F or more than forty-five years, all three branches of government have struggled with how to interpret the meaning of "waters of the United States" in the Clean Water Act. In 1972, Congress eliminated the requirement that waters must be navigable in the traditional sense —meaning they are capable of being used by vessels in interstate commerce—in order to be subject to federal water pollution regulation. Rather than use traditional tests of navigability, the 1972 amendments to the Federal Water Pollution Control Act, which came to be known as the Clean Water Act, redefined "navigable waters" to include "the waters of the United States, including the territorial seas." Disputes over the meaning of that phrase have been ongoing ever since the change. Some courts and commentators disagree on how the scope of federal jurisdictional waters changed over time as a result of interpretative approaches taken by the agencies responsible for administering the Clean Water Act—the Environmental Protection Agency (EPA) and the U.S. Army Corps of Engineers (Corps). This debate resurfaced during the Obama Administration when the Corps and EPA issued a rule, known as the Clean Water Rule, which substantially redefined "waters of the United States" in the agencies' regulations for the first time in more than two decades. While some argued that the Clean Water Rule constituted a major expansion of federal jurisdiction, others asserted that the agencies construed the term in a narrower fashion than in prior regulations. A vocal critic of the Clean Water Rule, President Trump shifted the executive branch's policy toward the meaning of "waters of the United States." In February 2017, President Trump issued an executive order directing EPA and the Corps to review and revise or rescind the Clean Water Rule. The agencies currently are in the process of carrying out the executive order, and they unveiled proposed regulations redefining "waters of the United States" in December 2018. As in nearly all prior attempts to define this phrase, however, observers disagree on whether the latest proposed definition correctly calibrates the scope of federal water pollution regulation. This report provides context for this debate by examining the history of major changes to the meaning of "waters of the United States" as expressed in federal regulations, legislation, agency guidance, and case law. The Clean Water Act is the principal law governing pollution of the nation's surface waters. Among other requirements, the act prohibits the unauthorized discharge of pollutants into "navigable waters," and requires persons wishing to discharge dredged or fill material into "navigable waters" to obtain a permit from the Corps. In its definition section, the act defines the term "navigable waters" to mean "waters of the United States, including its territorial seas." This single, jurisdiction-defining phrase applies to the entire law, including the national pollutant discharge elimination system (NPDES) permit program; permit requirements for disposal of dredged or fill material, known as the Section 404 program; water quality standards and measures to attain them; oil spill liability and prevention; and enforcement. The Clean Water Act itself does not expand further on the meaning of "waters of the United States." Instead, the Corps and EPA have expounded on this phrase through agency guidance and regulations, which federal courts have struck down on various occasions as failing to satisfy statutory or constitutional requirements. Federal authority to regulate waters within the United States primarily derives from the Commerce Clause, which gives Congress the power to "regulate commerce with foreign nations, and among the several states . . . ." Accordingly, federal laws and regulations regulating waters of the United States cannot cover matters that exceed that constitutional source of authority. Legal challenges to the Corps' and EPA's interpretation of "waters of the United States"—particularly those which were successful—often followed broader trends in interpreting the Commerce Clause. For a period after its enactment in 1972, courts generally interpreted the Clean Water Act as having a wide jurisdictional reach, but, in recent decades, the Supreme Court has emphasized that "the grant of authority to Congress under the Commerce Clause, though broad, is not unlimited." A time line of events in the evolution of the definition of "waters of the United States" is provided in the Appendix , and major events are shown in Figure 1 . Historically, federal laws regulating waterways, such as the Rivers and Harbors Appropriations Act of 1899 (Rivers and Harbors Act), exercised jurisdiction over "navigable water[s] of the United States[.]" The Supreme Court interpreted this phrase to govern only waters that were "navigable-in-fact"—meaning that they were "used, or are susceptible of being used, . . . as highways for commerce, over which trade and travel are or may be conducted in the customary modes of trade and travel on water." Beginning with the Federal Water Pollution Control Act of 1948, Congress began to use a different jurisdiction-defining phrase to regulate pollution of "interstate waters," which it defined as "all rivers, lakes, and other waters that flow across, or form a part of, a State's boundaries." Congress amended that legislation in 1961 to expand federal jurisdiction from "interstate waters" to "interstate or navigable waters[.]" The Federal Water Pollution Control Act Amendments of 1972, which came to be known as the Clean Water Act, again amended the jurisdictional reach of federal water pollution legislation. There, Congress exercised jurisdiction over "navigable waters," but provided a new definition of that phrase, stating: "The term 'navigable waters' means the waters of the United States, including the territorial seas." This subtle definitional change proved to have tremendous consequences for the jurisdictional scope of the Clean Water Act. In debating the 1972 amendments that created the Clean Water Act, some Members of Congress explained that they intended the revised definition to expand the law's jurisdiction beyond traditionally navigable or interstate waters. The conference report states that the "conferees fully intend that the term 'navigable waters' be given the broadest possible constitutional interpretation unencumbered by agency determinations which have been made or may be made for administrative purposes." And during debate in the House on approving the conference report, one