Opinion of the Court
States” is so “broad and unqualified” that, if viewed in isolation, it would extend to all water in the United States. Brief for Respondents 32. The EPA thus turns to the “significant nexus” test in order to reduce the clash between its understanding of “the waters of the United States” and the term defined by that phrase, i.e., “navigable waters.” As discussed, however, the meaning of “waters” is more limited than the EPA believes. See supra, at 14. And, in any event, the CWA never mentions the “significant nexus” test, so the EPA has no statutory basis to impose it. See Rapanos, 547 U. S., at 755–756 (plurality opinion).
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Second, the EPA’s interpretation gives rise to serious vagueness concerns in light of the CWA’s criminal penalties. Due process requires Congress to define penal statutes “ ‘with sufficient definiteness that ordinary people can understand what conduct is prohibited’ ” and “ ‘in a manner that does not encourage arbitrary and discriminatory enforcement.’ ” McDonnell v. United States, 579 U. S. 550, 576 (2016) (quoting Skilling v. United States, 561 U. S. 358, 402–403 (2010)). Yet the meaning of “waters of the United States” under the EPA’s interpretation remains “hopelessly indeterminate.” Sackett, 566 U. S., at 133 (Alito, J., concurring); accord, Hawkes Co., 578 U. S., at 602 (opinion of Kennedy, J.).
The EPA contends that the only thing preventing it from interpreting “waters of the United States” to “conceivably cover literally every body of water in the country” is the significant-nexus test. Tr. of Oral Arg. 70–71; accord, Brief for Respondents 32. But the boundary between a “significant” and an insignificant nexus is far from clear. And to add to the uncertainty, the test introduces another vague concept—“similarly situated” waters—and then assesses the aggregate effect of that group based on a variety of open-ended factors that evolve as scientific understandings
