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10 “ ‘undertaken representation of the interests of several Indian [T]ribes,’ ” so there was no need for the Court to hear from the Navajo. Id., at 107. In any event, the United States assured the Court, it would continue to apply “ ‘considerations of justice’ ” in its dealings with the Tribe. Ibid. The government conceded, however, “no evidence had been submitted on behalf of the Navajo Nation for uses from the mainstream.” Ibid. And it conceded that “such evidence would have had to be submitted in order for the Court to consider the issue of the Navajo Nation’s rights to the mainstream.” Ibid. As with their previous attempts to make their voices heard in the litigation, the Navajo’s motion to intervene was denied. Id., at 108.

In 1964, the litigation Arizona initiated more than a decade earlier culminated in a decree. See Arizona v. California, 376 U. S. 340. It allocated the Lower Basin Colorado River mainstream among various parties—including five other Tribes whose interests the United States did assert. See id., at 344–345. The decree also permitted the federal government to release water pursuant to certain “valid contracts” and applicable federal laws. Id., at 343; Brief for Federal Parties 7. But the Tribe’s rights remained in limbo. The United States never asserted any rights on the Navajo’s behalf; the Navajo never received an opportunity to assert them for themselves. Since 1964, the decree governing the Lower Basin has been modified at various points. See, e.g., Arizona v. California, 547 U. S. 150 (2006); Arizona v. California, 531 U. S. 1 (2000); Arizona v. California, 466 U. S. 144 (1984). But it has never been modified to address the Navajo.

In the intervening years, the Navajo have asked the federal government—repeatedly—to assess their rights in the mainstream of the Colorado. App. 109. In response to those inquiries, the Tribe received a letter from the Department of the Interior indicating that the Department still had not made “any decisions” about what water rights, if any, the