On May 27, 2015, the Environmental Protection Agency (EPA) and the U.S. Army Corps of Engineers (Corps) jointly announced a final rule defining the scope of waters protected under the Clean Water Act (CWA). The rule revises regulations that have been in place for more than 25 years. Revisions were proposed in March 2014 in light of Supreme Court rulings in 2001 and 2006 that interpreted the regulatory scope of the CWA more narrowly than the agencies and lower courts were then doing, and created uncertainty about the appropriate scope of waters protected under the CWA. In 2011, EPA and the Corps proposed guidance on policies for determining CWA jurisdiction to replace guidance previously issued in 2003 and 2008; all were intended to lessen confusion over the Court's rulings for the regulated community, regulators, and the general public. Those guidance documents sought to identify, in light of the Court's rulings, categories of waters that remain jurisdictional, categories not jurisdictional, and categories that require a case-specific analysis to determine if CWA jurisdiction applies. The 2011 proposed guidance identified similar categories as in the 2003 and 2008 documents, but it would have narrowed categories that require case-specific analysis in favor of asserting jurisdiction categorically for some types of waters. The 2015 rule replaces the existing 2003 and 2008 guidance, which had remained in effect because the 2011 proposed guidance was not finalized. The 2011 proposed guidance was extremely controversial, especially with groups representing property owners, land developers, and the agriculture sector, who contended that it represented a massive federal overreach beyond the agencies' statutory authority. Most state and local officials were supportive of clarifying the extent of CWA-regulated waters, but some were concerned that expanding the CWA's scope could impose costs on states and localities as their own actions (e.g., transportation projects) become subject to new requirements. Most environmental advocacy groups welcomed the proposed guidance, which would more clearly define U.S. waters that are subject to CWA protections, but some in these groups favored even a stronger document. Still, both supporters and critics of the 2011 proposed guidance urged the agencies to replace guidance, which is non-binding and not subject to full notice-and-comment rulemaking procedures, with revised regulations that define "waters of the United States." Three opinions in the 2006 Supreme Court Rapanos ruling similarly urged the agencies to initiate a rulemaking, as they did subsequently. In the 112 th and 113 th Congresses, a number of legislative proposals were introduced to bar EPA and the Corps from implementing the 2011 proposed guidance or developing regulations based on it; none of these proposals was enacted. Similar criticism followed almost immediately after release of the proposed rule in March 2014, with some Members asserting that it would result in job losses and damage economic growth. Supporters of the Administration, on the other hand, defended the agencies' efforts to protect U.S. waters and reduce frustration that has resulted from the unclear jurisdiction of the act. Support was expressed by environmental and conservation organizations, among others. The proposed rule was published in the Federal Register on April 21, 2014. The revised rule, announced on May 27, 2015, became effective August 28, 60 days after publication in the Federal Register . Judicial review of the rule began on July 13, 2015, but legal challenges were filed in multiple federal courts even before that date. On October 9, 2015, a federal court issued an order to stay implementation of the rule nationwide, pending further developments (see " Recent Developments "). Table 1 in this report provides a comparison of the existing regulatory language promulgated in 1986 that defines "waters of the United States" with language in the 2014 proposed rule and the 2015 final rule. The CWA protects "navigable waters," a term defined in the act to mean "the waters of the United States, including the territorial seas." Waters need not be truly navigable to be subject to CWA jurisdiction. Both the legislative history and the case law surrounding the CWA confirm that jurisdiction is not limited to traditional navigable waters, that is, waters that are, were, or could be used in interstate or foreign commerce. Waters that are jurisdictional are subject to the multiple regulatory requirements of the CWA: standards, discharge limitations, permits, and enforcement. Non-jurisdictional waters, in contrast, are not subject to these federal legal requirements. The act's single definition of "navigable waters" applies to the entire law. In particular, it applies to federal prohibition on discharges of pollutants except in compliance with the act's requirements (§301), requirements for point sources to obtain a permit prior to discharge (§§402 and 404), water quality standards and measures to attain them (§303), oil spill liability and oil spill prevention and control measures (§311), certification that federally permitted activities comply with state water quality standards (§401), and enforcement (§309). It impacts the Oil Pollution Act and other environmental laws, as well. The CWA leaves it to the agencies to define the term "waters of the United States" in regulations, which EPA and the Corps have done several times, most recently in 1986. According to the agencies, the new rule—which they now refer to as the Clean Water Rule—revises the existing administrative definition of "waters of the United States" in regulations consistent with legal rulings—especially the recent Supreme Court cases—and science concerning the interconnectedness of tributaries, wetlands, and other waters to downstream waters and effects of these connections on the chemical, physical, and biological integrity of downstream waters. The agencies assert that the rule also reflects their expertise and experience in administering the CWA, including making more than 120,000 case-specific jurisdictional determinations since 2008. The rule is particularly focused on clarifying the regulatory status of surface waters located in isolated places in a landscape (the types of waters with ambiguous jurisdictional status following the Supreme Court's 2001 ruling in SWANCC ) and small streams, rivers that flow for part of the year, and nearby wetlands (the types of waters affected by the Court's 2006 ruling in Rapanos ). In developing the rule, EPA and the Corps relied on a synthesis prepared by EPA's Office of Research and Development of more than 1,200 published and peer-reviewed scientific reports; the synthesis discusses the current scientific understanding of the connections or isolation of streams and wetlands relative to large water bodies such as rivers, lakes, estuaries, and oceans. The purpose of the scientific synthesis report was to summarize current understanding of these connections, the factors that influence them, and the mechanisms by which connected waters affect the function or condition of downstream waters. The document was reviewed by EPA's Science Advisory Board (SAB), which provides independent engineering and scientific advice to the agency and which completed its review in October 2014. A number of EPA's critics suggested that the agencies should have deferred developing or proposing a rule until a final scientific review document was complete. Some also expressed concern that the final report would not be available during the public comment period on the rule, which closed on November 14, 2014. Based on completion of the SAB review, EPA issued a final scientific assessment report in January 2015, saying that it would assist the agencies in developing the final rule. (See the Appendix for discussion of the connectivity report.) A key conclusion in the science report that was also emphasized by the SAB review is that streams and wetlands fall along a gradient of connectivity that can be described in terms of frequency; duration; magnitude; timing; and rates of change of water, material, and biotic fluxes to downstream waters. However, science cannot in all cases provide "bright lines" to interpret and implement policy. In the preamble to the final rule, EPA and the Corps acknowledge this point. ... the agencies' interpretive task in this rule ... requires scientific and policy judgment, as well as legal interpretation. The science demonstrates that waters fall along a gradient of chemical, physical, and biological connection to traditional navigable waters, and it is the agencies' task to determine where along that gradient to draw lines of jurisdiction under the CWA. In making this determination, the agencies must rely, not only on the science, but also on their technical expertise and practical experience in implementing the CWA during a period of over 40 years. In addition, the agencies are guided, in part, by the compelling need for clearer, more consistent, and easily implementable standards to govern the administration of the Act, including brighter line boundaries where feasible and appropriate. The final rule announced on May 27, 2015, retains much of the structure of the agencies' existing definition of "waters of the United States." Like the 2003 and 2008 guidance and the 2014 proposal, it identifies categories of waters that are and are not jurisdictional, as well as categories of waters that require a case-specific evaluation. The final rule revises parts of the 2014 proposed rule; the text box, below, lists the key changes in the final rule. Figure 1 illustrates waters that are jurisdictional by rule and waters that may be determined to be jurisdictional based on case-specific analysis.