 You don't know what move you're going to make until you're in the situation because the level of the river can go up and down. A set of boulders that are exposed that you can see to go around could be just now under the water as the water has risen. And so you don't even see to go around them, but they're just three inches under the water. And the draft of your kayak is deeper than three inches. So when you go over it, it flips you.