Gorsuch, J., dissenting
“it is impossible to believe that when … the Executive Department of this Nation created the [various] reservations” in the arid Southwest it was “unaware that … water from the [Colorado R]iver would be essential to the life of the Indian people and to the animals they hunted and the crops they raised.” Arizona I, 373 U. S., at 598–599. Nor does the United States dispute any of this. To the contrary, it acknowledges that the Navajo’s water rights very well “may … include some portion of the mainstream of the Colorado” that runs adjacent to their reservation. Tr. of Oral Arg. 33.
For our purposes today, that leaves just one question: Can the Tribe state a legally cognizable claim for relief asking the United States to assess what water rights they have? Not even the federal government seriously disputes that it acts “as a fiduciary” of the Tribes with respect to tribal waters it manages. Arizona II, 460 U. S., at 627–628. Indeed, when it comes to the Navajo, the United States freely admits that it holds certain water rights for the Tribe “in trust.” Tr. of Oral Arg. 40. And of course, that must be so given that the United States exercises pervasive control over much water in the area, including in the adjacent Colorado River. See Arizona I, 373 U. S., at 564–565.
Those observations suffice to resolve today’s dispute. As we have seen, that exact coupling—a fiduciary relationship to a specific group and complete managerial control over the property of that group—gives rise to a duty to account. See supra, at 16–17. The United States, we know, must act in a “legally [a]dequate” way when it comes to the Navajo’s water it holds in trust. Arizona II, 460 U. S., at 627. It follows, as the United States concedes, that the federal government could not “legally” dam off the water flowing to their Reservation, as doing so would “interfere with [the Tribe’s] exercise of their” water rights. Tr. of Oral Arg. 13. Implicit in that concession is another. Because Winters rights belong to the Navajo themselves, the United States
