The Clean Water Act was amended in in 1977 to add the phrase "adjacent wetlands" specifically because of contention on this point. This was after the Army Corps of Engineers and the EPA had fought and settled into an agreement on definitions and policy stemming the previous version and that understanding was known and referenced by congress during argument. The congressional research document which traces the history of this has already been linked on this post but I'll reiterate it here: https://crsreports.congress.gov/product/pdf/R/R44585

If you want to jump straight to the punchline, this is from the section on United States v. Riverside Bayview Homes, Inc in which the Supreme Court made clear that they were aware of this context and informed their ruling taking it into account: 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 statute107—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.108 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.109