Waterfalls are a problem going DOWN river.
If they were going UP river (against the current) how could they go over a waterfall?
Waterfalls only happen when water is going downhill.
Didn't anyone (actors, crew, producer, director) say "Waita minute!".
Terry Thomas
Actor and Film Unit Stills Photographer
Atlanta, Georgia USA
http://TerryThomasPhotos.GooglePages.com