Here you are on a nice float trip, something akin to Ruby Horsethief on the Colorado River stradding the Colorado/Utah border. Mellow. Tie the boats together and start drinking. Throw some sunscreen on and enjoy the heat. Put your life jacket on and take a swim. No big deal.
But then suddenly you hear it, the droll rumbling up ahead. The relaxation of your trip so far has dulled your wits, and you think that you'll just climb back in your raft and enjoy the mild excitement that awaits. The rumbling grows louder. You proceed slowly, non-shalantly rolling over the edge of the boat and enjoying the sunlight. The walls of the canyon seem to come alive with noise. Your heart, despite your mood, starts to race. The river starts to bend. You begin to trust your instinct. The smell of churned water fills the air. Panic sets in as the adrenaline injects itself into your veins. You've got to start paddling. Hard. Hard! No time to strap everything down! You've been too neglegent, and now, there's only time to make sure you get through alive! Paddling hard! Paddling!
...the fight is over. Time to assess what your laziness has cost you. You now know that, no matter how calm the river may be, you'll never lose your trust in its nature again.