Pursuant to provisions in the Clean Water Act (CWA), the Environmental Protection Agency (EPA) required the State of California to (1) identify the Garcia River as having insufficient pollution controls and (2) set “total maximum daily loads” (TMDLs) for pollution entering the River. A TMDL defined the maximum amount of a pollutant that could be discharged into a body of water from all sources. Section 303(d) of the CWA requires states to identify all waters for which certain “effluent limitations” are “not stringent enough” to implement applicable water quality standards. The phrase “effluent limitations” applies only to point sources of pollution under the CWA. However, the Garcia River was polluted only by nonpoint sources of pollution. Guido and Betty Pronsolino and others (plaintiffs) filed suit in federal court against Wayne Nastri, in his official capacity as regional administrator of the EPA, and others (defendants), challenging the EPA’s authority under the CWA to apply the identification and TMDL requirements to the River. The district court concluded the EPA had the statutory authority to act. Defendants appealed.