Arizona v. California

Background

Arizona sued California and several California water agencies, with Nevada, New Mexico, Utah, and the United States later joining the case, over how much water each state may use from the Colorado River. A Special Master ran a long trial, collected thousands of exhibits, and recommended a decree. The central legal question was how to read the 1928 Boulder Canyon Project Act and whether it or traditional state water rules should determine each Lower Basin State’s share.

Reasoning

The Court examined the Act’s language, its legislative history, and the Compact among the basin States and concluded Congress meant to allocate only mainstream Colorado River water among the Lower Basin States, reserving tributary waters to each State. The Court held that Congress fixed the basic split of the first 7,500,000 acre-feet of mainstream water (4,400,000 to California, 2,800,000 to Arizona, and 300,000 to Nevada) and authorized the Secretary of the Interior to make contracts under §5 to carry out deliveries. The Court ruled that the Act and those federal contracts, not state priority rules or the judicial equitable-apportionment doctrine, control the distribution of stored mainstream water. The Secretary must respect “present perfected rights,” including certain Indian reservation claims, may charge mainstream diversions above Lake Mead against a State’s share, and has discretion to choose reasonable methods to handle shortages.

Real world impact

Practically, the ruling makes federal contracts the key instrument for who receives stored mainstream Colorado River water. State water agencies, cities, farms, and Indian reservations will rely on the Secretary’s contracts for supply, and shortages will be managed by the Secretary’s chosen allocation methods rather than a single court-imposed proration formula.