The U.S. Supreme Court case Arkansas, et al. v. Oklahoma, et al., 1991 revolved around a dispute between the two states over water pollution standards and regulations under the Clean Water Act (CWA). The state of Oklahoma had set stringent water quality standards for its portion of the Illinois River to protect it from out-of-state pollution sources in Arkansas. However, Arkansas argued that these strict requirements were not necessary as they already complied with federal CWA provisions and their own state's environmental protection laws. The Supreme Court ruled in favor of Oklahoma stating that while compliance with technology-based effluent limitations is important under CWA, it does not automatically ensure compliance with downstream water quality standards set by other states like Oklahoma. Therefore, upstream polluters such as those in Arkansas could be required to meet more stringent controls if necessary to comply with downstream water quality standards.

In the dissenting opinion for Arkansas, et al. v. Oklahoma, et al., Justice Scalia argued that the majority's decision to uphold an Environmental Protection Agency (EPA) regulation was based on a misinterpretation of federal law. He contended that the Clean Water Act does not give EPA authority to impose water quality standards on upstream states like Arkansas in order to protect downstream states like Oklahoma from pollution. According to him, this interpretation would lead to absurd results where every state along a river could potentially be held responsible for meeting all other states' water quality standards downstream - something Congress never intended when it passed the Clean Water Act. Furthermore, he criticized the majority's reliance on legislative history and policy considerations rather than clear statutory language as basis for their ruling.