Cooperative federalism varies widely from program to program, and depends on the relationship each statute prescribes. The Clean Water Act (CWA), while providing ample room for state participation, is heavily federal and leaves little about this relationship to chance. Nonetheless, the federal-state interplay goes on in as many venues as there are states and clean water programs, and the tensions that arise are inevitable. The cases discussed here raise three different aspects of CWA federalism: inaction by the federal partner, inaction by the state partner, and collaborative action by both partners that is also being challenged on grounds of cooperative federalism. These questions are not abstract. They affect the most uncontrolled form of water pollution in America, and three of the most iconic ecosystems on the continent.