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Rh the Little Colorado, and the San Juan—border the reservation. To meet their water needs for household, agricultural, industrial, and commercial purposes, the Navajos obtain water from rivers, tributaries, springs, lakes, and aquifers on the reservation.

Much of the western United States is arid. Water has long been scarce, and the problem is getting worse. From 2000 through 2022, the region faced the driest 23-year period in more than a century and one of the driest periods in the last 1,200 years. And the situation is expected to grow more severe in future years. So even though the Navajo Reservation encompasses numerous water sources and the Tribe has the right to use needed water from those sources, the Navajos face the same water scarcity problem that many in the western United States face.

Over the decades, the Federal Government has taken various steps to assist the people in the western States with their water needs. The Solicitor General explains that, for the Navajo Tribe in particular, the Federal Government has secured hundreds of thousands of acre-feet of water and authorized billions of dollars for water infrastructure on the Navajo Reservation. See Tr. of Oral Arg. 5; see also, e.g., Consolidated Appropriations Act, 2021, Pub. L. 116–260, 134 Stat. 3227, 3230; Northwestern New Mexico Rural Water Projects Act, §§10402, 10609, 10701, 123 Stat. 1372, 1395–1397; Central Arizona Project Settlement Act of 2004, §104, 118 Stat. 3487; Colorado Ute Settlement Act Amendments of 2000, 114 Stat. 2763A–261, 2763A–263; Act of June 13, 1962, 76 Stat. 96; Act of Apr. 19, 1950, 64 Stat. 44–45.

In the Navajos’ view, however, those efforts did not fully satisfy the United States’s obligations under the 1868 treaty. The Navajos therefore sued the U. S. Department of the Interior, the Bureau of Indian Affairs, and other federal parties. As relevant here, the Navajos asserted a breach-of-trust claim arising out of the 1868 treaty and