The Chimehuín is where Argentine fly fishing effectively began — spring-fed clarity flowing off Lago Huechulafquen, sliding past Junín de los Andes through willow-lined banks and open pampa. The stretch around the Boca, where the river leaves the lake, has drawn anglers for the better part of a century chasing browns that cruise the current sipping duns off a flat, glassy surface. Downstream, the water gathers pace over gravel and volcanic cobble before slowing again toward the Collón Curá confluence. Fish here respond to careful, downstream presentations more than showy casting — a size 16 Parachute Adams or Elk Hair Caddis fished long and drag-free will out-fish anything flashier. Regulations bite hard: Zone I is fly-only water, and the reach down to the Collón Curá is strict catch-and-release on barbless singles, so every fish deserves the respect the river's reputation demands.

• Volcanic glacial