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8 the Colorado River, the Navajo Nation’s rights to use water from the Colorado River” have never been adjudicated. Id., at 36. The United States acknowledges that it holds certain water rights “in trust” for the Navajo. See Tr. of Oral Arg. 26, 40. It does not dispute that it exercises considerable control over the disposition of water from the Colorado River. And it concedes that the Navajo’s water rights “may … include some portion of the mainstream of the Colorado.” Id., at 33. But instead of resolving what the Navajo’s water rights might be, the United States has sometimes resisted efforts to answer that question.

The current legal regime governing the Colorado River began with a 1922 interstate compact between seven States. That agreement split the Colorado into two basins—an Upper Basin and a Lower Basin. See Colorado River Compact, Art. II, Colo. Rev. Stat. §37–61–101 (2022). The compact answered some high-level questions about which States could lay claim to which sections of the river. But it did not purport to “affec[t] the obligations of the United States of America to Indian [T]ribes.” Id., Art. VII. In that way, it left the Navajo with no insight into what water they could claim as their own.

Six years later, Congress entered the picture by passing the Boulder Canyon Project Act, 45 Stat. 1057, codified at 43 U. S. C. §§617–619b. That Act had a profound impact on the Lower Basin. It authorized the construction of the Hoover Dam and the creation of Lake Mead. §617. More than that, it gave the Secretary of the Interior substantial power to divvy up the resulting impounded water. Failing agreement among the States in the region, the law authorized the Secretary to enter into contracts for the delivery of water and provided that “[n]o person” may have water from the mainstream of the Colorado in the Lower Basin “except by contract.” §617d; see also Arizona v. California, 373 U. S. 546, 565 (1963) (Arizona I). In adopting this law, Congress hoped “to put an end to the long-standing dispute over