“Keep navigable waters clean.” It sounds straightforward, but in practice it’s anything but. For decades, the Environmental Protection Agency has struggled with that task, after the Clean Water Act in 1972 gave them the authority to regulate “navigable waters.” But “navigable” doesn’t cover small streams and wetlands, and water in those areas has a habit of flowing downhill into other, larger bodies of water. Whether the EPA could regulate all that water before it reached someplace navigable was never clear.