Representative explained that the definition "clearly encompasses all water bodies, including streams and their tributaries, for water quality purposes." Courts have frequently referred to the act's legislative history when interpreting its jurisdictional reach, but they have not always agreed on the import of this history. The Corps and EPA share responsibility for administering the Clean Water Act. Both agencies have administrative responsibilities under Section 404 of the act, and EPA administers most other Clean Water Act-related programs in partnership with U.S. states. Because of this shared jurisdiction, both agencies create regulations defining the waters subject to their regulatory jurisdiction. In the initial years following the enactment of the Clean Water Act, their respective definitions differed significantly. In May 1973, EPA issued its first set of regulations implementing the Clean Water Act's NPDES permit program. There, EPA defined the term "navigable waters" to include six categories of waterbodies. Three months prior to issuing these regulations, EPA's general counsel had provided an opinion on the meaning of "navigable waters" in the Clean Water Act. The general counsel's recommended definition largely mirrored EPA's 1973 regulatory definition, but with one critical difference: categories four through six of the general counsel's recommendation would have included interstate lakes, rivers, and streams that are utilized for interstate activities rather than intrastate waters used for such activities. EPA's definition of "navigable waters" in its non-NPDES water pollution regulations at the time also differed in certain ways from its May 1973 definition. The Corps' early implementation of the Clean Water Act differed considerably from EPA's regulations. After initially proposing regulations that simply repeated the statutory definition of "navigable waters," the Corps issued final regulations in April 1974 implementing Section 404 of the Clean Water Act. There, the Corps acknowledged the language from the conference report for the Clean Water Act as calling for the "broadest possible constitutional interpretation" of navigable waters, but concluded that the Constitution limited its jurisdiction to the same waters that it regulated under preexisting laws, such as the Rivers and Harbors Act. Based on this reasoning, the Corps defined "navigable waters" using language that generally limited its jurisdiction to waters that were navigable-in-fact. Less than one year after the Corps published its first regulations defining jurisdictional waters, the United States District Court for the District of Columbia struck them down as too narrow and inconsistent with the Clean Water Act. In Natural Resources Defense Council v. Callaway , the court held that because "Congress . . . asserted federal jurisdiction over the nation's waters to the maximum extent permissible under the Commerce Clause of the Constitution[,]" the definition could not be limited to "traditional tests of navigability[.]" The court ordered the Corps to produce new regulations that acknowledged "the full regulatory mandate" of the Clean Water Act. The Corps responded to Callaway on May 6, 1975, by publishing proposed regulations that offered four alternative methods of redefining the Corps' jurisdiction under the 1972 amendments. At the same time that it proposed these alternatives, the Corps published a press release stating that the holding of Callaway may require "the rancher who wants to enlarge his stock pond, or the farmer who wants to deepen an irrigation ditch or plow a field, or the mountaineer who wants to protect his land against stream erosion" to obtain federal permits. These events brought public and media attention to the breadth of jurisdiction under the Clean Water Act. They also created a disagreement between the Corps and EPA, and led to a series of subcommittee hearings in the House and Senate. In the aftermath of this public and congressional scrutiny, the Corps issued interim final regulations in 1975 in which it revised the definition of "navigable waters" for purposes of the Clean Water Act's Section 404 program by adopting much of the structure used in EPA's 1973 regulations. The Corps' definition also added "wetlands, mudflats, swamps, marshes, and shallows" that are "contiguous or adjacent to other navigable waters" and "artificially created channels and canals used for recreational or other navigational purposes that are connected to other navigable waters" to the definition of "waters of the United States." Finally, the Corps' 1975 interim regulations permitted federal regulation over all other waters that a Corps' district engineer "determines necessitate regulation for the protection of water quality" based on the Corps' technical standards and evaluation criteria. In 1977, the Corps issued final regulations reorganizing the definition of "waters of the United States" into five categories. The final category of the 1977 definition contained the Corps' most expansive definition of jurisdictional waters as of that time. A footnote to the Corps' regulations explained that the Category Five waters incorporate "all other waters of the United States that could be regulated under the federal government's Constitutional powers to regulate and protect interstate commerce." The Corps would continue to use this Commerce Clause-focused provision (with revisions) until the Clean Water Rule was published in 2015, and EPA would later adopt it in its regulations. After the Corps' 1975 and 1977 regulations, some Members of Congress introduced bills that sought to limit the Clean Water Act's jurisdiction to traditional, navigable-in-fact waters, but the proposed limiting legislation never became law. Instead, Congress amended the Federal Water Pollution Control Act through the Clean Water Act of 1977, which did not alter the jurisdictional phrase "waters of the United States." The original version of the Clean Water Act of 1977 introduced in the House would have limited the Corps' jurisdiction, and an amendment proposed in the Senate sought similar limitations. But the original Senate version, which generally retained the existing definition of "navigable waters," was adopted in conference and passed into law. The Clean Water Act of 1977, as enacted, contained certain exemptions from Section 404 permitting for "normal farming, silviculture, . . . ranching[,]" and other activities. While the 1977 legislation appeared to resolve temporarily some congressional dispute over the reach of