THE SUN BURNS a lot harsher at 9,000 feet, it turns out. And after running 8 wheezing miles up the side of Mount Sopris in Carbondale, Colo., I yearned to cool my sizzling skin. Dragging my feet back down the trail, I spotted a teasing patch of bright blue peeking through a stand of Aspen trees.
The trail kicked me out onto a narrow dirt road, which wound past empty campsites with ashes still smoking from the morning’s fires, and around a corner to the great reveal: Dinkle Lake, a local swimming hole with water that proved...