Copyright 2017 NPR. To see more, visit ARI SHAPIRO, HOST: President Trump has ordered a review of the Waters of the U.S. rule. It's an Obama-era regulation that says which waters the federal government can protect. As NPR's Nathan Rott reports, undoing the rule would be a long and difficult process. NATHAN ROTT, BYLINE: All federal protection over water goes back decades to the Clean Water Act. It says the federal government has jurisdiction over so-called navigable waters. Think big enough for a boat. But Jan Goldman-Carter, an attorney for the National Wildlife Federation who used to work for the EPA, says it was always meant to be broader. JAN GOLDMAN-CARTER: We cannot protect the downstream, float-a-boat Mississippi River unless we protect all the little streams and wetlands that feed into that river. ROTT: Which little streams and wetlands, though, was always unclear, and court decisions made it muddier. The Obama administration tried to clarify it with the Waters of the U.S. rule