On June 29, 2015, the U.S. Environmental Protection Agency and the U.S. Army Corps of Engineers published a final rule defining the scope of “waters of the United States” protected under the Clean Water Act (80 FR 37054). The Final Rule was developed partly in response to the U.S. Supreme Court’s decisions in U.S. v. Riverside Bayview Homes, Solid Waste Agency of Northern Cook County v. U.S. Army Corps of Engineers, and Rapanos v. United States. The Final Rule retains the definitions of traditional navigable waters, interstate waters, and territorial seas as jurisdictional waters. Within the definition of jurisdictional waters, the Final Rule includes impoundments and – for the first time – tributaries and all waters adjacent to tributaries. The Final Rule also modifies the definition of a tributary as a water “characterized by the presence of the physical indicators of a bed and banks and an ordinary high water mark.” The Final Rule, however, deletes the “waters of the United States” category of “other waters” whose use, degradation, or destruction would affect or could affect interstate or foreign commerce and replaces it with a new category of waters determined to have a “significant nexus” to a traditionally navigable water, interstate water, or the territorial sea. The Final Rule also requires a case-specific “significant nexus” test for certain categories of identified waters (i.e., prairie potholes, Carolina bays and Delmarva bays, pocosins, western vernal pools, and Texas coastal prairie wetlands) and for any waters found within a 100-year floodplain or within 4,000 feet of a water of the U.S. Based on prior regulation and practices, the Final Rule also expressly excludes a number of waters and features such as artificial lakes, waste treatment ponds, detention and retention basins, and percolation ponds for wastewater recycling. By clarifying which waters are subject to Clean Water Act jurisdiction, the Final Rule is meant to reduce the instances in which permitting authorities would make jurisdictional determinations on a case-by-case basis.