This article is an environmental history of Anaconda Copper Company’s disposal of hundreds of thousands of tons of toxic waste from its Potrerillos and El Salvador mines into Chile’s Río Salado and Bahía de Chañaral. First, it uncovers a long history of disputes between copper companies and workers who panned the river for tailings. This early water war in Chile was shaped by competing understandings of water’s legal status. While workers claimed rights under the water law’s definition of water as a bien nacional de uso común, mining companies invoked the mining code and contended that the river’s water and waste were private property under civil law. Mining companies claimed rivers’ water by treating rivers in legal terms as mines and property of the state, bienes fiscales, that could be conceded as private property. They argued that human engineering of rivers in dams and canals, and through pollution, made rivers into a commodity and a form of property akin to subsoil minerals. Second, the article describes how, during the social reformist government of Eduardo Frei (1964–1970) and the revolutionary government of Salvador Allende (1970–1973), the state asserted control over Chile’s waterways while balancing centralized state management of water in the name of development with local users’ claims of long-standing riparian use rights. Third, the article traces the long history of the state and mining companies treating water as an economic commodity, often superseding local use rights, and argues that this history built the foundation for the later privatization of water during the Pinochet dictatorship. The article demonstrates that the privatization of water in Chile under Pinochet had its origins in the resolution of the tension between water and civil law in favor of extending property rights to water and building as a subsidy to transnational mining companies. This meant rolling back state management of rivers and often eroding local users’ water rights. Finally, the article concludes by examining the town of Chañaral’s successful 1987 lawsuit against the El Salvador mine to win an injunction against further pollution of the Salado as part of a moment of broader Latin American “environmental constitutionalism” during the 1980s. While this legal victory reflected a significant change in environmental law and an emergent environmentalist movement in Chile and across Latin America, it struck a blow to hundreds of workers who depended on extracting tailings from the river for their livelihood and who responded with unsuccessful protests.