Opinion of the Court
Army Corps of Engineers (Corps) jointly enforce the CWA. The EPA is tasked with policing violations after the fact, either by issuing orders demanding compliance or by bringing civil actions. §1319(a). The Act also authorizes private plaintiffs to sue to enforce its requirements. §1365(a). On the front end, both agencies are empowered to issue permits exempting activity that would otherwise be unlawful under the Act. Relevant here, the Corps controls permits for the discharge of dredged or fill material into covered waters. See §1344(a). The costs of obtaining such a permit are “significant,” and both agencies have admitted that “the permitting process can be arduous, expensive, and long.” Hawkes Co., 578 U. S., at 594–595, 601. Success is also far from guaranteed, as the Corps has asserted discretion to grant or deny permits based on a long, nonexclusive list of factors that ends with a catchall mandate to consider “in general, the needs and welfare of the people.” 33 CFR §320.4(a)(1) (2022).
Due to the CWA’s capacious definition of “pollutant,” its low mens rea, and its severe penalties, regulated parties have focused particular attention on the Act’s geographic scope. While its predecessor encompassed “interstate or navigable waters,” 33 U. S. C. §1160(a) (1970 ed.), the CWA prohibits the discharge of pollutants into only “navigable waters,” which it defines as “the waters of the United States, including the territorial seas,” 33 U. S. C. §§1311(a), 1362(7), (12)(A) (2018 ed.). The meaning of this definition is the persistent problem that we must address.
B
Michael and Chantell Sackett have spent well over a decade navigating the CWA, and their voyage has been bumpy and costly. In 2004, they purchased a small lot near Priest Lake, in Bonner County, Idaho. In preparation for building a modest home, they began backfilling their property with
