It wasn't until recently that I had the opportunity to get a real experience in some fast moving water. Being a surfer, I feel proud to have overcome my fear of large waves and open ocean. However, this was a totally different animal.
First, I thought my boat was broken. I was paddling seemingly straight, but discovered that my boat would turn, then turn more, and ultimately rotate along with me before turning completely sideways. Turns out that kayaks have no rudder or any other type of controlling device. This means that unless you're actively keeping to your line, you are in danger of turning into the current and getting flipped.
Second, flipping over means you either you recover or evacuate. There's a way to explain how to flip back upright after turning over; I have not met anyone that can do that. It is basically a maneuver that you learn by practice - and I definitely don't recommend below-freezing water with chunks of ice in it that gives you ice cream headaches pretty much upon contact. I got up and out once; that was enough practice.
However, I will admit that I have a new found respect and admiration for fast moving water. It wasn't until I had the experience of floating down after my kayak clutching my paddle with my other hand, or putting my skirt on incorrectly and taking on water to become a "submarine," that I realized just how little control I really had. I believe it was only after that experience that I could truly feel the power when standing in front of a great waterfall.
So I will admit that I am intimidated looking at Nahanni.