The Chamokane Creek and its tributaries formed the Chamokane Basin, which was located partially within the Spokane Indian Reservation (reservation) in the state of Washington. Many non-Indians resided on the reservation. The state of Washington sought to regulate water usage by non-Indians living on the reservation. The United States (plaintiff), acting on behalf of the Spokane Tribe (tribe), sued, arguing that the state had no authority to regulate on tribal land. The district court ruled for the state of Washington, and the United States appealed to the Ninth Circuit.