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Colorado River water supplies are shared by seven western states, the Federal Government, several American Indian tribes and the Republic of Mexico. Arizona obtains nearly forty percent of its water supply from the Colorado River for municipalities and industries, agriculture, Indian tribes and environmental purposes.
Modern use of Colorado River water for irrigation began in the late 1800s when water was diverted for use in California’s Imperial Valley. By 1901, some 100,000 acres of farmland were irrigated with Colorado River water in the Imperial Valley. Competition for Colorado River water supplies has increased steadily in response to population growth. Competition for the supplies of the Colorado River has resulted in decades of political and legal confrontation and compromise. But even after Congress and the U.S. Supreme Court agreed to the apportionment of the river, this competition continues.
Arizona has the right to the consumptive use of up to 2.8 million acre-feet annually. This is Arizona’s largest renewable water supply. This supply is further apportioned, approximately one-half to mainstream Colorado River water users, and one-half to Central Arizona Project (CAP) water users.
Arizona Colorado River water has a unique priority system. Present Perfected Rights, as described in the Supreme Court Decree in Arizona v. California are the most senior or first priority rights. Federal Reservations and Perfected Rights established before September 30, 1968 are second priority. Third priority water rights are held by water users that executed contracts with the United States on or before September 30, 1968. Second and third priority rights are coequal. Fourth priority rights are held by water users with contracts, Secretarial Reservations or other rights established with the United States after September 30, 1968. The CAP water supply is primarily fourth priority. Fifth priority water users have contracts for any unused Arizona entitlement water, while sixth priority water users have contracts for any surplus apportionment of water. For more Arizona Colorado River entitlement and priority information, click here.