the Clean Water Act, disagreement arose between the Corps and EPA over which agency had final authority to determine which waters were subject to Section 404 permit requirements. EPA independently defined the jurisdictional reach of the Clean Water Act as it related to programs like NPDES and oil pollution prevention, but it incorporated the Corps' definition into its regulations related to Section 404 permits. At the same time, however, EPA separately expanded on that definition in an appendix to its Section 404 regulations. The U.S. Attorney General ultimately intervened in 1979 and provided a legal opinion that EPA has final administrative authority to determine the reach of the term "navigable waters" for purposes of Section 404. The Corps and EPA eventually executed a Memorandum of Agreement in 1989 resolving that EPA would act as the lead agency responsible for developing programmatic guidance and interpretation of the scope of jurisdictional waters, and the Corps would be responsible for most case-specific determinations on whether certain property was subject to Section 404. Although it took the agencies 10 years after the Attorney General's opinion to agree formally on a division of responsibilities, the Corps and EPA streamlined and harmonized the regulatory definition of "waters of the United States" well before that. In May 1980, EPA issued regulations redefining the term among its consolidated permit requirements, and the Corps adopted EPA's definition in interim regulations two years later . The Corps issued final regulations in 1986 that did not change the regulatory definition, and the two agencies continued to use this core definition (with modifications) until they published the Clean Water Rule in 2015 . The Supreme Court reviewed a legal challenge to the Corps' application of "waters of the United States" for the first time in 1985 in United States v. Riverside Bayview Homes, Inc . There, the Corps sought to enjoin a property owner from discharging fill material on his wetlands located one mile from the shore of Lake St. Clair in Michigan, a 468-square-mile, navigable-in-fact lake that forms part of the boundary between Michigan and Ontario, Canada. The Corps argued that, by defining "waters of the United States" to include wetlands that are "adjacent to" other jurisdictional waters, including navigable-in-fact waters like Lake St. Clair, its regulations required the landowner to obtain a Section 404 permit before discharging fill material. Before the case reached the Supreme Court, the Sixth Circuit concluded that it must construe the Corps' regulatory definition narrowly in order to avoid a potential violation of the Fifth Amendment prohibition on the taking of private property for public use without just compensation. Applying this method of interpretation, the Sixth Circuit construed the Corps' regulations so as not to include the wetlands at issue, and it avoided reaching a decision on whether the Corps' regulations were constitutional. The Supreme Court reversed. Although it acknowledged that on a "purely linguistic level" it may seem unreasonable to classify lands , wet or otherwise, as waters , the Supreme Court called such a plain language approach "simplistic." Further, it rejected the lower courts' concerns over the constitutionality of the Corps' regulations as "spurious." Instead of applying a narrow approach to avoid constitutional implications, the Court gave deference to the Corps' position, and concluded that because "[w]ater moves in hydrological cycles" rather than along "artificial lines," it was reasonable for the Corps to conclude that "adjacent wetlands are inseparably bound up with the 'waters' of the United States . . . ." The Court also cited legislative history from the passage of the Clean Water Act and the amendments in 1977—in which the term "adjacent wetlands" was added to the statute —as support for its conclusion that Congress intended for the Clean Water Act to have a broad jurisdictional reach which included the adjacent wetlands at issue. In concluding that adjacent wetlands could reasonably be covered, however, the Court also emphasized that it did not express any opinion on the Corps' authority to regulate discharges of fill material into wetlands that are not adjacent to bodies of open water. Following Riverside Bayview Homes , the Corps and EPA engaged in rulemaking in which they interpreted the Clean Water Act to govern all waters which were used or may have been used by migratory birds crossing state lines. The agencies did not redefine "waters of the United States" through this interpretation, which came to be known as the Migratory Bird Rule, but instead stated that the Migratory Bird Rule was a "clarification" of the existing regulatory definition. The agencies also continued to adjust their interpretation of the definition of "waters of the United States" in the late 1980s by, among other things, excluding nontidal drainage and irrigation ditches, artificial lakes or ponds used for irrigation and stock watering, reflecting pools, and swimming pools. In 1993, the agencies jointly revised their regulations to exclude "prior converted cropland"—areas that were previously drained and converted to agricultural use—from jurisdictional waters. In addition to disputes over the textual definition of "waters of the United States," disagreement surrounding the technical standards used to delineate the physical boundaries of jurisdictional waters, particularly wetlands , arose in the late 1980s. The Corps issued the first wetlands delineation manual in 1987 (1987 Manual), but EPA published its own manual the following year which used an alternative technical analysis. Differences among these and other wetlands manuals led to the preparation of an interagency Federal Manual for Identifying and Delineating Jurisdictional Wetlands in January 1989 (Federal Manual). Some observers criticized aspects of the Federal Manual, including the methodology it employed for identifying and delineating jurisdictional waters. Some also argued that the Federal Manual improperly expanded the scope of federal regulations of wetlands. Disagreements ultimately led to congressional action in 1991 in the form of appropriations legislation that prohibited the Corps from using funds to identify jurisdictional waters using the Federal Manual. The following year, Congress mandated that the Corps use the 1987 Manual until a new manual was published after public notice and comment. The interagency group proposed revisions to the Federal Manual, which