You’re “asleep” at the oars, enjoying the scenery or focusing on that big rapid coming up, when suddenly you find your raft on a rock you didn’t see under the water. The water wells up over it, then drops off the other side, curling back on itself in what may be a big, powerful wave. It’s a guessing game. Dragging your boat on the rock can twist the raft, rip it, or make it difficult for you to hit the wave below, leading to trouble.