In the latest chapter of a decades-long fight, the Environmental Protection Agency (EPA) on April 15 ended its period for comment on a much-anticipated new rule to define “waters of the United States”—or WOTUS, as the controversial phrase has become known.

Congress first used the phrase in 1972, when it signed much of the Clean Water Act (CWA) into law. The CWA prohibited the discharge of any pollutant into “waters of the United States” without a permit issued by the EPA or Army Corps. Congress used WOTUS to define the scope of federal water-quality regulation, but it did not define WOTUS.

Since 1972, the WOTUS definition has alternated between controversy and ambiguity. In 1977, the EPA and Army Corps adopted an expansive definition for WOTUS that included any wetlands adjacent to traditionally navigable waters. The Supreme Court considered this definition in major cases in 1985 and 2001, and rejected them in 2006 with its Rapanos v. United States decision.

Instead of rejecting and replacing the 1977 definition with a clear alternative, the Court’s Rapanos holding splintered between two camps. In one camp, led by Justice Antonin Scalia, four justices insisted that only “relatively permanent, standing, or flowing bodies of water” qualified as WOTUS. In the other, Justice Anthony Kennedy contended that WOTUS included any wetland that had a “significant nexus” to flowing, navigable waters. By the Marks rule, Justice Kennedy’s “significant-nexus” test controlled.

In 2015, under the Obama administration, the EPA and Army Corps embraced the “significant-nexus” test in new rules to define WOTUS. Almost immediately, however, the rules met resistance, and the Sixth Circuit Court of Appeals stayed their effect nationally in 2016. The Supreme Court rejected this nationwide stay on procedural grounds, but two other courts’ stays remained effective in fifteen states.

The Trump administration joined the fray in February 2017, when President Trump issued an executive order directing his agencies to consider rescinding the “significant-nexus” test. Their initial attempt to delay implementation of the “significant-nexus” test failed in the courts, but, in February 2019, the EPA and Army Corps proposed a new rule that would redefine WOTUS by Justice Scalia’s Rapanos approach. Critics of the proposed rule have claimed that it would remove federal regulation over 60% of the nation’s waterways.

Public comment for the proposed rule closed in April 2019, and the administration will soon take next steps in the ongoing fight to define WOTUS.