The Malleo carries a reputation among Patagonian dry-fly rivers: a small, technical stream running out of the Lanín volcano's shadow, gin-clear over pale gravel, where trout see everything and forgive nothing. Runs and pocket water stay narrow and readable, ideal for careful downstream drifts with an Elk Hair Caddis or small Stimulator worked tight to undercut banks and boulder seams. Within Lanín National Park, the upper river above the Route 23 bridge is fly-only, barbless-hook, catch-and-release throughout — every fish goes back to spawn again in this cold, snowmelt-fed water. Access leans toward lodges and local guides who know which bend holds a selective riser and which one just looks like it should. Expect small, precise flies, long leaders, and trout that punish sloppy presentation more than they reward big patterns.

• Volcanic glacial