Arizona is joining Nevada's legal challenge to the federal government's new Colorado River rules, adding the state facing the deepest near-term cuts to the fight over how federally mandated cuts are divided among the seven basin states.

Arizona Gov. Katie Hobbs (D) and the Arizona Department of Water Resources announced Friday that the state has intervened in the August lawsuit Nevada filed against the U.S. Department of the Interior. Arizona officials said the federal plan fails to hold the Upper Basin states of Colorado, Utah, Wyoming and New Mexico accountable for delivering water to Lake Mead and could lead to Lower Basin cuts of up to 3 million acre-feet a year.

"We are drawing a line in the sand," Hobbs said in a statement. She said Arizona would continue working with Nevada and California to carry out their own two-year Lower Basin agreement while the case moves through court.

Tom Buschatzke, the department's director and Arizona's lead Colorado River negotiator, said intervening was necessary to protect the state's legal rights to the river while keeping a negotiated outcome with California and Nevada on the table.

Nevada, the Colorado River Commission of Nevada and the Southern Nevada Water Authority filed suit Aug. 24, becoming the first state to challenge Interior's final plan released Aug. 21. 

Under the federal framework, Nevada's 300,000 acre-foot allocation could be cut by as much as 213,556 acre-feet, or about 71 percent, if Lower Basin shortages reach the framework's outer limit — leaving Southern Nevada with less than 86,500 acre-feet a year. 

An acre-foot is about 326,000 gallons, roughly enough to cover a football field in a foot of water or supply two to three households for a year. Southern Nevada used about 198,000 acre-feet in 2025, according to the water authority.

Gov. Joe Lombardo (R) said in a statement Friday that Arizona's decision is "an important step in challenging a federal plan that unfairly burdens the Lower Basin." He said Nevada remains committed to a negotiated solution but will not accept a framework that puts the state's water supply at risk.

Southern Nevada Water Authority General Manager John Entsminger said Arizona shares Nevada's concerns about flaws in the federal environmental review and the process used to develop the post-2026 framework. He said the agency is still working with California and Arizona to finalize a Lower Basin agreement to reduce near-term demands on Lake Mead.

The three Lower Basin states proposed that agreement in May, committing to conserve more than 3.2 million acre-feet through 2028. Interior adopted the near-term split for 2027 and 2028, under which Arizona will have to reduce its use by 760,000 acre-feet a year, California by 440,000 and Nevada by 50,000.

Arizona officials said the stakes are especially high for the state because the Central Arizona Project, which carries river water to Phoenix and Tucson, holds relatively junior water rights. Without the Lower Basin agreement, they said, the project's Colorado River allocation could be cut by 77 percent over the next two years.

Arizona said it plans to challenge parts of the federal government's final environmental review and the final plan itself, which it argues unlawfully fails to require releases from Upper Basin reservoirs to Lake Mead.

Nevada's lawsuit has already drawn pushback upstream. Utah's House speaker criticized the suit shortly after it was filed, saying it made a difficult situation harder.