The details are confusing—especially since I haven't actually read the opinions—but this morning the Supreme Court issued a ruling on the difference between "navigable waters" and unnavigable wetlands:
A plurality of the Supreme Court concluded on Monday that the Clean Water Act's protection of "waters of the United States" is limited to those bodies of water that are "permanent, standing or continously flowing," and thus does not embrace channels through which water flows only some of the time. And, the Court added, "navigable waters" under the Act ordinarily is no broader than U.S. waters. The decision appeared to rule out protection against filling-in or pollution of wetlands not part of actual waterways. The actual impact of the plurality opinion by Justice Antonin Scalia appears to have been qualified somewhat by a lengthy concurrence by Justice Anthony M. Kennedy, who supplied a fifth vote for the result. After Scalia announced his opinion, Kennedy discussed his separate views.
The ruling came in the consolidated cases of Rapanos v. U.S. and Carabell v. Army Corps of Engineers. The Reason Foundation's Shikha Dalmia looked at Rapanos last October.