Cite as: 530 U. S. 392 (2000)
Opinion of Rehnquist, C. J.
the claim or demand, but as to any other admissible matter which might have been offered for that purpose." Nevada v. United States, 463 U. S. 110, 129-130 (1983) (quoting Cromwell v. County of Sac, 94 U. S. 351, 352 (1877)).
In Arizona II, we recognized that the general principles of res judicata apply to our 1964 decree even though the decree expressly provided for modification in appropriate circumstances. In so doing, we noted the importance of the certainty of water rights in the Western United States. "A major purpose of this litigation, from its inception to the present day, has been to provide the necessary assurance to States of the Southwest and to various private interests, of the amount of water they can anticipate to receive from the Colorado River system. . . . If there is no surplus of water in the Colorado River, an increase in federal reserved water rights will require a 'gallon-for-gallon reduction in the amount of water available for water-needy state and private appropriators.' " 460 U. S., at 620-621 (quoting United States v. New Mexico, 438 U. S. 696, 699 (1978)). Thus, we concluded that allowing recalculation of the amount of practicably irrigable acreage "runs directly counter to the strong interest in finality in this case." 460 U. S., at 620. We also noted that treating the 1964 calculation as final comported with the clearly expressed intention of the parties and was consistent with our previous treatment of original actions, allowing modifications after a change in the relevant circumstances.
This reasoning is equally applicable to the United States' and the Tribe's claim for additional water for the disputed boundary lands. Even though the exact claim was not actually litigated in Arizona v. California, 373 U. S. 546 (1963) (Arizona I), the United States could have raised the boundary claim and failed to do so. Indeed, in the proceedings before Special Master Rifkind, the counsel for the United States affirmatively represented that "[t]he testimony . . . as reflected by these maps and by the other testimony will de-
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