well if i was near a river and I was close to dying, I would go ahead and drink. That being said, a lot of rivers contain protozoa and parasites that would only cause further dehydration once the diarrhea hits you, only causing you to die quicker and more painfully. http://en.wikipedia.org/wiki/Waterborne_diseases but if i was going to go hiking in the grand canyon you probably want to have a lot of water or a water filtration kit with you?!