We were camped in a narrow corridor of black cliffs that actually felt like a box, but it hardly felt claustrophobic. A whisper of breeze carried the rich, muddy scent of the river. Wispy clouds turned orange over a narrow slice of sky as daylight slowly filled the atmosphere. A cacophony of birdsong emanated from a curtain of greenery behind camp, building in intensity along with the coming light. It all felt very tropical.

Three of us — Melanie Bell, a university biostatistics professor; her husband, photographer Bill Hatcher; and I — had convened at the Gila River without any visions of a tropical getaway. This was a desert river, and that title in itself was sort of an oxymoron, so we didn’t know what to expect. I hadn’t floated the Gila in decades, and that was on the receding waters of a flood, when the restless river roiled across its wide bed, filling the canyon bottom, covering the bushes and some of the trees, swiftly ushering me through a wide desert landscape — or so I recalled. This time was bound to be different. When I left my house in Flagstaff, the Gila...