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20 Dictionary 25; see Webster’s Third New International Dictionary 26 (1976); see also Oxford American Dictionary & Thesaurus 16 (2d ed. 2009) (listing “adjoining” and “neighboring” as synonyms of “adjacent”). But “construing statutory language is not merely an exercise in ascertaining ‘the outer limits of a word’s definitional possibilities,’ ” FCC v. AT&T Inc., 562 U. S. 397, 407 (2011) (alterations omitted), and here, “only one … meanin[g] produces a substantive effect that is compatible with the rest of the law,” ''United Sav. Assn. of Tex. v. Timbers of Inwood Forest Associates, Ltd.'', 484 U. S. 365, 371 (1988). Wetlands that are separate from traditional navigable waters cannot be considered part of those waters, even if they are located nearby.

In addition, it would be odd indeed if Congress had tucked an important expansion to the reach of the CWA into convoluted language in a relatively obscure provision concerning state permitting programs. We have often remarked that Congress does not “hide elephants in mouseholes” by “alter[ing] the fundamental details of a regulatory scheme in vague terms or ancillary provisions.” Whitman v. American Trucking Assns., Inc., 531 U. S. 457, 468 (2001). We cannot agree with such an implausible interpretation here.

If §1344(g)(1) were read to mean that the CWA applies to wetlands that are not indistinguishably part of otherwise covered “waters of the United States,” see supra, at 14, it would effectively amend and substantially broaden §1362(7) to define “navigable waters” as “waters of the United States and adjacent wetlands.” But §1344(g)(1)’s use of the term “including” makes clear that it does not purport to do—and in fact, does not do—any such thing. See National Assn. of Home Builders v. Defenders of Wildlife, 551 U. S. 644, 662–664, and n. 8 (2007) (recognizing that implied amendments require “ ‘clear and manifest’ ” evidence of congressional intent). It merely reflects Congress’s assumption that certain “adjacent” wetlands are part of “waters of the United States.”