This article explores the development of public trust principles from early Roman and British law through modern U.S. law as a public commons approach to natural resource management, especially with regard to waterways. It then analyzes the tension between the common pool approach underlying the public trust regulation of waterways and the contrasting theoretical premises of American laws that regulate private use of the water within them—especially the privatization model embedded in the doctrine of prior appropriations, which assigns perpetual rights to withdraw from the watercourse on a first-in-time basis. The public trust doctrine, the protagonist of much modern environmental advocacy in the United States, creates a set of public rights and responsibilities with regard to waterways and other natural resource commons. For hundreds if not thousands of years, the common law has recognized that some resources are so critical that they cannot be owned by anyone in particular—instead, they must belong to everyone together. Accordingly, most versions of the doctrine obligate the state to manage resource commons for the benefit of the public. Over the centuries, the doctrine gradually evolved from an affirmation of sovereign authority over these resources to a recognition of sovereign responsibility to protect them for present and future generations. In recent decades, it has evolved substantially in many U.S. states to address a broader variety of natural resources and a broader scope of public values associated with them, including not only navigational but also ecological values. Part I of the article introduces the public trust doctrine as a creatue of common law, constitutional law, and perhaps as an underlying feature of sovereign authority more generally, tracing its history from the Justinian statement of the jus publicum through the British Magna Carta to early American affirmations in both state and federal supreme courts. Parts II and III review the acceptance of public trust principles in modern U.S. law, their ratification in many state constitutions, and the questions that remain open about the extent to which the doctrine applies at the federal level—including its role as a background principle in constitutional takings analyses, and the extent to which it constrains even federal sovereign authority. Part IV explores the intersection of the doctrine with state water allocation law, reviewing the broad mechanics of the riparian and appropriative rights doctrines that can conflict with public trust principles. Conflict is especially pronounced between the public commons model that underlies the public trust and the privatization model embedded in the western doctrine of prior appropriations, as demonstrated by distinct approaches to reconciling them in California, Idaho, and Nevada. The analysis reveals how the unresolved relationship between these two sets of doctrines create ongoing friction in the water governance regimes that follow them, enabling us to better understand the core conflicts within water disputes—and, ideally, prevent them in the future. It concludes with reflections on the evolution of public commons doctrines and contrasting approaches to environmental rights, and the role they can play in helping us locate the dynamic equipoise between the conflicting values at stake in contemporary environmental disputes