received over 100,000 comments, but that proposal was never finalized, and no interagency wetlands manual was created. In contrast to the agencies' attempt to align jurisdictional waters with what they interpreted to be the outer reaches of the Commerce Clause in the 1980s, a series of court cases beginning in the late 1990s caused the Corps and EPA to modify their interpretation of "waters of the United States." For much of the 20th century, the Supreme Court broadly construed the Commerce Clause to give Congress discretion to regulate activities which "affect" interstate commerce, so long as its legislation was reasonably related to achieving its goals of regulating interstate commerce. In the 1995 case of United States v. Lopez , however, the Supreme Court struck down a federal statute for the first time in more than 50 years based purely on a finding that Congress exceeded its powers under the Commerce Clause. In Lopez , the Court held the Commerce Clause did not provide a constitutional basis for federal legislation criminalizing possession of a firearm in a school zone because the law neither regulated a commercial activity nor contained a requirement that the firearm possession be connected to interstate commerce. The Court revisited its prior Commerce Clause cases and sorted Congress's commerce power into three categories: (1) regulation of channels of commerce, (2) regulation of instrumentalities of commerce, and (3) regulation of economic activities which not only affect but "substantially affect" interstate commerce. Lopez set the backdrop for a series of major opinions limiting federal jurisdiction under the Clean Water Act. The United States Court of Appeals for the Fourth Circuit issued the first in the series of decisions limiting the jurisdictional reach of the Clean Water Act in 1997. Following a seven-week trial in United States v. Wilson , a jury convicted three defendants of violating Section 404 for knowingly discharging fill material into wetland property located approximately 10 miles from the Chesapeake Bay and 6 miles from the Potomac River in Maryland. On appeal to the Fourth Circuit, the defendants challenged their conviction on the grounds that the portion of the Corps' regulatory definition of "waters of the United States"—which included all waters "the use, degradation or destruction of which could affect interstate or foreign commerce"—exceeded the Corps' statutory authority in the Clea n Water Act and Congress's constitutional authority in the Commerce Clause. Relying in part on the holding in Lopez , the Fourth Circuit agreed with a portion of the defendants' arguments and ordered a new trial. The court reasoned that, under Lopez , the regulated conduct must "substantially affect" interstate commerce in order to invoke the Commerce Clause power. Because the Corps purported to regulate waters that "could affect" interstate commerce—without regard to whether there was any actual effect, substantial or otherwise—the Fourth Circuit concluded that the Corps exceeded its authority. Although the Fourth Circuit strongly suggested that the Corps' assertion of jurisdiction exceeded the constitutional grant of authority under the Commerce Clause, it ultimately invalidated the challenged portion of the regulations solely on the ground that it exceeded the congressional authorization under the Clean Water Act. As Wilson never reached the Supreme Court, it was only binding precedent in the Fourth Circuit, and the stricken language remained in the regulations of the Corps and EPA until the release of the 2015 Clean Water Rule. Although the Corps did not modify its regulatory definition of "waters of the United States" in response to Wilson , it did publish guidance in March 2000 on the effect of the decision on its Section 404 jurisdiction. The Corps explained that, within the Fourth Circuit only , "isolated waters" must be shown to have an actual connection to interstate or foreign commerce. "Isolated waters," in Clean Water Act parlance, are waters that are not navigable-in-fact, not interstate, not tributaries of the foregoing, and not hydrologically connected to such waters—but whose use, degradation, or destruction could affect interstate commerce. The 2000 guidance also provided clarification on certain nontraditional waters that the Corps considered part of the "waters of the United States." Jurisdictional waters, the Corps explained, included both intermittent streams , which have flowing water supplied by groundwater during certain times of the year, and ephemeral streams , which have flowing water only during and for a short period after precipitation events. The Corps also deemed drainage ditches constructed in jurisdictional waters to be subject to the Clean Water Act except when the drainage was so complete that it converted the entire area to dry land. In 2001, the Supreme Court took up another challenge to the jurisdictional reach of the Clean Water Act in Solid Waste Agency of Northern Cook County v. U.S. Army Corps of Engineers ( SWANCC ), revisiting the issue for the first time since its 1995 decision in Riverside Bayview Homes . In SWANCC , the Court evaluated whether Clean Water Act jurisdiction extended to an abandoned sand and gravel pit which contained water that had become a habitat for migratory birds. Citing the legislative history of the 1972 amendments and the Clean Water Act of 1977, the Corps had argued that the Clean Water Act can extend to such isolated waters under the Migratory Bird Rule. In a 5-4 ruling, the Court rejected the Corps' position, and held that the Corps' assertion of jurisdiction over isolated waters based purely on their use by migratory birds exceeded its statutory authority. The SWANCC Court's conclusion was informed, in part, by Lopez and another landmark Commerce Clause decision issued five years later, United States v. Morrison , in which the Court held that Congress lacked constitutional authority under the Commerce Clause to enact portions of the Violence Against Women Act. In light of this jurisprudence, the SWANCC Court concluded that allowing the Corps to assert jurisdiction under the Migratory Bird Rule raised "serious constitutional questions" about the limits of Congress's authority and "would result in significant impingement of States' traditional and primary power of land and water use." Rather than interpret the Clean Water Act in a way that would implicate these "significant constitutional and federalism questions[,]" the Court concluded that Congress's use of the phrase "navigable