The Lower Basin states of the Colorado River are Arizona, California, and Nevada. Together they share 7.5 million acre-feet of mainstem water each year, split 4.4 million to California, 2.8 million to Arizona, and 300,000 to Nevada, with Arizona also keeping exclusive use of the Gila River and its tributaries inside its borders.1National Academies of Sciences. Colorado River Basin Water Management Those numbers have held on paper for nearly a century. What is changing fast is how much of that water actually exists, how much each state is being required to leave in Lake Mead, and what rules will replace the current operating framework when it expires on September 30, 2026.
How The 7.5 Million Acre-Feet Got Split Among The Three States
The 1922 Colorado River Compact gave the Lower Basin 7.5 million acre-feet a year, plus another 1 million acre-feet to account for Gila River diversions in Arizona, but it never divided that water among Arizona, California, and Nevada.2University of Arizona Water Resources Research Center. Sharing Colorado River Water: History, Public Policy, and the Colorado River Compact Congress supplied the state-by-state formula six years later in the Boulder Canyon Project Act of 1928, which authorized Hoover Dam and set the 4.4/2.8/0.3 split still in use.1National Academies of Sciences. Colorado River Basin Water Management California accepted the ceiling in its 1929 Limitation Act, agreeing “irrevocably and unconditionally” to live within 4.4 million acre-feet.3Wyoming Water Plan. Law of the River — Boulder Canyon Project Act
Arizona refused to go along for years. The Supreme Court finally resolved the fight in Arizona v. California on June 3, 1963, holding that the Boulder Canyon Project Act was itself a congressionally mandated apportionment scheme and that the Secretary of the Interior carried it out through delivery contracts. Federal law, not state prior-appropriation doctrine, controls who gets Lower Basin mainstream water, and in a shortage the Secretary has discretion to devise reasonable sharing methods rather than making uniform proportional cuts.4Justia. Arizona v. California, 373 U.S. 546 The same decision recognized federal reserved water rights for five tribes along the Lower Colorado — Fort Mohave, Fort Yuma (Quechan), Chemehuevi, Colorado River, and Cocopah — with priority dates tied to when each reservation was created.5U.S. Department of Justice. Arizona v. California
How Each State Uses Its Share
California: 4.4 Million Acre-Feet, Mostly To Agriculture
California holds the most senior rights in the Lower Basin. Internally, the 1931 Seven-Party Agreement ranks users by priority. Agricultural users in the Imperial and Coachella Valleys sit at the top, with the first three priorities drawing up to 3,850,000 acre-feet combined. The Metropolitan Water District of Southern California, which serves the Los Angeles coastal plain, holds Priority 4 at 550,000 acre-feet a year.6U.S. Bureau of Reclamation. Seven-Party Agreement
The Imperial Irrigation District alone holds present perfected rights to 2.6 million acre-feet a year under the 1964 Supreme Court decree, making it the single largest user of Colorado River water in the American West.7Imperial Irrigation District. IID History California used well over its 4.4 million acre-feet entitlement for years before the Quantification Settlement Agreement, finalized October 10, 2003, forced the state back inside its ceiling for up to 75 years. Under that deal, IID transfers up to 200,000 acre-feet a year to the San Diego County Water Authority and up to 103,000 acre-feet to the Coachella Valley Water District and the Metropolitan Water District, all funded by conservation and efficiency gains.8Imperial Irrigation District. QSA Water Transfer Four tribal nations in California hold diversion rights of 156,522 acre-feet a year, deducted from the state’s 4.4 million.9Public Policy Institute of California. The Colorado River
Arizona: 2.8 Million Acre-Feet, Delivered Mostly Through The CAP
Arizona’s mainstem water moves largely through the Central Arizona Project, a 336-mile canal system that carries river water from Lake Havasu to Phoenix and Tucson. The CAP’s internal priority system protects municipal, industrial, and tribal supplies first, so when shortage cuts hit, agricultural users absorb almost all of them.10Central Arizona Project. Colorado River Operations
