- Arizona filed a motion on October 16 to join Nevada’s lawsuit against federal water officials.
- Federal rules could reduce Arizona’s Colorado River water supply by 760,000 acre-feet annually in 2027 and 2028.
- Arizona argues that federal officials are failing to enforce Upper Basin water delivery requirements.
- The state wants to protect a proposed conservation agreement with California and Nevada.
- Arizona and Nevada disagree on whether the court should immediately set aside the federal operating rules.
Tuesday, October 20, 2026 — Arizona has taken its fight over Colorado River water to federal court, joining Nevada in a legal challenge that could affect how much water reaches farms, cities, tribes and businesses across the Southwest.
On October 16, 2026, Governor Katie Hobbs and the Arizona Department of Water Resources announced that Arizona had filed a motion to intervene in Nevada’s lawsuit against the U.S. Department of the Interior and the Bureau of Reclamation.
The dispute centers on federal rules for operating Lake Powell and Lake Mead after 2026. Arizona argues that those rules place too much responsibility for water shortages on the three Lower Basin states while failing to enforce water delivery requirements for the four Upper Basin states.
The lawsuit could affect Colorado River operations through 2036.
Arizona Challenges Federal Water Cuts.
Under federal operating guidelines issued in August 2026, Arizona, California and Nevada face combined water reductions of 1.25 million acre-feet annually in 2027 and 2028.
Arizona would absorb the largest share.
According to the state’s court filing, the annual reductions would be divided as follows:
- Arizona: 760,000 acre-feet.
- California: 440,000 acre-feet.
- Nevada: 50,000 acre-feet.
Arizona’s share represents approximately 60% of the total reductions imposed on the Lower Basin.
An acre-foot is the amount of water needed to cover one acre of land to a depth of one foot.
The state also warns that the federal framework could eventually produce Lower Basin reductions of up to 3 million acre-feet annually.
Governor Hobbs criticized the federal approach in her October 16 announcement.
“Arizona has always been willing to do our part to protect the Colorado River, but we will not accept a forced federal plan that solves this crisis on the backs of our families, farmers, and businesses,” Hobbs said
.
She said Arizona would continue working with California and Nevada on a separate conservation agreement while pursuing its legal challenge.
A Dispute Over Upper Basin Water Deliveries.
At the center of the lawsuit is a disagreement over how federal officials must apply the Colorado River Compact of 1922 and the laws that followed it.
The compact divides the Colorado River Basin into two regions.
The Upper Basin includes Colorado, New Mexico, Utah, and Wyoming. The Lower Basin includes Arizona, California, and Nevada.
Under the compact, the Upper Basin must allow at least 75 million acre-feet of water to pass Lee Ferry, the dividing point between the two basins, over any consecutive 10-year period.
Arizona’s court filing argues that an additional obligation related to the United States’ water treaty with Mexico raises that minimum to 82.5 million acre-feet over 10 years.
The state maintains that federal officials must operate upstream reservoirs in a way that respects those requirements.
Arizona argues that the new operating rules fail to do so.
The filing cites federal projections showing that water passing Lee Ferry could total about 80.4 million acre-feet over 10 years under average water conditions.
Under critically dry conditions, that figure could fall to 73.9 million acre-feet.
Both figures fall below the 82.5 million acre-feet that Arizona contends federal law requires.
These are Arizona’s legal arguments, not court findings.
Lake Powell Releases Are Another Concern.
Arizona’s lawsuit also challenges how much water the federal government plans to release from Lake Powell.
Under earlier operating guidelines, federal officials generally used 8.23 million acre-feet as the minimum annual release from Lake Powell, with larger releases possible when reservoir conditions allowed.
The new guidelines place greater emphasis on protecting Lake Powell’s water level.
Under critically dry conditions, releases could drop to 6 million acre-feet annually. In extraordinary circumstances, they could fall as low as 5 million acre-feet.
The guidelines aim to keep Lake Powell above 3,500 feet, initially targeting a minimum of 3,510 feet.
Arizona argues that reducing releases from Lake Powell could prevent enough water from reaching Lake Mead to satisfy the Upper Basin’s legal obligations.
The state’s challenge involves both the amount of water released and the federal government’s authority to adopt rules that Arizona believes conflict with existing Colorado River law.
A Separate Agreement Could Conserve Water.
Despite the lawsuit, Arizona continues to support a proposed two-year conservation agreement with California and Nevada.
Under that agreement, the three Lower Basin states would conserve more than 3.2 million acre-feet of Colorado River water during 2027 and 2028.