waters" in the Clean Water Act "has at least the import of showing us what Congress had in mind for enacting the [act]: its traditional jurisdiction over waters that were or had been navigable in fact or which could reasonably be made so." Based on this reading, the Court concluded that Congress did not intend to invoke the outer limits of the Commerce Clause in the Clean Water Act, and the Corps could not rely on the Migratory Bird Rule as a basis for jurisdiction. In contrast to Riverside Bayview Homes , the SWANCC Court focused less on the legislative history of the Clean Water Act, and instead emphasized the Corps' original interpretation of the 1972 amendments in which it limited its jurisdiction to navigable-in-fact waters. Although the Riverside Bayview Homes Court found that classical "navigability" was of "limited import" in determining Clean Water Act jurisdiction, the SWANCC Court distinguished that case as focused on "wetlands adjacent to navigable waters." The ponds which formed in the abandoned gravel pits in SWANCC were " not adjacent to open water[,]" and therefore lacked the requisite "significant nexus" to traditionally navigable waters necessary for jurisdiction under the Clean Water Act, the Court concluded. SWANCC did not go as far as the Fourth Circuit, however, in striking down an entire subsection of the definition of "waters of the United States." It limited its holding to the Migratory Bird Rule, which the Corps described as an effort to "clarify" its regulatory definition. But while its direct holding was arguably narrow, SWANCC 's rationale was much broader and called into question whether the Corps and EPA could assert jurisdiction under the Clean Water Act over many wholly intrastate isolated waters. The relationship between SWANCC 's limited holding and the Court's broader rationale generated considerable litigation over the scope of the Clean Water Act. The general counsels for the Corps and EPA added their voices to the post- SWANCC debate in a joint memorandum issued on the last full day of the Clinton Administration, January 19, 2001. Combining the "significant nexus" language from SWANCC with the existing regulatory definition of "waters of the United States," the agencies concluded that they could continue to exercise jurisdiction over isolated waters so long as the use, degradation, or destruction of those waters could affect other "waters of the United States." The potential effect on or degradation on existing jurisdictional waters, the agencies reasoned, established the "significant nexus" mentioned in SWANCC . In January 2003, the Corps and EPA issued a notice of proposed rulemaking regarding how field staff should address jurisdictional issues in the Clean Water Act and which contained a revised joint memorandum on the effect of SWANCC . The agencies later abandoned that proposed rulemaking effort, leaving unanswered questions over federal jurisdiction over isolated waters after SWANCC . These uncertainties caused the Corps and EPA to shift their attention to alternative bases for jurisdiction in defining "waters of the United States"—such as "adjacent wetlands"—and set the stage for the Supreme Court's next encounter with a Clean Water Act jurisdictional dispute in Rapanos v. United States . Rapanos involved a consolidation of two cases on appeal from the Sixth Circuit— Rapanos and Carabell —both of which concerned the breadth of the Clean Water Act's jurisdiction over "adjacent" wetlands. In Carabell , landowners challenged whether Section 404 jurisdiction extends to "wetlands that are hydrologically isolated from any of the 'waters of the United States[,]'" and Rapanos presented the similar question of whether this jurisdiction includes nonnavigable wetlands "that do not even abut a navigable water." In both cases, collectively referred to as Rapanos , the Sixth Circuit upheld the Corps' assertion of jurisdiction over the wetland property in question. Many anticipated that Rapanos would provide clarity on the disputes following SWANCC . And although a majority of five Justices agreed that the Sixth Circuit decision was flawed, they were not able to agree on a single, underlying standard which would govern future jurisdictional disputes. Instead, a four-Justice plurality opinion, authored by Justice Scalia, and an opinion by Justice Kennedy, writing only for himself, proposed two alternative tests for evaluating jurisdictional waters. With no controlling rationale from the majority, lower courts interpreting Rapanos struggled with the question of what analysis to apply in Clean Water Act jurisdictional disputes. When a majority of the Supreme Court agrees only on the outcome of a case and not on the ground for that outcome, the holding of the Court which lower courts must follow "may be viewed as that position taken by those Members who concurred in the judgments on the narrowest grounds." While this rule may appear straightforward, it is not always self-evident how courts should identify which Justice's opinion rests on the "narrowest grounds." Some courts have held that Justice Kennedy's "significant nexus" test is the narrowest ruling to be derived from Rapanos . Others concluded that waterbodies that satisfy either the plurality test or the "significant nexus" test satisfy Rapanos and may be deemed jurisdictional. Of the nine circuits that have addressed the issue, all have applied Justice Kennedy's significant nexus test either alone or in combination with the plurality's test, and none have applied the plurality approach alone. Still, some courts and observers have criticized the significant nexus test as vague and difficult to implement. The Corps and EPA offered their own interpretation of Rapanos through guidance to field officers in 2007, which the agencies revised and replaced after public comment in 2008. The 2008 guidance adopted the view taken by some lower courts that jurisdiction exists over any waterbody that satisfies either the plurality approach or the significant nexus test. The agencies further deconstructed the jurisdictional analysis into three categories: (1) waters that are categorically jurisdictional; (2) waters that may be deemed jurisdictional on a case-by-case basis; and (3) waters that are excluded from jurisdiction under the Clean Water Act. In 2011, the Corps and EPA sought comments on proposed changes to the 2008 guidance, which the agencies acknowledged would increase the number of waters regulated under the Clean Water Act in comparison to its earlier post- Rapanos guidance. The potential