In 2006, when the court last took up this issue, no majority was able to agree on how to define “waters of the United States.” Writing for a plurality of four justices in U.S. v. Rapanos, Justice Antonin Scalia defined the term narrowly to include only relatively permanent, standing or continuously flowing bodies of water such as streams, oceans, rivers and lakes. Waters of the U.S., he contended, should not include “ordinarily dry channels through which water occasionally or intermittently flows.”

Acknowledging that wetlands present a tricky line-drawing problem, Scalia proposed that the Clean Water Act should reach “only those wetlands with a continuous surface connection to bodies that are waters of the United States in their own right.”

In a concurring opinion, Justice Anthony Kennedy took a very different approach. “Waters of the U.S.,” he wrote, should be interpreted in light of the Clean Water Act’s objective of “restoring and maintaining the chemical, physical, and biological integrity of the Nation’s waters.”

Accordingly, Kennedy argued, the Clean Water Act should cover wetlands that have a “significant nexus” with navigable waters – “if the wetlands, either alone or in combination with similarly situated lands in the region, significantly affect the chemical, physical, and biological integrity of other covered waters more readily understood as ‘navigable.’”

Neither Scalia’s nor Kennedy’s opinion attracted a majority, so lower courts were left to sort out which approach to follow. Most applied Kennedy’s significant nexus standard, while a few held that the Clean Water Act applies if either Kennedy’s standard or Scalia’s is satisfied.

Regulators have also struggled with this question. The Obama administration incorporated Kennedy’s “significant nexus” approach into a 2015 rule that followed an extensive rulemaking process and a comprehensive peer-reviewed scientific assessment. The Trump administration then replaced the 2015 rule with a rule of its own that largely adopted the Scalia approach.

The Biden administration responded with its own rule defining waters of the United States in terms of the presence of either a significant nexus or continuous surface connection. However, this rule was promptly embroiled in litigation and will require reconsideration in light of Sackett v. EPA.

The Sackett decision adopts Scalia’s approach from the 2006 Rapanos case. Writing for a five-justice majority, Justice Samuel Alito declared that “waters of the United States” includes only relatively permanent, standing or continuously flowing bodies of water, such as streams, oceans, rivers, lakes – and wetlands that have a continuous surface connection with and are indistinguishably part of such water bodies.

None of the nine justices adopted Kennedy’s 2006 “significant nexus” standard. However, Justice Brett Kavanaugh and the three liberal justices disagreed with the majority’s “continuous surface connection” test. That test, Kavanaugh wrote in a concurrence, is inconsistent with the text of the Clean Water Act, which extends coverage to “adjacent” wetlands – including those that are near or close to larger water bodies.

“Natural barriers such as berms and dunes do not block all water flow and are in fact evidence of a regular connection between a water and a wetland,” Kavanaugh explained. “By narrowing the Act’s coverage of wetlands to only adjoining wetlands, the Court’s new test will leave some long-regulated adjacent wetlands no longer covered by the Clean Water Act, with significant repercussions for water quality and flood control throughout the United States.”

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