Arizona also holds the largest tribal entitlement on the river. The Colorado River Indian Tribes hold 662,402 acre-feet a year from the mainstem, part of a total entitlement of 719,248 acre-feet. The Colorado River Indian Tribes Water Resiliency Act, settled in 2024, authorized the tribe to enter conservation, leasing, and storage agreements, but only with entities in the Lower Basin inside Arizona.11Arizona Mirror. Colorado River Indian Tribes Sign Historic Water Rights Settlement
Nevada: 300,000 Acre-Feet, And Not Even All Of It
Nevada has the smallest share. The Southern Nevada Water Authority manages it for the Las Vegas metropolitan area, roughly 2.3 million residents. Even after adding about 876,000 residents since 2002, Southern Nevada has cut per capita water use by 58% through mandatory outdoor watering restrictions, turf-replacement rebates, and recycling of indoor water. By the end of 2024, actual consumptive use came in at 212,400 acre-feet, well below the full 300,000. The authority has also banked more than 2.2 million acre-feet in storage programs across Nevada, Arizona, California, and Lake Mead as a hedge against future cuts.12Southern Nevada Water Authority. Drought and Shortage
Tribal Rights Sit On Top Of The State Shares
Across the whole Colorado River Basin, 22 tribal nations hold rights to roughly 3.2 million acre-feet a year, about a quarter of the basin’s average supply. Twelve tribes still have unresolved claims, and as those settle, the volume dedicated to tribal use will grow, reducing water available to non-tribal users.13Native American Rights Fund. Tribal Interests in the Colorado River In the Lower Basin, the present perfected rights established through Arizona v. California carry priority dates that in many cases predate the 1922 Compact, making them senior to most other claims on the river.5U.S. Department of Justice. Arizona v. California
What Shortage Means Right Now: The 2026 Tier 1 Cuts
The 2007 Interim Guidelines built an elevation-based system of mandatory reductions tied to Lake Mead’s level, requiring Arizona, Nevada, and Mexico to take cuts before California. Because of its senior rights, California is not required to reduce deliveries or contribute water under a Tier 1 shortage.14Colorado River Commission of Nevada. Drought and Colorado River Declared Shortage The Lower Basin Drought Contingency Plan, signed May 20, 2019, layered additional conservation on top, and the first-ever Lower Colorado shortage was declared in January 2022, forcing Arizona to cut 320,000 acre-feet, Nevada 13,000, and Mexico 50,000.15U.S. Bureau of Reclamation. Drought Contingency Plan
For 2026, the Bureau of Reclamation projected Lake Mead at 1,055.88 feet — a Level 1 shortage. That requires Arizona to reduce deliveries by 512,000 acre-feet (about 18% of its total Colorado River supply and 30% of normal CAP deliveries), Nevada by 21,000 acre-feet, and Mexico by 80,000 acre-feet.16U.S. Bureau of Reclamation. Annual Operating Plan for Colorado River Reservoirs
The May 2026 Three-State Proposal
On May 1, 2026, Arizona, California, and Nevada submitted a joint proposal for short-term operations covering 2026 through 2028, designed as a bridge until post-2026 guidelines are finalized. It commits to at least 3.2 million acre-feet of total savings, including 1.25 million acre-feet in annual reductions during 2027 and 2028: Arizona 760,000 acre-feet, California 440,000, and Nevada 50,000. Another 700,000 to 1 million acre-feet of system conservation is targeted if federal funding and cost-sharing come through.17Arizona Department of Water Resources. Lower Division States’ Proposal for Short-Term Operations
The plan also tightens the rules for Intentionally Created Surplus, the mechanism states use to bank conserved water in Lake Mead. Starting in 2027, a 3% evaporation charge would apply to most stored water, and 10% of newly created surplus would be converted to system water. Withdrawals would be tied to Lake Mead’s elevation and suspended entirely if the lake drops to or below 1,000 feet. The proposal also contemplates a new conservation pool for Indian tribes in Arizona of roughly 280,000 acre-feet through 2028, accounted for within Arizona’s storage framework.17Arizona Department of Water Resources. Lower Division States’ Proposal for Short-Term Operations It remains subject to approval by the Arizona Legislature and the governing boards of California and Nevada water agencies, and all parties have reserved their rights under the Law of the River.18Colorado River Board of California. Lower Basin States Proposal: 3.2 MAF Through 2028
