On January 8, 2013, the U.S. Supreme Court, in a unanimous decision reversing the Ninth Circuit Court of appeals, held that “the flow of water from an improved portion of a navigable waterway into an unimproved portion of the very same waterway does not qualify as a discharge of pollutants under the CWA. Because the decision below  cannot be squared with that holding, the Court of Appeals judgment must be reversed.” LA County Flood Control District v. NRDC.

Pursuant to its MS4 permit, the Flood Control District had been monitoring instream flows in improved sections of the Los Angeles and San Gabriel Rivers. Plaintiffs in the case had argued that the District illegally discharged pollutants by pumping stormwater from the improved portion of the river systems into unimproved portions of the same waterway. The Court held that such activity does not qualify as a “discharge of a pollutant” under the Clean Water Act. Shortly before oral argument, the District was issued a new MS4 permit that requires end-of-pipe pollutant monitoring at individual MS4 discharge points.