The Clean Water Act (CWA) is the principal federal law governing pollution of the nation's surface waters. The CWA protects "navigable waters," defined in the statute as "waters of the United States, including the territorial seas." The scope of this term— waters of the United States , or WOTUS—determines which waters are federally regulated and has been the subject of debate for decades. The CWA does not define the term. Thus, in implementing the CWA, the Army Corps of Engineers and U.S. Environmental Protection Agency (EPA) have defined the term in regulations. For much of the past three decades, regulations promulgated by the Corps and EPA in 1986 and 1988, respectively, have been in effect. In 2001 and 2006, the Supreme Court issued rulings pivotal to the definition of WOTUS— Solid Waste Agency of Northern Cook County v. U.S. Army Corps of Engineers and Rapanos v. United States , respectively. Both rulings interpreted the scope of the CWA more narrowly than the Corps and EPA had done previously in regulations and guidance, but they created uncertainty about the intended scope of waters that are protected by the CWA. The Court's decision in Rapanos , split 4-1-4, yielded three different opinions. The four-Justice plurality decision, written by Justice Scalia, states that the dredge and fill provisions in the CWA apply only to wetlands connected to relatively permanent bodies of water (streams, rivers, lakes) by a continuous surface connection. Justice Kennedy, writing alone, demanded a "significant nexus" between a wetland and a traditional navigable water, using a case-by-case test that considers ecological connection. Justice Stevens, for the four dissenters, would have upheld the existing reach of Corps/EPA regulations. In response to the rulings, the agencies developed guidance in 2003 and 2008 to help clarify how EPA regions and Corps districts should implement the Court's decisions. This guidance identified categories of waters that remained jurisdictional or not jurisdictional and required a case-specific analysis to determine whether jurisdiction applies. The guidance did not resolve all interpretive questions, and diverse stakeholders requested a formal rulemaking to revise the existing rules. Accordingly, the Corps and EPA proposed a rule in April 2014 defining the scope of waters protected under the CWA. On June 29, 2015, the Corps and EPA finalized the rule—known as the Clean Water Rule or WOTUS rule. It reflects over 1 million public comments on the 2014 proposed rule as well as input provided through public outreach efforts that included over 400 meetings with diverse stakeholders. The 2015 Clean Water Rule retained much of the structure of the agencies' prior definition of WOTUS. It focused on clarifying the regulatory status of waters with ambiguous jurisdictional status following the Supreme Court's rulings, including isolated waters and streams that flow only part of the year and nearby wetlands. As explained in the 2015 Clean Water Rule's preamble, the Corps and EPA used Justice Kennedy's "significant nexus" standard in developing the rule as well as the plurality opinion (written by Justice Scalia) in establishing boundaries on the scope of jurisdiction. The 2015 Clean Water Rule identified categories of waters that are and are not jurisdictional as well as waters that require a case-specific evaluation (see Figure 1 ). Broad categories under the final rule include the following: Jurisdictional by rule in all cases . Traditional navigable waters, interstate waters, the territorial seas, and impoundments of these waters are jurisdictional by rule. All of these waters were also jurisdictional under pre-2015 rules. Jurisdictional by rule, as defined . Two additional categories—tributaries and adjacent waters—are jurisdictional by rule if they meet definitions established in the 2015 Clean Water Rule. According to the rule's preamble, the definitions ensure that the rule covers waters that meet the significant nexus standard. Tributaries, under pre-2015 rules, were jurisdictional by rule without qualification but lacked a regulatory definition. The 2015 Clean Water Rule newly defined tributaries . Tributaries that meet the new definition are jurisdictional by rule. Similarly, "adjacent waters"—including wetlands, ponds, lakes, oxbows, impoundments, and similar waters that are adjacent to traditional navigable waters, interstate waters, the territorial seas, jurisdictional tributaries, or impoundments of these waters—are jurisdictional by the 2015 Clean Water Rule if they meet the rule's established definition. Under the 2015 Clean Water Rule, adjacent means "bordering, contiguous, or neighboring" one of the aforementioned waters. The rule established a definition of neighboring that set new limits for the purposes of determining adjacency. Neighboring is defined to include waters (1) located within 100 feet of the ordinary high water mark (OHWM) of a traditional navigable water, interstate water, the territorial