Petrohué begins as the outflow of Lago Todos los Santos and immediately turns into something extraordinary — a broad, glacial-turquoise river sliding past the black volcanic cone of Osorno, inside Vicente Pérez Rosales National Park. The colour comes from rock flour ground fine by ice; the water stays cold and heavy through the season, which suits streamers and sculpin patterns fished deep along seams and boulder pockets more than delicate dry-fly work. Brown and rainbow trout hold in the softer margins, and chinook salmon run through in season, adding a different kind of weight to the take. It's a river that rewards a boat and a guide who knows which channel is fishing that week — the volume and colour change fast, and a wrong read costs you a day.

• Volcanic glacial