The Los Angeles County Flood Control District (the District) (defendant) operated a municipal separate storm sewer system, known as an MS4, that collected, transported, and discharged storm water from concrete-lined sections of the Los Angeles and San Gabriel Rivers to un-lined, natural sections of the same waterways. Because storm water is often polluted, the District had obtained the required permits pursuant to the Clean Water Act (CWA) before discharging the storm water. The Natural Resources Defense Council, Inc. (NRDC) and the Santa Monica Baykeeper (plaintiffs) filed suit in federal district court claiming the District had repeatedly violated its permit by allowing water-quality standards to be exceeded. The district court granted summary judgment to the District. Plaintiffs appealed. The court of appeals reversed and concluded that a “discharge of a pollutant” under the CWA occurred when the water detected at upstream monitoring stations flowed out of the concrete channels and into un-lined portions of the same waterway. The District appealed. The U.S. Supreme Court granted certiorari to review.