On March 25, 2014, the Environmental Protection Agency (EPA) and the U.S. Army Corps of Engineers (Corps) jointly proposed a rule defining the scope of waters protected under the Clean Water Act (CWA). The proposal would revise regulations that have been in place for more than 25 years. Revisions are proposed in light of 2001 and 2006 Supreme Court rulings that interpreted the regulatory scope of the CWA more narrowly than previously, but created uncertainty about the precise effect of the Court's decisions. According to the agencies, the proposed rule would revise the existing administrative definition of "waters of the United States" consistent with legal rulings and science concerning the interconnectedness of tributaries, wetlands, and other waters and effects of these connections on the chemical, physical, and biological integrity of downstream waters. Waters that are "jurisdictional" are subject to the multiple regulatory requirements of the CWA. Non-jurisdictional waters are not subject to those requirements. This report describes the proposed rule and includes a table comparing the existing regulatory language that defines "waters of the United States" with the proposed revisions. The proposal is particularly focused on clarifying the regulatory status of waters located in isolated places in a landscape. It does not modify some categories of waters that currently are jurisdictional by rule (traditional navigable waters, interstate waters and wetlands, the territorial seas, and impoundments). The proposed rule would replace EPA-Corps guidance that was issued in 2003 and 2008, which has guided agency interpretation of the Court's rulings but also has caused considerable confusion. Beyond the categories of waters that would be categorically jurisdictional under the proposal are "other waters." The regulatory term "other waters" applies to wetlands and non-wetland waters such as prairie potholes that are not considered traditionally navigable or meet other of the proposed rule's jurisdictional definitions. Much of the controversy since the Supreme Court rulings has focused on the degree to which "other waters" are jurisdictional. According to the agencies' analyses, 17% of these "other waters" would be categorically jurisdictional under the proposal, but "other waters" that are not categorically jurisdictional would continue to need case-by-case evaluation. The rule also lists waters that would not be jurisdictional, such as prior converted cropland and certain ditches. It makes no change to existing statutory exclusions, such as CWA permit exemptions for normal farming and ranching activities. Proposed changes would increase the asserted scope of CWA jurisdiction, in part as a result of expressly declaring some types of waters categorically jurisdictional (such as all waters adjacent to a jurisdictional water), and also by application of definitions, which would give larger regulatory context than previously to some waters, such as tributaries. Nevertheless, the agencies believe that the proposal does not exceed the CWA's coverage or protect new types of waters that have not been protected historically. While it would enlarge jurisdiction beyond that under the existing EPA-Corps guidance, they believe that it would not enlarge jurisdiction beyond what is consistent with the Supreme Court's narrow reading of jurisdiction and would reduce jurisdiction over some waters, as a result of exclusions and exemptions. Others may disagree. The agencies estimate that the rule would assert CWA jurisdiction over approximately 3% more U.S. waters (e.g., by including additional "other waters"), compared with current field practice, but 5% less than prior to the Supreme Court rulings (by specifically excluding some waters).