Cite as: 530 U. S. 392 (2000)
Opinion of the Court
raised a boundary lands claim for the Fort Yuma Reservation in the Arizona I proceedings based on facts known at that time, just as it did for the Fort Mojave and Colorado River Reservations, but deliberately decided not to do so, just as it did with respect to the "omitted lands." In Arizona II, this Court rejected the United States' claim for water rights for the "omitted lands," emphasizing that "[c]ertainty of rights is particularly important with respect to water rights in the Western United States" and noting "the strong interest in finality in this case." 460 U. S., at 620. Observing that the 1964 decree determined "the extent of irrigable acreage within the uncontested boundaries of the reservations," id., at 621, n. 12, the Court refused to reconsider issues "fully and fairly litigated 20 years ago," id., at 621. The Court concomitantly held that the Tribes were bound by the United States' representation of them in Arizona I. 460 U. S., at 626-627.
The Special Master rejected the State parties' preclusion argument. He brought out first the evident reason why the United States did not assert water rights claims for the Fort Yuma Reservation boundary lands in Arizona I. At that point in time, the United States was bound to follow the 1936 Margold Opinion, see supra, at 402, which maintained that the Tribe had no claim to those lands. "[I]t is clear," the Master stated, "that the later Secretary of the Interior opinion arbitrarily changing [the Margold] decision was a circumstance not known in 1964, thus constituting an exception to the application of the rule of res adjudicata." Special Master McGarr Memorandum Opinion and Order No. 4, pp. 6-7 (Sept. 6, 1991). Characterizing the question as "close," the Master went on to conclude that "the Tribe is not precluded from asserting water rights based on boundary land claims on [sic] this proceeding, because although the U. S. on behalf of the Tribe failed to assert such claims in the proceeding leading to the 1964 decree, a later and then unknown circum-
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