In the case of State of Arizona v. State of California et al., 2000, the Supreme Court was asked to resolve a dispute over water rights in the Colorado River among several Western states and Native American tribes. The issue at hand was whether or not certain Indian reservations had reserved water rights under federal law, and if so, how much they were entitled to. The court ruled that five Indian reservations along the river did indeed have such rights dating back to when their lands were first set aside by the government. Furthermore, it held that these rights took precedence over those claimed by other users under state law because they dated from earlier times. However, determining exactly how much water each reservation could claim proved more difficult due to various factors including changes in population size and agricultural needs since their establishment.

In the dissenting opinion for the case of STATE OF ARIZONA v. STATE OF CALIFORNIA, et al., 2000, Justice Stevens argued that Arizona's claim was not barred by res judicata because it arose from a different transactional nucleus of facts than those litigated in previous cases. He believed that the Court had misapplied this doctrine and failed to consider whether there were new factual or legal theories presented by Arizona which could have potentially altered prior judgments. Furthermore, he disagreed with majority’s interpretation of Boulder Canyon Project Act (BCPA), arguing that it did not grant California an entitlement to surplus water beyond its apportionment under Colorado River Compact when such surplus is available. Instead, he contended BCPA only provided a mechanism for allocating any such surplus among Lower Basin States but did not create substantive rights to water itself.