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Rh into farmers. They must pass through the intermediate stage of herdsmen. They must first become pastoral, then agricultural.’ ” Id., at 269.

Despite all this, “[f]or the Navajos the treaty signified not defeat, but victory, and not disappearance, but continuation.” Iverson 36. “The agreement allowed [them] to return to a portion of their home country.” Ibid. Nor would that “portion” remain so confined. The Navajo often struggled to stay on the narrow tract of land the United States provided. Commission Report 9. In practice, the federal government often tolerated (and sometimes encouraged) the Navajo to live and tend to livestock off reservation to preserve their self-sufficiency. Kessell 271. These arrangements continued until the 1930s, when Congress first “enact[ed] legislation defining the exterior boundaries of the Navajo Reservation.” Id., at 272. Over the ensuing decades, Congress would go on to extend the reservation’s boundaries repeatedly. See, e.g., Act of June 14, 1934, 48 Stat. 960; Act of Feb. 21, 1931, ch. 269, 46 Stat. 1204; Act of May 23, 1930, ch. 317, 46 Stat. 378.

Fast forward to the present. Today, the Navajo Reservation has become “the largest Indian reservation in the United States,” with over “17 million acres,” and over “300,000 members.” App. 90. Its western boundary runs alongside a vast stretch of the Colorado River. Id., at 91. Yet even today, water remains a precious resource. “Members of the Navajo Nation use around 7 gallons of water per day for all of their household needs”—less than one-tenth the amount the average American household uses. Id., at 101. In some parts of the reservation, as much as 91% of Navajo households “lack access to water.” Id., at 102.

That deficit owes in part to the fact that no one has ever assessed what water rights the Navajo possess. For instance, “[a]lthough the Navajo Reservation is adjacent to