Whitewater Rafting in the Grand Canyon Anyone?

I've heard stories about Grand Canyon expeditions that include whitewater rafting.  Has anyone gone on such an adventure?

Apparently, tourists and thrill-seekers ride atop donkeys through the Canyon to campsites and spaces for whitewater rafting.  That sounds like so much fun.  I'd love to try it, especially if I didn't have to descend the Canyon on foot.