Rapanos v. United States, 547 US 715, that federal authority to regulate activities impacting on wetlands, under the 1972 Clean Water Act, is limited to those wetlands that have a "significant nexus" to navigable waters, and it defined "navigable" far more narrowly than past court decisions. The ruling did not specify or suggest what constitutes a "significant nexus." Justice Kennedy, concurring in the ruling, invited the US Army Corps of Engineers (the federal agency with authority for wetlands permitting under the Clean Water Act) to promulgate a new rule specifying what might constitute a "significant nexus."
This morning's New York Times has an article about the unintended consequences of the Court's ruling in Rapanos. According to that article, the Rapanos ruling, by restricting what counts as a "navigable" water, may have put as many as 45% of major polluters outside the reach of the Clean Water Act.
Senate Democrats have introduced legislation that would, in effect, reverse Rapanos and extend the reach of federal regulatory authority under the Clean Water Act to its previous extent. Section 3(8)(C) of the proposed Clean Water Restoration Act would define "waters of the United States" to include virtually all waters, including purely intrastate lakes, ponds, potholes, and mudflats. Already, an impressive array of lobbies has come out against the bill, which has been voted out of the Committee on Environment and Public Works. The coming legislative fight to overturn Rapanos will probably be the most significant one on any environmental issue, other than climate change, likely to come before the current Congress.