Our pilot, David Kunkel, asked me to retrieve his oxygen bottle from under my seat, and when I handed it to him he gripped the plastic breathing tube with his teeth and opened the valve. We had taken off from Boulder not long before and were flying over Rocky Mountain National Park, 30 miles to the northwest. “People don’t usually think altitude is affecting them,” he said. “But if you ask them to count backward from a hundred by sevens they have trouble.” What struck me at that moment was not how high we were but how low: As Kunkel banked steeply to the right to give us a better view of a stream at the bottom of a narrow valley, his wingtip appeared to pass just feet from the jagged declivity beneath us. The other passenger, sitting in the copilot’s seat and leaning out the window with a big camera, was Jennifer Pitt, the director of the 蜜柚APP’s Colorado River Program. Pitt is in her forties. She has long brown hair, which she had pulled back into...