enlargement of jurisdiction spawned congressional attention, including a letter signed by 41 Senators requesting that the agencies abandon the effort. Some Members of Congress introduced prohibitions on funding related to the draft guidance in several appropriations bills, but those provisions were never enacted. Instead, the agencies abandoned pursuit of the 2011 draft guidance in favor of their 2015 effort at defining the scope of "waters of the United States," the Clean Water Rule. The Corps and EPA issued the Clean Water Rule in May 2015 in an effort to clarify the bounds of jurisdictional waters in the wake of SWANCC and Rapanos . The agencies relied on a synthesis of more than 1,200 published and peer-reviewed scientific reports and over 1 million comments on the proposed version of the rule. The Clean Water Rule contains the same three-tier structure from the agencies' 2008 joint guidance, identifying waters that (1) are categorically jurisdictional, (2) may be deemed jurisdictional on a case-by-case basis if they have a significant nexus with other jurisdictional waters, and (3) are categorically excluded from the Clean Water Act's jurisdiction. In an effort to reduce uncertainty about the scope of federal jurisdiction, the agencies sought to increase categorical jurisdictional determinations and reduce the number of waterbodies subject to the case-specific significant nexus test. The Clean Water Rule was the subject of significant debate among observers, stakeholders, and Members of Congress, and a 2015 Government Accountability Office (GAO) report found that EPA violated publicity or propaganda and antilobbying provisions in prior appropriations acts through its promotion of the Clean Water Rule on social media. The 114th Congress also took steps to block its implementation. In January 2016, the Senate and House passed a resolution of disapproval seeking to nullify the Clean Water Rule under the Congressional Review Act. However, President Obama vetoed that resolution, and a procedural vote in the Senate to override the veto failed. The Obama Administration intended the Clean Water Rule to take effect on August 28, 2015, but 31 states and 53 non-state plaintiffs, including industry associations, environmental groups, and others, filed suit challenging its legality. The plaintiffs argued, among other things, that the rule exceeded the agencies' statutory and constitutional authority and did not comply with the rulemaking requirements in the Administrative Procedure Act (APA). Environmental groups, seven states, and the District of Columbia intervened in defense of the rule. Before any court could address the merits of the claims, however, an impasse arose over what court was the proper forum for the litigation. Whereas some plaintiffs filed suit in federal district courts, others argued that a judicial-review provision in Section 509 of the Clean Water Act gave the U.S. circuit courts of appeals direct appellate-level review over challenges to the Clean Water Rule. At the district court level, some courts dismissed their suits, concluding that the courts of appeals had exclusive jurisdiction. But one district court—the District Court for the District of North Dakota (District of North Dakota)—ruled that it had jurisdiction to review the Clean Water Rule. In August 2015, the District of North Dakota concluded that the rule was likely to be struck down on the merits, and it granted a motion for preliminary injunction, temporarily barring the Clean Water Rule's implementation in 13 western states. (The court later added another state, Iowa, to the scope of injunction.) In the parallel litigation at the appellate level, a Judicial Panel on Multidistrict Litigation consolidated and transferred all circuit court cases to the United States Court of Appeals for the Sixth Circuit (Sixth Circuit). In the consolidated, appellate-level litigation, the Sixth Circuit concluded that the agencies should not apply the Clean Water Rule during the pendency of the legal challenges, and it issued a nationwide stay of the rule. The Sixth Circuit also concluded that it—and not the district courts—had exclusive jurisdiction over the challenges to the Clean Water Rule, setting the stage for the Supreme Court to address the threshold question of which court or courts possess jurisdiction to hear the Clean Water Rule cases. In National Association of Manufacturers (NAM) v. Department of Defense , the Supreme Court disagreed with the Sixth Circuit and concluded that the Clean Water Act did not provide direct appellate-level jurisdiction over the pending cases. Section 509 of the Clean Water Act lists seven categories of agency actions subject to direct appellate review, Justice Sotomayor explained in an opinion for the unanimous Court, but a legal challenge to a rule defining "waters of the United States" does not fall within those categories. "Congress has made clear that rules like the [Clean Water] Rule must be reviewed first in federal district court[,]" the Court concluded. While NAM resolved the threshold question of which courts can hear challenges to the Clean Water Rule, it did not address the merits of the challenges themselves. Merits challenges soon resumed at the district court level after the 2018 NAM decision. In the interim, while the jurisdictional issue was being litigated, the legal landscape had changed as a result of the Trump Administration's shift in United States' policy toward the jurisdictional reach of the Clean Water Act. The Trump Administration opposes the Clean Water Rule, and it is in the process of attempting to rescind the rule and replace it with new regulations elaborating on the meaning of "waters of the United States." Less than two months after taking office, President Trump issued Executive Order 13778 directing EPA and the Corps to revise or rescind the Clean Water Rule. The executive order instructs the agencies to review the Clean Water Rule for consistency with the Administration's policy to "ensure that the Nation's navigable waters should be kept free from pollution, while at the same time promoting economic growth, minimizing regulatory uncertainty, and showing due regard for the role of the Congress and the States under the Constitution." The executive order also provides that EPA and the Corps "shall consider" interpreting the jurisdictional reach of the Clean Water Act in a manner consistent with Justice Scalia's plurality opinion in Rapanos . EPA and the Corps intend to carry out Executive Order 13778 through a two-step process. First, they