Water pools into puddles here and there in a landscape covered with decaying leaves and peppered with bare trees. The earth sinks with each step in this seldom-trod swamp near a park in the Wicomico County hamlet of Bivalve.

Although this mucky spot lies just a few dozen yards from the Nanticoke River and is undeniably damp, it used to rest outside the jurisdiction of America's nearly 50-year-old clean water rule. That's because the area remains unconnected to the river and only brims with standing water at certain times of the year.

Some environmental advocates fear that rolling back the law could weaken efforts to restore the Chesapeake Bay. If the federal government can't protect headwaters of its tributaries from harm, then waters farther downstream, including the bay itself, will suffer, said Jay Ford, executive director of the Virginia's Eastern Shorekeeper.