seas, jurisdictional tributary, or impoundment of these waters; (2) located in the 100-year floodplain and within 1,500 feet of the OHWM of a traditional navigable water, interstate water, the territorial seas, jurisdictional tributary, or impoundment of these waters; or (3) located within 1,500 feet of the high tide line of a traditional navigable water or the territorial seas and waters located within 1,500 feet of the OHWM of the Great Lakes. Under pre-2015 rules, adjacent was defined to mean "bordering, contiguous, or neighboring" and did not include specific limits on "neighboring." Waters requiring a case-specific evaluation . Some types of waters—but fewer than under practices used prior to the 2015 Clean Water Rule—would remain subject to a case-specific evaluation of whether or not they meet the standards for federal jurisdiction. This case-specific evaluation examines whether the water has a significant nexus to traditional navigable waters, interstate waters or wetlands, or the territorial seas. Similarly situated waters (i.e., prairie potholes, Carolina bays and Delmarva bays, pocosins, western vernal pools, and Texas coastal prairie wetlands) are combined for the purposes of a significant nexus analysis. In addition, the 2015 Clean Water Rule provides that two other categories of waters are subject to case-specific significant nexus analysis: (1) waters within the 100-year floodplain of a traditional navigable water, interstate water, or the territorial seas; and (2) waters within 4,000 feet of the high tide line or the OHWM of a traditional navigable water, interstate water, the territorial seas, impoundments, or jurisdictional tributary. Exclusions . Certain waters would be excluded from CWA jurisdiction. Some were restated exclusions under pre-2015 rules (e.g., prior converted cropland). Some have been excluded by practice and would be expressly excluded by rule for the first time (e.g., groundwater and some ditches). Some were new in the final rule (e.g., stormwater management systems). The 2015 Clean Water Rule did not affect existing statutory exclusions—that is, exemptions for existing "normal farming, silviculture, and ranching activities" and for maintenance of drainage ditches (CWA §404(f)) as well as for agricultural stormwater discharges and irrigation return flows (CWA §402(l) and CWA §502(14)). Much of the controversy since the Supreme Court's rulings has centered on instances that have required CWA permit applicants to seek a case-specific analysis to determine if CWA jurisdiction applies to their activity. The Corps and EPA's stated intention in promulgating the Clean Water Rule was to clarify questions of CWA jurisdiction in view of the rulings while also reflecting their scientific and technical expertise. Specifically, they sought to articulate categories of waters that are and are not protected by the CWA, thus limiting the water types that require case-specific analysis. Industries that are the primary applicants for CWA permits and agriculture groups raised concerns over how broadly the 2014 proposed rule would be interpreted. They contended that the proposed definitions were ambiguous and would enable agencies to assert broader CWA jurisdiction than is consistent with law and science. The final 2015 Clean Water Rule added and defined key terms, such as tributary and significant nexus , and modified the proposal in an effort to improve clarity, but the concerns remained. Some local governments that own and maintain public infrastructure also criticized the 2014 proposed rule. They argued that it could increase the number of locally owned ditches under federal jurisdiction because it would define some ditches as WOTUS under certain conditions. Corps and EPA officials asserted that the proposed exclusion of most ditches would decrease federal jurisdiction, but the issue remained controversial. The final 2015 Clean Water Rule excluded most ditches and expressly excluded stormwater management systems and structures from jurisdiction. Some states supported a rule to clarify the scope of CWA jurisdiction, but there was no consensus on the 2014 proposed rule or the final 2015 Clean Water Rule. Many states asserted that the changes would too broadly expand federal jurisdiction, some believed that the agencies did not adequately consult with states, and some were largely supportive. Environmental groups generally supported the agencies' efforts to protect waters and reduce uncertainty. Still, some argued that the scope of the 2014 proposed rule should be further expanded—for example, by designating additional categories of waters and wetlands (e.g., prairie potholes) as categorically jurisdictional. The final 2015 Clean Water Rule did not do so. Instead, such waters would require case-specific analysis to determine if jurisdiction applies. Corps and EPA officials under the Obama Administration defended the 2014 proposed rule but acknowledged that it raised questions they believed the final 2015 Clean Water Rule clarified.