Arizona water heavyweight joins Colorado River lawsuit against Trump administration

Another powerhouse in Arizona water, the Central Arizona Project, plans to join a lawsuit against the Trump administration over its plan for dividing up the Colorado River’s water supply with steep cuts for several states.

The lawsuit, originally filed by Nevada, takes aim at the federal government for its recently announced management plan for key Colorado River reservoirs, lakes Mead and Powell. The 10-year plan includes heavy, mandatory water cuts for the three Lower Basin states — Arizona, California and Nevada — during dry years. Arizona announced its plans to join the lawsuit this month.

One of the central complaints for the downstream water agencies is that the plan calls on upstream states, like Colorado, to voluntarily cut back on water use, but does not require mandatory cuts.

The Central Arizona Project, which provides Colorado River water to the Phoenix metropolitan area, asked to join the lawsuit to be able to defend its interests in the basin. No hearing date was set for the case, and a judge had yet to rule on the two motions out of Arizona to join the lawsuit as of Thursday.

“The actions by the federal government disregard the Colorado River Compact and other laws governing the Colorado River,” Brenda Burman, the project’s general manager, said in a news release Tuesday. “Those of us who share this precious resource have no choice but to turn to the courts to correct those errors.” 

In August, Nevada was the first state to sue over new water management rules for the Colorado River Basin, which provides water to more than 35 million people, after states failed to negotiate a unified proposal to submit to the Trump administration. Arizona announced its plans to join the lawsuit Oct. 16.

The states have been fighting for three years over how to manage the basin’s key reservoirs after fall 2026. They were tasked with replacing a set of water management rules which were established in 2007 and expired Sept. 30. 

Federal officials said they would defer to the states — if they could reach an agreement. The seven basin states missed deadline after deadline, and without a unified proposal in hand by August, the Trump administration announced its plans for how to manage the reservoirs starting Oct. 1.

The federal plan set guardrails for the next 10 years, outlining minimum and maximum dam releases and water cuts for the downstream states. Every two years, officials will decide specific releases and cuts to better respond to changing water conditions.

Under the 2027-2028 management plan, downstream states have to cut their use by 1.25 million acre-feet each year. That’s a 20% cut through 2028, plus voluntary conservation goals. The states are still deciding by how much each state will reduce its use.

Their forced cuts could increase to 3 million acre-feet per year under the federal guardrails. Such reductions could effectively dry up the Central Arizona Project canal system, according to Arizona officials.

One acre-foot, or nearly 326,000 gallons, roughly equals the annual water use of two to three households.

Upstream states, like Colorado, will aim to conserve water to help avoid crisis lows at Lake Powell.

Both Lake Powell, on the Utah-Arizona border, and Lake Mead, on the Arizona-Nevada border, have been at historically low levels and have struggled to recover with warming temperatures and continuous water demands from communities and industries.

Lower Basin officials have called the federal plan “fundamentally flawed and unlawful.”

“But the US decision puts the entire burden of sacrifice on the shoulders of three states,” Arizona said in its Oct. 16 news release. “We are joining Nevada in challenging that framework.”

Nevada’s lawsuit says the federal government should have analyzed how the basin’s water system would function if mandatory conservation and water cuts were included. Without that, the analysis was insufficient, according to Nevada.

Arizona and Nevada officials argue the federal government didn’t adequately analyze the impact of its post-2026 management plan on Lower Basin economies. 

They have also called on the U.S. District Court of Nevada to weigh in on requirements for Upper Basin states.

But that’s where legal arguments will come into play. 

The Secretary of the Interior has the authority as “water master” of the Lower Basin to decide shortages for the downstream states. It does not have that authority for upstream states, like Colorado, which use the Upper Colorado River Commission to coordinate on basin issues, according to Upper Basin water experts.

If Nevada wins in court, federal agencies might have to scrap the existing plan or redo parts of it. So far, Colorado and its three sister states in the Upper Colorado River Basin won’t be directly impacted by the lawsuit.

Colorado officials declined Thursday to comment on the case.

Who takes cuts has been a near-constant tension point in negotiations. 

While Lower Basin states have argued everyone needs to share water cuts, Upper Basin officials say they already have water cuts imposed by nature and do not use their full share of Colorado River water. Upstream states also allege that their downstream counterparts have repeatedly gone over their legal water allowances.

Both sub-basins are legally allocated 7.5 million acre-feet of Colorado River water (not counting some provisions for tributary use in the Lower Basin) under the 1922 Colorado River Compact.