This Comment examines the legal framework for state protection of nonfederal waters and its implications for cooperative federalism. After a brief overview and legal background, it identifies some recent state actions that attempt to fill gaps in coverage created by changes in federal interpretations of the Clean Water Act. It then summarizes the current scope of state regulation of waters in every state, in order to discern the likely impact of changes at the federal level on the status of waters in the states. Finally, updating an analysis done by the Environmental Law Institute in 2013, it examines legal constraints on the ability of state agencies to engage in gap-filling regulations; and discusses the implications of impending changes that may result from federal court decisions, including the U.S. Supreme Court’s pending consideration of Sackett v. Environmental Protection Agency in its October 2022 term.