When John Wesley Powell became the first person to navigate the entire Colorado River, through the Grand Canyon, he completed what Lewis and Clark had begun nearly 70 years earlier--the final exploration of continental America. The son of an abolitionist preacher, a Civil War hero (who lost an arm at Shiloh), and a passionate naturalist and geologist, in 1869 Powell tackled the vast and dangerous gorge carved by the Colorado River and known today...