Cite as: 521 U. S. 1 (1997)
Opinion of the Court
quently been submerged. Apart from daily shifts in the tide and seasonal shifts in sea level, the feature itself changes height. Report 275, 280-283, 309, n. 66. This phenomenon may be at least in part attributable to what the United States' expert witness termed "ice collapse." Dinkum Sands is formed by layers of ice and gravel mixed with ice. As the summer months approach, ice within Dinkum Sands melts and the feature slumps in elevation. 7 Tr. 986-987, 8 Tr. 1060-1062 (July 23, 1984). Alaska and the United States agree that "high-tide" under Article 10(1) should be defined as "mean high water," an average measure of high water over a 19-year period. Cf. United States v. California, 382 U. S. 448, 449-450 (1966) (per curiam) (entering decree defining an island as "above the level of mean high water" and defining mean high water as "the average elevation of all the high tides occurring over a period of 18.6 years"); Borax Consol., Ltd. v. Los Angeles, 296 U. S. 10, 26-27 (1935) (approving definition of "mean high tide line" based on "average height of all the high waters . . . over a considerable period of time," at least 18.6 years). They disagree over how frequently a feature of variable elevation such as Dinkum Sands must be above mean high water to qualify as an island. Based on the drafting history of Article 10, the Master concluded that an island must "generally," "normally," or "usually" be above mean high water. Report 302. Applying this standard, the Master reviewed historical hydrographic and cartographic evidence and the results of a joint monitoring project conducted by the parties in 1981 and 1982. He concluded that Dinkum Sands is frequently below mean high water and therefore is not an island. Id., at 310.
Alaska excepts to this conclusion on three grounds. First, Alaska challenges the legal conclusion that Article 10(1) requires an island to be above mean high water at least "generally," "normally," or "usually." Second, Alaska disputes the Master's factual finding that Dinkum Sands is fre-
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