an important note: the afore mentioned rivers are not "naturally navigable". at least not in all seasons for all types of cargo. there is a series of locks and dams built to allow heavy freight traffic to get much further up river than would otherwise be possible, or only possible by portage of cargo from one basin to the next. there is also a channelization accomplished by dredging, jetty construction, and levy construction that keeps the shipping lanes open. this is all largely managed by army corps of engineers. so whatever you think of these inland waterways, they are currently propped up by fiat. it may well be that they could be transitioned to honest funding sources, tolls for example. also, many of these rivers represent borders between states, eg Missouri and Illinois. this is not currently a problem as fedgov redistributes from all to pay the bills, but if Missouri and Illinois were no longer in the same political union, it could get confusing and dangerous really fast.