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18 promised large numbers of animals to the Tribe. Art. XII, id., at 670. Those guarantees take as a given that the Tribe could access water sufficient to live, tend crops, and raise animals in perpetuity.

As we have seen, “the history of the treaty, the negotiations, and the practical construction adopted by the parties” may also inform a treaty’s interpretation. Choctaw Nation, 318 U. S., at 432. And here history is particularly telling. Much of the Navajo’s plight at Bosque Redondo owed to both the lack of water and the poor quality of what water did exist. General Sherman appreciated this point and expressly raised the availability of water in his negotiations with the Tribe. Treaty Record 5. Doubtless, he did so because everyone had found the water at Bosque Redondo insufficient and because the Navajo’s strong desire to return home rested in no small part on the availability of water there. Id., at 3, 8. Because the Treaty of 1868 must be read as the Navajo “themselves would have understood” it, Mille Lacs Band, 526 U. S., at 196, it is impossible to conclude that water rights were not included. Really, few points appear to have been more central to both parties’ dealings.

What water rights does the Treaty of 1868 secure to the Tribe? Remarkably, even today no one knows the answer. But at least we know the right question to ask: How much is required to fulfill the purposes of the reservation that the Treaty of 1868 established? See Nevada v. United States, 463 U. S. 110, 116, n. 1 (1983) (citing cases). We know, too, that a Tribe’s Winters rights are not necessarily limited to the water sources found within the corners of their reservation. Winters itself involved a challenge to the misappropriation of water by upstream landowners from a river that ran along the border of tribal lands. 207 U. S., at 576. And here the Navajo’s Reservation likewise stands adjacent to a long stretch of the Colorado River flowing through both its Upper and Lower Basins. App. 91. Finally, we know that