The Environmental Protection Agency (EPA) and the Department of the Army (specifically, the United States Army Corps of Engineers (USACE)) published a revised definition of "waters of the United States" ("WOTUS") on February 14, 2019 in the Federal Register. (84 FR 4154) (FRL-9988-15-OW); EPA-HQ-OW-2018-0149.

​Under the newly published draft rule, Waters of the Unites States (WOTUS) are defined as tidally influenced navigable waters used in interstate or foreign commerce; all interstate waters and wetlands; other waters such as intrastate lakes, rivers, streams (including intermittent streams), mudflats, sandflats, wetlands, sloughs, prairie potholes, wet meadows, playa lakes, or natural ponds used for interstate or foreign commerce or recreation or their tributaries; any waters with fish or shellfish harvested for interstate or foreign commerce; those waters used for industrial purposes in interstate commerce; the territorial seas; and wetlands adjacent to waters other than waters that are themselves wetlands.

By contrast, the current 2015 rule defines WOTUS as all the waters defined above, but also includes specific inclusion of prairie potholes, Carolina bays and Delmarva bays, pocosins, Western vernal pools, and Texas coastal prairie wetlands—all of which are a specific type of regionalized wetland that may or may not be connected to navigable waters. The 2015 WOTUS rule also includes all waters located within the 100-year floodplain of any of the above waters, all waters located within 4,000 feet of the high tide line or ordinary high water mark of any of the above waters, and any waters that are determined on a case-specific basis to have a "significant nexus" to any water identified above. It is the vast expansion of the definition of WOTUS triggered by the "significant nexus" language in the 2015 rule that has caused much of the confusion over the scope of activities which require a National Pollutant Discharge Elimination System (NPDES) permit from the EPA under the Clean Water Act.

This revised definition is intended to replace the 2015 rule definition of WOTUS adopted by the EPA and USACE and return to the pre-Obama era rule. Chiefly, it removes the regionalized wetlands, the waters in the 100-year floodplain or within 4,000 feet of water, and, most importantly, those waters with a "significant nexus" from the WOTUS definition. The 2015 WOTUS rule definition has been the subject of almost continued litigation since its adoption due it its more expansive definition of WOTUS which prohibits unpermitted discharges to any waters which may have a "significant nexus" to navigable waters (the prior rule required some physical connection to navigable waters via a "discrete conveyance" such as a pipe, ditch or canal or abutting wetlands before triggering permitting). The United States Supreme Court is set to consider whether to grant certiorari review in two cases (County of Maui, Hawaii v. Hawaii Wildlife Fund, et al., U.S. Supreme Court Case Number 18-260 and Kinder Morgan Energy Partners, L.P., et al v. Upstate Forever, et al., U.S. Supreme Court Case Number 18-268) where third party citizen suits were brought under the Clean Water Act (CWA) after unpermitted pollution discharges were made to groundwater that eventually could reach navigable waters. Under the pre-2015 WOTUS rule (to which the revised definition seeks to return) such discharges were outside the scope of the CWA and did not require a federal NPDES permit (although most states prohibit discharges of pollutants to groundwater without a permit). The U.S. Supreme Court was scheduled to consider granting certiorari in these cases at its February 15, 2019 conference. Publication of the new rule may definitely affect the decision to bring the matters up for certiorari consideration at this conference. In accordance with the requirements for federal rulemaking, the public has until April 15, 2019 to file any comments on the revised rule definition.