proposed to issue regulations that rescind the Clean Water Rule and recodify the definition of "waters of the United States" that was in place before the agencies issued that rule in 2015. Second, they proposed to engage in a separate rulemaking process to develop new regulations that will define the jurisdictional reach of the Clean Water Act. In July 2017, EPA and the Corps provided notice and sought comment on a proposed rule (Step One Proposal) rescinding the Clean Water Rule and replacing it with the same text that existed before the Clean Water Rule was promulgated. In 2018, the agencies issued a supplemental notice expanding on their legal rationale for repealing the Clean Water Rule and clarifying that the Step One Proposal is intended to rescind permanently the Clean Water Rule in its entirety. According to the supplemental notice, a full repeal is necessary because the Clean Water Rule exceeded the agencies' statutory authority by adopting an interpretation of Justice Kennedy's Rapanos opinion that was inconsistent with the Clean Water Act and the opinion itself. The agencies also argued that the complex legal landscape created by litigation surrounding the Clean Water Rule has undermined the Clean Water Rule's goal of providing greater clarity regarding the scope of "waters of the United States." The public comment period for the proposed repeal closed on August 13, 2018. In December 2018, EPA and the Corps unveiled a second proposed rule (Step Two Proposal) that would complete the second step of the repeal and revise process by creating new regulations that substantively redefine "waters of the United States." According to EPA and the Corps, the Step Two Proposal is intended to provide "predictability and consistency by increasing clarity as to the scope of 'waters of the United States' federally regulated" under the Clean Water Act. The agencies also intend the Step Two Proposal to "clearly implement" the Clean Water of Act's objectives of restoring and maintaining the quality of the nation's waters while respecting state and tribal authority over land and resources. The Step Two Proposal would define "waters of the United States" to include six categories of waterbodies. The Step Two Proposal would mark a significant change from post- Rapanos interpretations of "waters of the United States" because it would eliminate the case-by-case "significant nexus" evaluation that has been part of EPA and the Corps' guidance and regulations since 2007. According to the agencies, improvements to the definitions of "adjacent wetland" and "tributary" in the Step Two Proposal would eliminate the need for case-specific significant nexus tests. Under the Clean Water Rule, a wetland is adjacent to jurisdictional waters (and therefore subject to Clean Water Act regulation itself) if, among other potential criteria, it meets certain distance requirements from the ordinary high water mark of other jurisdictional waters. The Step Two Proposal would largely eliminate the distance evaluation and define "adjacent wetlands" as those wetlands that "abut" ( i.e. , touch) or have a "direct hydrological surface connection with" other jurisdictional waters. Tributaries under the Step Two Proposal must contribute flow to traditionally navigable waters through other jurisdictional waters or non-jurisdictional waters that convey downstream perennial or intermittent flows. Under the Clean Water Rule, by contrast, a tributary is any water that contributes flow to jurisdictional waters that have a bed, bank, and ordinary high water mark. In addition to the two-step repeal and replace plan, the Trump Administration has engaged in a third rulemaking process designed to suspend the Clean Water Rule until February 2020. While the Clean Water Rule states that it is effective as of August 28, 2015, EPA and the Corps published a separate final rule (Applicability Date Rule), which adds a new "applicability date" of February 6, 2020, to the Clean Water Rule. The Trump Administration's impetus for the Applicability Date Rule is derived, in part, from the Supreme Court's NAM v. Department of Defense decision. Prior to NAM , the Sixth Circuit's nationwide stay of the Clean Water Rule prevented EPA and the Corps from applying the Clean Water Rule anywhere in the United States. But after NAM concluded that challenges to the rule must begin in federal district courts, the Sixth Circuit dismissed the consolidated appellate-level challenges and vacated its stay. With no nationwide stay in place and with the step-one repeal rule still in proposed form, the Clean Water Rule could have reverted into effect in states that were not subject to a district court injunction. Seeking to prevent reactivation of the Clean Water Rule in some parts of the country, EPA and the Corps promulgated the Applicability Date Rule in an effort to suspend the Clean Water Rule while the agencies undertake the two-step repeal and revise process. Like many prior rules related to the definition of "waters of the United States," litigants challenged the Applicability Date Rule in federal courts. In late 2018, two federal district courts determined that EPA and the Corps did not comply with administrative rulemaking requirements in promulgating the Applicability Date Rule. By declining to consider comments on the substantive merits of the pre-Clean Water Rule regulations, the agencies deprived the public of a "meaningful opportunity" to comment on the Applicability Date Rule in violation of the Administrative Procedure Act, the courts held. Both courts issued orders vacating the Applicability Date Rule nationwide. As a consequence, there currently is no instrument (either a final rule or court order) that bars application of the Clean Water Rule on a nationwide basis. The multitude of legal challenges related to "waters of the United States" has created a complex legal landscape for the 116th Congress. Because both rules in the Trump Administration's rescind-and-replace process are still in proposed form, the Obama Administration's Clean Water Rule remains the current regulation defining waters of the United States. However, post- NAM challenges to the Clean Water Rule have proceeded at the U.S. district court level, and three federal district courts have entered preliminary injunctions barring application of the Clean Water Rule during the pendency of the suits. At the same time, these district courts have limited the scope of their injunction to the specific states that brought legal challenges to the Clean Water Rule. The ultimate