I am a whitewater guide on the Hudson River. Your guide can definitely make your trip, and a good guide can make the trip exciting regardless of the water level. Likewise, a good guide (in any activity) can significantly add to your experience.
I don't know if there should be a set rule with regards to tipping a guide. I would say to simply tip appropriately based on what you feel your guide has added to your experience, especially compared to other guides you have used or seen in action. I have seen great guides have lousy days when they were in lousy moods, and quite frankly, if I was a cutomer, I would'nt want to tip them jack. I've seen rookie guides who don't know the river well provide an exceptional experience for their group.
Simply ask yourself if your guide added significantly to your experience, and tip accordingly. Some folks who come on our trips may not be able to afford to tip as graciously as they would like, in which case a sincere thank you will do just fine.
John