I made this website that shows the drainage area for any point in the lower 48. You can also get an elevation graph which is useful to boaters as you can see where the steep sections in a river are. Have a look, let me know what you think. It’s not quite public yet and I would like some feedback on usability and additional features.
Disclaimer: The elevation graph tries to do a good job but it’s not perfect as the underlying GPS coordinates and the elevation data only has a certain resolution.