result is that the Clean Water Rule currently is enjoined in 28 states, but it is the current enforceable regulation in 22 states, the District of Columbia, and U.S. territories. While finalization of the Trump Administration's Step One and Step Two Proposals could bring greater uniformity to this fragmented legal landscape, those rules are also likely to engender new litigation. The focus of future lawsuits, if filed, is likely to depend on the rulemaking process and content of the final rules. But observers expect critics to challenge whether EPA and the Corps considered sufficient scientific data and provided an adequate rationale to depart from prior agency guidance and regulations that utilized Justice Kennedy's "significant nexus" test. While critics of that test argue that it is too unpredictable for the average landowner to determine whether a waterbody is part of the "waters of the United States," opponents of the Trump Administration's policy contend that the Step Two Proposal would also introduce new technical definitions that ordinary landowners would not be able to implement without hiring a specialist. Because the "waters of the United States" debate hinges on the meaning of a statutory term, Congress could enact legislation that seeks to define the jurisdictional reach of the Clean Water Act more clearly. Some Members of the 115th Congress introduced legislation that would have amended the Clean Water Act by providing a narrower definition of "waters of the United States." Other legislation introduced in the 115th Congress would have repealed the Clean Water Rule or allowed EPA and the Corps to repeal the Clean Water Rule without regard to the requirements of the Administrative Procedure Act. While none of the proposed legislation in the 115th Congress was enacted, at least one bill introduced in the 116th Congress proposes to repeal the Clean Water Rule and narrow the Clean Water Act's definition of jurisdictional waters. The debate over the jurisdictional reach of the Clean Water Act implicates complex and overlapping concerns of environmental protection, statutory interpretation, federalism, and constitutional law. While judicial interpretations of "waters of the United States" generally have followed broader trends in understanding of the scope of the Commerce Clause, the Supreme Court's inability to identify a unified rationale in Rapanos has caused significant confusion and debate over the outer reaches of the Clean Water Act in the following years. Both the Obama Administration (in the Clean Water Rule) and the Trump Administration (in its rescind and revise process) have sought to provide clarity by promulgating new definitions of "waters of the United States" in EPA and the Corps' regulations. But both Administrations' efforts have faced criticism and legal challenges from certain stakeholders, creating a fragmented legal landscape for the 116th Congress in which "waters of the United States" means different things in different parts of the nation. Because the "waters of the United States" debate hinges on the meaning of a statutory term, Congress could provide greater clarity and uniformity by amending the Clean Water Act to define its jurisdictional scope more clearly, but legislative proposals thus far have not been enacted. 
For more than forty-five years, all three branches of government have struggled with how to interpret the meaning of "waters of the United States" in the Clean Water Act. In a shift from early water pollution legislation, the 1972 amendments to the Federal Water Pollution Control Act, which came to be known as the Clean Water Act, eliminated the requirement that federally regulated waters must be capable of being used by vessels in interstate commerce. Rather than use traditional navigability tests, the 1972 amendments redefined "navigable waters" for purposes of the Clean Water Act's jurisdiction to include "the waters of the United States, including the territorial seas." Disputes over the proper meaning of that phrase have been ongoing since that change. Federal authority to regulate waters within the United States primarily derives from the Commerce Clause, and accordingly, federal laws and regulations concerning waters of the United States cannot cover matters which exceed that constitutional source of authority. During the first two decades after the passage of the Clean Water Act, courts generally interpreted the act as having a wide jurisdictional reach. In recent decades, however, the Supreme Court has emphasized that "the grant of authority to Congress under the Commerce Clause, though broad, is not unlimited." This modern Commerce Clause jurisprudence has informed federal courts' approach to interpreting which "waters" are subject to the Clean Water Act. At the same time, the Supreme Court has not always provided clear rules for determining whether a particular waterbody is a water of the United States. In its most recent case on the issue, Rapanos v. United States, the High Court issued a fractured 4-1-4 decision with no majority opinion providing a rationale for how to evaluate jurisdictional disputes. Some courts and commentators disagree on how the scope of federal jurisdictional waters changed over time as a result of interpretative approaches taken by the agencies responsible for administering the Clean Water Act—the Environmental Protection Agency (EPA) and the U.S. Army Corps of Engineers (Corps). This debate resurfaced during the Obama Administration when the Corps and EPA issued a rule, known as the Clean Water Rule, which substantially redefined "waters of the United States" in the agencies' regulations for the first time in more than two decades. While some argued that the Clean Water Rule constituted a large-scale expansion of federal jurisdiction, others asserted that the agencies construed the term in a narrower fashion than in prior regulations. A vocal critic of the Clean Water Rule, President Trump shifted the executive branch's policy toward the meaning of "waters of the United States." In February 2017, President Trump issued an executive order directing EPA and the Corps to review and revise or rescind the Clean Water Rule. The agencies currently are in the process of carrying out the executive order, and they unveiled proposed regulations redefining "waters of the United States" in December 2018. As in nearly all prior attempts to define this phrase, observers disagree on whether the latest proposed definition correctly calibrates the scope of federal jurisdiction to regulate water pollution.