Chasing Alexander Supertramp

Eva Holland: Alaska's Teklanika River runs fast and cold. One chilly, drizzly day in early September, I stood on its eastern bank and watched two young hikers strip down to boxer shorts and sneakers, stuff their clothes into drybags and then into backpacks, and attempt to cross. Three more hikers stoked a small fire a few feet away, in case their friends fell in and needed to warm up fast; the plan was for two members of the group to tackle the river first, with the other three following if the first pair succeeded.

Scott Wilkerson settled his pack on his back. Minutes earlier, when the 22-year-old had agreed to go first, he'd joked, "I volunteer as tribute!" Now he turned to me and…