The Grand River Ride

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The first six people known to have successfully traveled the 446-km stretch of the Colorado River within the Grand Canyon were led in 1869 by a one-armed U.S. Civil War veteran named John Wesley Powell. Braving the huge waves and submerged rocks in little wooden boats, they barely survived. These days, most boats are made of sturdy rubber, the guides are experienced oarsmen, and tourists pay for the thrill. The river averages a chilly 5.6C, and July's rainy season churns up sediment that turns the Colorado chocolate brown.

But the water's perils and the sun's searing...