizona has been preparing for possible litigation: state lawmakers set aside millions of dollars for legal action, and the water department hired outside counsel. Phoenix and the Central Arizona Project have also taken steps to prepare for potential court fights.

The lawsuit adds uncertainty for communities and water users that depend on the river while states and federal officials continue negotiations over a lasting allocation plan.