Under the Clean Water Act (CWA), 33 U.S.C. §§ 1251 et seq., the United States Environmental Protection Agency (EPA) (defendant) treated Indian tribes as states when implementing and enforcing certain provisions of the CWA. The Isleta Pueblo Indians set their own water-quality standards for water flowing through their reservation and adopted more stringent standards than those required by the EPA. The City of Albuquerque (plaintiff) is situated upstream from the Isleta Pueblo Indian reservation. The City of Albuquerque received a National Pollution Discharge Elimination System (NPDES) permit for the discharge of water from its waste treatment facilities into the Rio Grande river, which flows through the Isleta Pueblo Indian reservation. In the 1990s, the EPA sought to revise Albuquerque’s NPDES permit in response to the Isleta Pueblo Indians’ heightened water-quality standards. Albuquerque sued the EPA over its approval of the Isleta Pueblo Indians’ higher standards. The EPA argued that the CWA did not authorize tribes to set higher water standards than those required by federal law and that tribal requirements could not be enforced beyond reservation boundaries. The district court denied Albuquerque’s request for a permanent injunction and granted summary judgment for the EPA. Albuquerque appealed.