Pyramid Lake was located within the boundaries of the Paiute Indian reservation. Tribal members have traditionally lived on the lakeshore and fished in it. The lake was fed by the Truckee River. The lake required 385,000 acre-feet of water per year to maintain its level. A decreased level endangers the native fish and upsets the salinity balance of the lake to the point where its continued use is threatened. The Department of Interior contracted with the Truckee-Carson Irrigation District (district) to supply it with water. Morton, the Secretary of the Interior, (secretary) (defendant) issued a regulation diverting 378,000 acre-feet of water from the Truckee River to supply the district. The Pyramid Lake Paiute Tribe of Indians (Paiute Tribe) (plaintiffs) sued Morton, contending that the amount of water diverted was an arbitrary abuse of discretion that violated the government’s duties to the Paiute Tribe.