In Arizona v. California, the Supreme Court settled a decades-long fight over the lower Colorado River by ruling that the Boulder Canyon Project Act of 1928 already set the rules for dividing the water. The 1963 decision gave California 4.4 million acre-feet, Arizona 2.8 million acre-feet, and Nevada 300,000 acre-feet of the first 7.5 million acre-feet of the river’s mainstream flow.1Arizona Department of Water Resources. 1963 Supreme Court Decision The Court also protected water rights for five Indian reservations and named the Secretary of the Interior as the river’s water master.2U.S. Department of Justice. Arizona v. California
Arizona filed the case in 1952, asking the Supreme Court to decide how much water each lower-basin state could legally take. The 1922 Colorado River Compact had split water between the upper and lower basins but did not divide the lower-basin share among the states, and that gap drove the litigation.2U.S. Department of Justice. Arizona v. California
How the Court Divided the Mainstream
The justices did not apply the usual equitable apportionment analysis, which weighs historical use and fairness between states. They held that Congress had already done the work in the Boulder Canyon Project Act, and that statute controlled.3Legal Information Institute. Arizona v. California, 460 U.S. 605 The first 7.5 million acre-feet of mainstream flow below Lee Ferry go to the three states in the fixed shares above: 4.4 million to California, 2.8 million to Arizona, and 300,000 to Nevada.1Arizona Department of Water Resources. 1963 Supreme Court Decision
Deliveries run through federal water contracts, which the Court confirmed as the legal mechanism for getting each state its share.3Legal Information Institute. Arizona v. California, 460 U.S. 605
Why Tributaries Stayed Out of the Count
California argued that a state’s tributaries, including Arizona’s Gila River, should count against its mainstream allocation. If accepted, that reading would have sharply cut what Arizona could draw from the main channel.3Legal Information Institute. Arizona v. California, 460 U.S. 605
The Court rejected the argument. The federal division applies only to the mainstream below Lee Ferry, and each state keeps exclusive use of its own internal tributaries without those volumes reducing its mainstream share.3Legal Information Institute. Arizona v. California, 460 U.S. 605
Reserved Water Rights for Five Reservations
The decision applied the Winters Doctrine, established in a 1908 case, which holds that when the federal government creates a reservation it also sets aside enough water to make the land productive. Those rights attach on the date the reservation is created and carry a high priority.2U.S. Department of Justice. Arizona v. California
To quantify the water, the Court used the practicably irrigable acreage standard: the volume needed to irrigate all reservation land that can reasonably be farmed. On that basis it protected rights for five reservations:2U.S. Department of Justice. Arizona v. California
- Chemehuevi
- Cocopah
- Colorado River
- Fort Mohave
- Fort Yuma (Quechan)
These are treated as present perfected rights. In shortage years, the Secretary of the Interior is generally required to satisfy them before serving other users.3Legal Information Institute. Arizona v. California, 460 U.S. 605
The Interior Secretary as Water Master
The ruling placed the Secretary of the Interior in charge of managing deliveries on the lower Colorado. In drought or shortage, the Secretary decides how much water each state actually receives.1Arizona Department of Water Resources. 1963 Supreme Court Decision
That authority is broad but not unlimited. The Secretary must work within the Court’s decree and the federal statutes governing the river, which keeps a single administrator responsible for shortage decisions across the lower basin.1Arizona Department of Water Resources. 1963 Supreme Court Decision