Pursuant to the requirements of the Clean Water Act (CWA), the City of Fayetteville, Arkansas (defendant) filed an application with the Environmental Protection Agency (EPA) seeking a National Pollution Discharge Elimination System (NPDES) permit for its new sewage treatment plant. An NPDES permit requires pollution dischargers to place limits on the type and quantity of pollutants that may be released into the water. The EPA issued the permit and authorized the treatment plant to discharge effluent into a stream in the northwestern portion of the State, but provided for alteration if downstream water quality standards fell below acceptable levels. The discharged pollutants traveled downstream from the Fayetteville site, entered the Illinois River, and eventually into waters located in the State of Oklahoma (plaintiff). Oklahoma challenged the permit as violating state water quality standards requiring that no degradation of water quality be allowed. An Administrative Law Judge (ALJ) affirmed the issuance of the permit because the discharge had no more than a “de minimis impact” on Oklahoma’s waters. On petition for review, the EPA’s Chief Judicial Officer concluded the permit should be upheld so long as there was no “detectable” violation of Oklahoma’s water quality standards. Thereafter, both states sought review in federal court. The court of appeals reversed EPA’s issuance of the permit. The U.S. Supreme Court granted certiorari to review.