Arizona joins Nevada's Colorado River lawsuit, with Mexico's water share at stake

Arizona moved on Oct. 16, 2026, to join Nevada's federal lawsuit against the U.S. Interior Department's Colorado River operating plan, opening a three-state fight over how 1.25 million acre-feet of annual cuts are divided among the river's lower basin. Mexico, whose own share is fixed by a separate treaty, sits downstream of every number under dispute.

Nevada sued in late August to block the plan, which Interior finalized that month and which sets the reductions for 2027 and 2028. Inside Climate News reported that Arizona's filing argues its interests are distinct from Nevada's and cannot be entrusted to another state protecting its own claims. California, the largest consumer of Colorado River water, has not joined the case. The Los Angeles Times reported that Lake Mead and Lake Powell are at their lowest levels since they first began filling.

The stakes are measured in acre-feet, the unit that governs the river: one acre-foot is 325,851 gallons, roughly enough to supply one to three households for a year. Under the federal plan, Arizona absorbs 760,000 acre-feet of the cuts, California about 440,000 and Nevada 50,000, according to Inside Climate News. The Los Angeles Times reported that the two-year reductions work out to about 31% for Arizona, 12% for California and 28% for Nevada, with the framework allowing total cuts of up to 40% through 2036.

Mexico's position rests on the 1944 Water Treaty, known in Spanish as the Tratado de Aguas de 1944, which guarantees the country 1.5 million acre-feet a year, a volume confirmed by the University of Arizona Water Resources Research Center. On Sept. 2, 2026, commissioners of the International Boundary and Water Commission signed Minute 334 in El Paso, accepting a 250,000 acre-foot reduction, about 17%, for 2027 and 2028, KUER reported. Most of that water irrigates farmland in the Mexicali Valley in Baja California.

Arizona Gov. Katie Hobbs said the state is "drawing a line in the sand." The case proceeds alongside implementation of the first round of cuts, which apply in 2027 and 2028, the same two years Mexico agreed to absorb under Minute 334.

Sources: The Arizona-Nevada-California water war is heating up, and all three states will end up parched · Arizona Joins Lawsuit Challenging the Federal Government's Plan for the Colorado River · US and Mexico sign new Colorado River sharing deal · US and Mexico Release Updated Colorado River Sharing Agreement

This story was written with the help of artificial intelligence from verified sources and reviewed by a human editor before publication.