As we headed down the shoot of one of the largest and most toilsome rapids in North America, we knew we were in trouble. Our guide barked orders from behind as if we had met our match. We could all hear a distinct change in his voice half way through the rapid, and a few of us sensed something wasn’t right. Just then, I saw it, a boulder that dwarfed our raft, and we were hopelessly on a collision course with it. Everyone in the raft was paddling with every bit of force they could muster, but the current was too overwhelming. The tail ....