I can still picture Katy. A petite, brown-eyed, 16-year-old Texan sitting cross-legged on a large rock on the far bank of the river, her elbows propped on her knees, her chin resting in her hands, pouting.

“Don’t worry, Katy,” I shouted over the rushing water. “It’s just walking.” She smiled wanly and waved.

It was our personal joke.

Katy Brain and I were in the heart of the Absaroka Range in northwest Wyoming, in the fourth week of a monthlong course run by the National Outdoor Leadership School, the premier outfit in wilderness education. Katy had struggled almost from the start, clinging to an unhappiness that had tried my patience at times. A few days earlier we’d been making dinner, and she was crying. I finally said to her in frustration, “Katy, we’re just walking here, okay? We get up and we walk to the next place, and then we do it again the next day. It’s just walking. And in a week it will be over.” She settled into a sullen silence for the remainder of the night. But the next evening when Katy was returning from the bear hang, I overheard her say to another student, in a disarmingly confident tone, “It’s just walking, that’s all we’re doing here. We’re just walking.” I smiled to myself and felt a paternal satisfaction.

It’s a memory I can’t shake.

Our group included 15 students and three instructors. On June 26, 1996, day 24 of our course, the class was divided into three independent, student-led units, with instructions to hike to a prearranged spot on the map and regroup with the guides in the evening for dinner. The Absaroka Range is rugged wilderness, carved out of stratified volcanic and metamorphic rock, and the planned route for the day had us hugging a drainage until we reached the confluence of the South Buffalo Fork of the Snake River, then following it for several miles to the meeting spot. It was a straightforward plan—follow the rivers and nothing could go wrong.

Katy’s group of five...