Under the Clean Water Act (Act) implementing regulations require permits before channeled stormwater runoff from logging roads can be discharged into the navigable waters of the United States. The Environmental Protection Agency (EPA), has issued a regulation defining the term “associated with industrial activity” to cover only discharges “from any conveyance that is used for collecting and conveying storm water and that is directly related to manufacturing, processing or raw materials storage areas at an industrial plant.” The EPA interprets its regulation to exclude the type of stormwater discharges from logging roads at issue here. Under the rule, any discharge from a logging-related source that qualifies as a point source requires an NPDES permit unless some other federal statutory provision exempts it from that coverage. Congress has exempted certain discharges of stormwater runoff. Congress directed the EPA to continue to require permits for stormwater discharges “associated with industrial activity.” The statute does not define that term, but the EPA adopted a regulation which did. P sued under the Clean Water Act's citizen-suit provision. P alleged that D caused discharges of channeled stormwater runoff into waterways and had not obtained NPDES permits, and so, the suit alleged, they had violated the Act. The District Court dismissed concluding that NPDES permits were not required because the ditches, culverts, and channels were not point sources of pollution under the Act and the EPA’s Silvicultural Rule. The Court of Appeals reversed. It ruled that the discharges were from point sources and not exempt from the NPDES permitting scheme by the Industrial Stormwater Rule. The Court granted certiorari.