Cite as: 521 U. S. 1 (1997)
Opinion of the Court
still be above "mean high water" and therefore qualify as an island.
What Alaska seeks is insular status not for a feature that is submerged at abnormally high states of tide, but for a feature that rises above and falls below mean high water— a tidal datum that has already accounted for the tidal abnormalities about which the drafters of Article 10(1) were concerned. Even if Article 10(1)'s drafting history could support insular status for a feature that slumps below mean high water because of an abnormal change in elevation, it does not support insular status for a feature that exhibits a pattern of slumping below mean high water because of seasonal changes in elevation. Alaska nevertheless contends that there is support for according island status to features more "ephemera[l]" than Dinkum Sands. See Alaska Exceptions Brief 45-50. The authorities Alaska cites all predate the Convention and are therefore unhelpful in construing Article 10(1). Alaska also relies on an analogy to the "mudlumps" of the Mississippi delta, features whose status under the Convention has never been determined. See Report of Special Master in United States v. Louisiana, O. T. 1974, No. 9 Orig., p. 4 (filed July 31, 1974) (concluding that Louisiana's Submerged Lands Act grant could be measured from two mudlumps, but not deciding whether the mudlumps were islands under Article 10(1) or low-tide elevations under Article 11(1)); United States v. Louisiana, 420 U. S. 529 (1975) (overruling exceptions).
In sum, the Convention's drafting history suggests that, to qualify as an island, a feature must be above high water except in abnormal circumstances. Alaska identifies no basis for according insular status to a feature that is frequently below mean high water.
B
In disputing the Master's factual conclusion that Dinkum Sands is "frequently below mean high water," Report 39, Alaska relies on three cartographic sources. First, two nau-
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