In 1972, a bi-partisan Congress enacted the Clean Water Act “to restore and maintain the chemical, physical, and biological integrity of the Nation's waters.” Almost fifty years have passed since Congress enacted the law and, during that time, the Supreme Court has played a significant role in the administration and evolution of the law. Since the dawn of the environmental era in the 1970's, the Supreme Court has heard more cases involving the Clean Water Act than any other environmental law. However, the manner in which the Court has analyzed the law has changed substantially over the last half century. This article reviews the shift in the Court's interpretation of the Act ver time. A review of the thirty cases that the Court has heard that involve statutory interpretation of the Clean Water Act show that the Court, during the early years of the law, focused heavily on legislative history and the purpose of the law in Section 101(a) and interpreted the law to carry out that purpose. Over time, though, the Court adopted a more textualist approach to interpreting the Clean Water Act and, beginning with the Rehnquist Court, the Court began to focus on protecting States' rights. In contrast to the Court's early opinions, opinions from the past few decades do not generally discuss the water quality protection purposes of Section 101(a) of the Clean Water Act. Instead, to the limited extent that the Court focuses on purposes of the law, it cites language in Section 101(b) of the law that discusses a Congressional policy to preserve and protect States' rights. A review of the Court's Clean Water Act cases also shows that, as the Court has moved to a more textualist approach to statutory interpretation, it has become more ideologically divided and the outcomes of the cases are more frequently those that could be characterized as anti-environmental. In addition, while early Supreme Court Clean Water Act decisions often adopted a rhetorical tone sympathizing with the government's efforts to advance public rights with limited resources, more recent decisions tend to adopt a tone of skepticism or even hostility toward government regulation.The shift in the Supreme Court's interpretation of the Clean Water Act is troubling because it coincides with Congressional disengagement in oversight of the law. In the first few decades after the Clean Water Act was enacted, Congress was vigilant in responding to Supreme Court and lower court interpretations of the law, and frequently legislated to affirm or overturn those interpretations. That is no longer the case, either for the Clean Water Act or most other environmental laws. If the Supreme Court adopts an interpretation of the law that conflicts with the water quality protection goals and purposes of the law, Congress is no longer likely to step in to correct the Court's mistake. The lack of concern demonstrated by the Supreme Court and Congress toward interpreting and applying the Clean Water Act to meet the Section 101(a) goals to protect water quality could be counterbalanced to some degree by aggressive implementation of the law by EPA and the U.S. Army Corps of Engineers to carry out those goals. Chevron deference to the agencies' interpretations of the law could provide a minor bulwark against the erosion of the law. However, courts are increasingly finding ways to avoid applying Chevron to agency decisions. Even if courts continued to aggressively apply Chevron to agency actions, though, deferring to the actions that the EPA and the Corps have taken over the past few years would not advance the water quality protection goals of the Clean Water Act because the agencies have increasingly emphasized the protection of States' rights policy of the law in Section 101(b) in their decision-making at the expense of the water quality protection goals of Section 101(a). The agencies' recent navigable waters protection rule and EPA's policy reversal regarding discharges to groundwater in the County of Maui, Hawaii v. Hawaii Wildlife Fund case are just a few examples of the agencies' policy shift.