Opinion of the Court
The EPA and the Corps initially promulgated different interpretations of “the waters of the United States.” The EPA defined its jurisdiction broadly to include, for example, intrastate lakes used by interstate travelers. 38 Fed. Reg. 13529 (1973). Conversely, the Corps, consistent with its historical authority to regulate obstructions to navigation, asserted jurisdiction over only traditional navigable waters. 39 Fed. Reg. 12119 (1974). But the Corps’ narrow definition did not last. It soon promulgated new, much broader definitions designed to reach the outer limits of Congress’s commerce power. See 42 Fed. Reg. 37144, and n. 2 (1977); 40 Fed. Reg. 31324–31325 (1975).
Eventually the EPA and Corps settled on materially identical definitions. See 45 Fed. Reg. 33424 (1980); 47 Fed. Reg. 31810–31811 (1982). These broad definitions encompassed “[a]ll … waters” that “could affect interstate or foreign commerce.” 40 CFR §230.3(s)(3) (2008). So long as the potential for an interstate effect was present, the regulation extended the CWA to, for example, “intrastate lakes, rivers, streams (including intermittent streams), mudflats, sandflats, wetlands, sloughs, prairie potholes, wet meadows, playa lakes, or natural ponds.” Ibid. The agencies likewise took an expansive view of the CWA’s coverage of wetlands “adjacent” to covered waters. §230.3(s)(7). As noted, they defined “adjacent” to mean “bordering, contiguous, or neighboring” and clarified that “adjacent” wetlands include those that are separated from covered waters “by manmade dikes or barriers, natural river berms, beach dunes and the like.” §230.3(b). They also specified that “wetlands” is a technical term encompassing “those areas that are inundated or saturated by surface or ground water at a frequency and duration sufficient to support, and that under normal conditions do support, a prevalence of vegetation typically adapted for life in saturated soil conditions.” §230.3(t). The Corps released what would become a 143-page manual to guide officers when they determine whether
