Under §401 of the Clean Water Act, 33 U.S.C §13·n, to obtain a federal license for any activity that results in a "discharge into the navigable waters," the license applicant must obtain a certification from the state in which the activity takes place that the discharge complies with several aspects of state water-quality regulation under the Clean Water Act. A common setting in which this requirement has been applied is when a hydropower dam seeks to be relicensed by the Federal Energy Regulatory Commission (FERC).
Robert H. Abrams, Can States Regulate Hydropower Dams as Dischargers Pursuant to Their Clean Water Certification Authority? 2005-06 Preview U.S. Sup. Ct. Cas. 251 (2006)