The U.S. Supreme Court case Idaho ex rel. Evans, Governor of Idaho, et al. v. Oregon et al., 1979 revolved around a dispute between the states of Idaho and Oregon over fishing rights in the Columbia River which runs through both states. The river is home to various species of salmon that were traditionally caught by commercial fishermen from both states until stocks began to decline due to overfishing and environmental changes in the mid-20th century. In an attempt to conserve fish populations, Oregon imposed stricter regulations on its fishermen than did Idaho leading to a conflict between the two states' interests. Idaho filed suit against Oregon claiming that these conservation measures violated their equal protection rights under Article IV Section 2 Clause 1 (the Privileges and Immunities Clause) as well as interstate commerce laws because they disproportionately affected Idaho's fishermen. However, after reviewing evidence presented by both sides including scientific data about fish migration patterns and population trends along with historical catch records from each state's fisheries departments, the court ruled in favor of Oregon stating that their actions were justified given compelling conservation needs for preserving dwindling salmon stocks.

In the dissenting opinion for Idaho ex rel. Evans v. Oregon, Justice William Rehnquist disagreed with the majority's decision to uphold a lower court ruling that allocated fishing rights between states based on historical catch data rather than equal division of resources. He argued that this approach was inconsistent with previous Supreme Court decisions and violated principles of interstate equity by favoring one state over another without sufficient justification. Furthermore, he contended that using past catch data as a basis for allocation unfairly penalized states like Idaho which had taken conservation measures in the past to protect fish populations while rewarding those like Oregon and Washington who had not done so to the same extent. In his view, an equal division would have been more equitable and consistent with precedent.