Los Angeles County Flood Control District v. Natural Resources Defense Council, Inc., 568 U.S. 78

District operates a municipal separate storm sewer system (MS4) and holds an NPDES permit for discharging storm water.
NRDC and Baykeeper sued under CWA §505 alleging water-quality measurements show permit violations due to upstream discharges.
District moved for summary judgment; district argued polluted water at downstream stations did not prove an MS4 discharge at upstream points.
Ninth Circuit concluded pollutants discharged when water flowed from the concrete-lined channels to downstream unlined portions, imposing liability on District.
Court granted certiorari to decide whether flow from an engineered channel within a river constitutes a ‘discharge of pollutants’ under the CWA, with agreement that the answer is no.
Supreme Court held that transfer within the same water body from improved to unimproved portions is not a discharge under the CWA and reversed the Ninth Circuit.

Issue	Plaintiff's Argument	Defendant's Argument	Held
Does flow from an engineered channel within a river qualify as a pollutant discharge?	NRDC/Baykeeper: downstream exceedances prove upstream discharge liability.	District: flow between parts of the same water body is not a discharge under the CWA.	No discharge; Ninth Circuit reversed.

South Fla. Water Mgmt Dist. v. Miccosukee Tribe, 541 U.S. 95 (U.S. 2004) (transfer of polluted water within same water body not a discharge)
Catskill Mountains Chapter of Trout Unlimited, Inc. v. New York, 273 F.3d 481 (2d Cir. 2001) (illustrative analogy on the meaning of 'add' in discharge context)
United States v. Detroit Timber & Lumber Co., 200 U.S. 321 (1905) (syllabus note on treatment of syllabus; context for 'discharge' concept)