At age sixty-seven, Colin Fletcher, the guru of backpacking in America, undertook a rigorous six-month raft expedition down the full length of the Colorado River—alone. He needed "something to pare the fat off my soul...to make me grateful, again, for being alive." The 1,700 miles between the Colorado's source in Wyoming and its conclusion at Mexico's Gulf of California contain some of the most spectacular vistas on earth, and Fletcher...