Arizona v. California

Background

The United States intervened and the State of Arizona brought the original suit against California water users; Nevada and several Indian tribes also participated. The case follows an earlier decree entered in 1964 and amended in 1966. On joint motion, the parties asked the Court to enter a supplemental decree that lists who has what present perfected rights to use mainstream Colorado River water and the priority dates tied to those rights.

Reasoning

The Court addressed whether to approve the parties’ agreed list of present perfected water rights and their priority dates. The Court granted the joint motion and entered the supplemental decree, which sets annual diversion amounts, acreage-based diversion measurements, and priority dates for many claimants, including irrigation districts, cities, Federal establishments, and numerous Indian reservations. The decree also specifies unit diversion quantities for five Indian reservations, requires that those reservations’ listed quantities be provided for first in certain shortage situations, and preserves adjustments tied to final reservation-boundary determinations. The Court appointed Judge Elbert P. Tuttle as Special Master to manage further filings and proceedings, denied one tribe’s motion to intervene insofar as it opposed the decree, and referred other intervention matters to the Special Master. Justice Marshall took no part.

Real world impact

This order fixes the currently recognized annual water shares and priority dates for many users on the Colorado River, shaping who may divert water and in what order during shortages. The decree is subject to later adjustments if reservation boundaries or other facts change, and the Special Master will oversee follow-on proceedings and evidence that can further refine or implement these allocations.