This is the route, with some extrapolations, of the book
River Horse by William Least Heat-Moon: across the United States by as much river, lake and stream as possible.
Related Links:
Try using the tour feature of Google Earth to get a feel for floating down the rivers. (select a Route, then F10)
Note how many "Lewis and Clark" bridges and other references there are along the route!
Route
Missouri-Yellowstone Confluence
Salmon-Snake Confluence