568 U.S. 597 (2013)
Decker v. Northwest Environmental Defense Center sits at the intersection of environmental protection, administrative law, and citizen enforcement.
Do stormwater discharges from logging roads that are collected and conveyed by ditches and culverts require NPDES permits as discharges "associated with industrial activity" under EPA's stormwater regulations, and is a citizen suit to enforce such permitting requirements procedurally proper and not moot in light of EPA's subsequent regulatory amendment?
Under the Clean Water Act, the discharge of pollutants from any point source to navigable waters requires an NPDES permit. 33 U.S.C. §§ 1311(a), 1342. A "point source" is any discernible, confined, and discrete conveyance, including a ditch or conduit. 33 U.S.C. § 1362(14). For stormwater, Congress requires permits for discharges "associated with industrial activity." 33 U.S.C. § 1342(p)(2)(B). EPA's Industrial Stormwater Rule defines that term, 40 C.F.R. § 122.26(b)(14), while the Silvicultural Rule, 40 C.F.R. § 122.27, identifies limited silvicultural operations that yield point source discharges; other silvicultural runoff is treated as nonpoint source. Courts defer to an agency's reasonable interpretation of its own ambiguous regulations unless plainly erroneous or inconsistent with the regulation (Auer/Seminole Rock deference). A citizen suit may enforce effluent standards or permit requirements, 33 U.S.C. § 1365(a), and CWA § 1369(b)(1) channels direct challenges to certain EPA actions to the courts of appeals but does not bar enforcement suits that do not seek to invalidate a regulation.
Reversing the Ninth Circuit, the Supreme Court held that, under EPA's reasonable interpretation of its industrial stormwater regulation, stormwater discharges from logging roads that are channeled through ditches and culverts are not discharges "associated with industrial activity," and therefore do not require NPDES permits. The citizen suit was not an impermissible collateral attack on EPA's rules, and the case was not moot despite EPA's intervening amendment.
For environmental law, Decker narrows the circumstances under which NPDES permits are required for forestry-related stormwater, signaling EPA's preference to regulate logging road runoff through best management practices and targeted designations rather than across-the-board permitting. Practitioners must now look first to EPA's designations under § 402(p) and state programs for BMP enforcement in the forestry context. For administrative law, Decker is a prominent reaffirmation of Auer/Seminole Rock deference: when a regulation is ambiguous, an agency's reasonable interpretation of its own rule controls, even if advanced in litigation and followed by a clarifying amendment. The separate opinions presaged later debates culminating in Kisor v. Wilkie (2019), which constrained but did not eliminate Auer deference. Decker also underscores the viability of CWA citizen suits to enforce permitting obligations and clarifies that such suits are not necessarily precluded by § 1369(b)(1) or mooted by subsequent regulatory changes.