After September 30, 2026: A Federal Framework If The States Can’t Agree
Both the 2007 Interim Guidelines and the Drought Contingency Plan expire at the end of 2026. The Bureau of Reclamation released a draft Environmental Impact Statement on January 16, 2026 laying out five alternatives for future operations.19University of Arizona Water Resources Research Center. Reclamation Releases Draft EIS on Colorado River Options The seven basin states missed both a November 2025 and a February 2026 deadline for consensus.20U.S. Bureau of Reclamation. Interior Department Advances Post-2026 Colorado River Operations
The Lower Basin states have offered reductions to their own entitlements of 27% for Arizona, 10% for California, and 17% for Nevada. The Upper Basin states — Colorado, New Mexico, Wyoming, and Utah — have refused to propose cuts, arguing they already use far below their 7.5 million acre-feet entitlement and should not be penalized for climate-driven declines in river flow.21The American Prospect. Wild Western Water Wars As of June 2026, the Bureau’s acting commissioner said the federal government will impose a 10-year operating framework by the end of summer if the states cannot agree, with new guidelines needing to take effect October 1. Reclamation would reevaluate the framework every two years, and a future state consensus could still replace it.22Inside Climate News. Colorado River Federal Management Plan
The Compact Call That Could Reach The Supreme Court
The deeper legal fight is over Article III(d) of the 1922 Compact. The Lower Basin states read it as a firm delivery obligation: the Upper Basin must ensure 75 million acre-feet flow past Lee Ferry over every rolling 10-year period, plus roughly half of Mexico’s 1.5 million acre-feet treaty share, for 82.5 million acre-feet per decade. Arizona has said publicly it is prepared to litigate if that threshold is breached. The Upper Basin reads the same provision as a non-depletion obligation, meaning its states cannot cause flows to drop below the threshold through their own consumption, but are not required to deliver a fixed volume regardless of hydrology. Upper Basin consumptive use has stayed between 3.5 and 4.5 million acre-feet a year, well below the 7.5 million entitlement, while natural flows have fallen roughly 20% from 20th-century averages.23Water Desk. Dwindling Water Supply, Legal Questions Push Colorado River Into Wildly Uncharted Territory
Federal forecasts suggest the 10-year cumulative flow at Lee Ferry could drop below 82.5 million acre-feet as early as 2027, which would give the Lower Basin states a factual hook for a lawsuit. Any such case would fall under the Supreme Court’s original jurisdiction over interstate water disputes. States are already building war chests: Arizona has set aside $3 million, the Central Arizona Project $6 million, Utah $6 million, and Colorado has been hiring water attorneys.21The American Prospect. Wild Western Water Wars
Why All Of This Is Happening Now: The Reservoirs
As of February 2026, Lake Powell sat at 3,533 feet, 32 feet lower than the same date in 2025 and only about 43 feet above the 3,490-foot minimum power pool at which Glen Canyon Dam can no longer generate electricity. Reclamation modeling indicated Powell could reach that level by December 2026. Lake Mead stood at 1,065 feet, roughly 170 feet above the 895-foot dead pool at which water can no longer flow through Hoover Dam.24Colorado Politics. Bureau of Reclamation Plans to Move Forward on Colorado River Operations Mead has fallen about 160 feet since 2000.12Southern Nevada Water Authority. Drought and Shortage
To buy time, the federal government has cut releases from Powell to Mead from a historical norm of 7.48 million acre-feet down to 6 million acre-feet in water year 2026, and has released supplemental water from Flaming Gorge Reservoir upstream.22Inside Climate News. Colorado River Federal Management Plan Whether that holds the system together through 2028 depends on Rocky Mountain snowpack and on whether the seven states and the federal government land a long-term framework before the current rules expire on September 30, 2026.