Just because it's a federal law doesn't mean the state has it wrong. I haven't read the law, and, honestly, don't know what the definition of navagible waters is, or who is charged with deciding with deciding what navagible water is, but I doubt that all flowing water is considered navagible water. And it could be up to a local governmental body to determine what water is navagible. If somebody wants to post a link to the relevant statute/code provisions, I'd love to take a look at it (too lazy/busy to hunt it down myself).