Don't underestimate the quality of photos that a disposable camera can take. They are remarkably inexpensive, lightweight, and worry-free and take excellent photographs. I suggest taking your regular camera, plus one of the waterproof 'disposables' made by Kodak or Fuji. This way, you can take photographs while on the river safari or near the waterfall without worrying about destroying your more expensive camera. Carrying around a couple rolls too many is not that big of a deal.
Film can be very expensive in developed and less-developed countries.
If it's your first trip and you like to take pictures, six rolls will suffice for a two month travel adventure. Of course, some things just beg for a photo, like the Leaning Tower of Pisa, but try to keep these shots to a minimum. If your are traveling alone, this may be one of your few options. But be careful who you ask. If the person looks like a faster runner than you, don't bother.