The Henrys Fork comes out of the ground rather than down from it — Big Springs near Island Park pours a hundred-odd million gallons a day of constant cold water straight out of the porous volcanic rock, and the river is essentially full-grown from its first mile. It runs across the Island Park caldera as the most famous spring creek in America: the Railroad Ranch water at Harriman is a wide, smooth, weed-rich glide over volcanic sand and gravel, flat as a tabletop and clear as gin, where the trout sip tiny mayflies and refuse anything that drags a millimeter. Above and below it shows other faces — the white violence of Box Canyon, the falls at Mesa and Sheep Falls. The geology is all rhyolite and ash, and the spring flow keeps it even-tempered. Wading the Ranch is easy; the humility is the hard part.

Wading: Soft weed beds and exposure on glassy flats