THE RIVER had us trapped in a rocky eye where it lunged and growled around the stern of our boat. I stood up on a slippery rock, breathing as if I’d just run a record marathon, and looked at the Old Guide. He sat on another rock, his shoulders bowed, as exhausted as I was. The first two days had been rough enough, but today we’d taken our worst beating. “You can’t convince me,” I said, “that you or anyone else ever floated through this canyon.” …