The Clean Water Act (CWA) governs activities that may affect “waters of the United States” – sometimes called “navigable waters.” These activities include water quality planning and discharge regulation by the US Environmental Protection Agency (EPA) and delegated states, and regulation of projects that may lead to “dredge and fill” of waters, through permits issued by the US Army Corps of Engineers. However, CWA provides no definition of this critical term, leaving agencies and courts to define and apply the term, and thereby to define their authority to control actions that affect water quality. In 2001, and again in 2006, the U.S. Supreme Court issued decisions upending decades of established practice by eliminating some types of water bodies from CWA control, and casting others into ambiguous status. First, in Solid Waste Agency of Northern Cook County v. U.S. Army Corps of Engineers [SWANCC], the Court ruled that the Corps lacks jurisdiction over “isolated” waters and wetlands that are not “adjacent” to navigable waters—such as “prairie potholes,” mudflats, and freshwater seasonal ponds. Then, in Rapanos v. United States, the Court ruled that the Corps has jurisdiction over non-adjacent wetlands when it can demonstrate on a case-by-case basis that there is a “significant nexus” between the wetlands and navigable waters (in addition, opinions signed by eight of the justices required that the wetlands be at least “relatively permanent.”).