According to the governor’s office, the agreement is intended to stabilize the river system and prevent deeper federally imposed reductions.
Arizona officials warn that the federal approach could reduce the Central Arizona Project’s Colorado River allocation by 77% over the two-year period.
The Central Arizona Project delivers Colorado River water to communities and other users across central and southern Arizona.
Arizona Department of Water Resources Director Tom Buschatzke said the lawsuit is necessary to protect the state’s legal rights while preserving the possibility of an agreement among the Lower Basin states.
“Intervention in the Nevada lawsuit is necessary at this time for Arizona to protect its legal rights to Colorado River water while preserving a path forward to achieving a collaborative outcome with California and Nevada to protect the Colorado River system,” Buschatzke said.
Finalizing the Lower Basin Agreement remains a priority for Arizona, according to the October 16 announcement.
Arizona and Nevada Want Different Court Remedies.
Although Arizona and Nevada agree on several major legal issues, their positions differ.
Nevada filed its original lawsuit on August 24, 2026. The plaintiffs include the State of Nevada, the Colorado River Commission of Nevada, and the Southern Nevada Water Authority.
The lawsuit challenges three federal actions: the final environmental impact statement issued in July, the federal decision adopting the new operating framework in August, and the specific operating guidelines for 2027 and 2028.
Both states contend that federal officials failed to follow Colorado River law and adequately address important concerns during the decision-making process.
However, the states disagree about what should happen to the 2027 and 2028 guidelines if the court finds them unlawful.
Nevada wants the court to set those guidelines aside.
Arizona wants the court to send the guidelines back to federal officials for reconsideration while temporarily leaving them in effect.
Arizona argues that canceling the guidelines immediately would leave water users uncertain about how much water they could expect over the next two years.
The state wants the court to set aside the broader environmental review and federal decision, but preserve the short-term operating guidelines while those decisions are reconsidered.
Arizona’s proposed complaint also disputes Nevada’s interpretation of how shortages should be divided among the Lower Basin states.
Those differences are among the reasons Arizona wants to participate directly in the lawsuit rather than rely on Nevada to represent its interests.
Farmers, Cities and Tribes Face Uncertainty.
Arizona’s court filing identifies several groups that could be affected by reduced Colorado River deliveries.
They include farmers, municipal water providers, tribal communities and businesses.
The state specifically raises concerns about agricultural production, manufacturing and the semiconductor industry.
Arizona also argues that federal officials did not adequately examine the economic and social consequences of the proposed reductions.
Those concerns are part of the state’s challenge under the National Environmental Policy Act, which governs how federal agencies evaluate environmental effects before making major decisions.
Arizona contends that the federal environmental review did not sufficiently consider the consequences for its communities and industries.
The lawsuit asks the court to examine whether federal officials properly evaluated those effects and considered reasonable alternatives.
What Happens Next?
Arizona filed its motion in the U.S. District Court for the District of Nevada on October 16, 2026.
The case is identified as State of Nevada, Colorado River Commission of Nevada, and Southern Nevada Water Authority v. Doug Burgum and the U.S. Department of the Interior, Case No. 2:26-cv-02665.
Arizona is asking the court for permission to join the existing lawsuit as a plaintiff. This legal process is known as intervention.
The state argues that its interests are substantial, that the lawsuit directly affects its Colorado River water rights, and that Nevada cannot fully represent Arizona’s separate legal concerns.
Arizona also argues that allowing it to participate would avoid the possibility of overlapping lawsuits involving the same federal decisions.
The October 16 filing requests to join the case, not a court ruling on the legality of the federal operating guidelines.
The state is pursuing that legal challenge while continuing negotiations with California and Nevada over the proposed Lower Basin conservation agreement.
The dispute involves federal operating decisions for 2027 and 2028, as well as the broader framework governing Lake Powell and Lake Mead through 2036.
Sources and Citations
1. Office of Arizona Governor Katie Hobbs. October 16, 2026. Governor Katie Hobbs and ADWR Join Nevada in Colorado River Lawsuit to Block Federal Cuts to Lower Basin Water Supplies. Official news release.
2. State of Arizona. October 16, 2026. Motion to Intervene of the State of Arizona and Memorandum of Points and Authorities in Support, with proposed Complaint in Intervention. U.S. District Court, District of Nevada, Case No. 2:26-cv-02665-GMN-NJK, Document 15 and accompanying exhibits.
3. Arizona Department of Water Resources. Laws, Rules and Policies Overview: Colorado River Litigation. October 16, 2026. Agency notice confirming Arizona’s filing in the Nevada federal court case.
