The Filo Hua Hum runs close to the Chilean frontier, a glacial-fed freestone river carrying volcanic gravel down from high country into the Lácar drainage — remote enough that it sees a fraction of the traffic its more famous neighbours around Junín and San Martín de los Andes attract. The character is classic northern Patagonian freestone: broken runs, boulder pockets and cold, oxygenated water holding browns and rainbows that haven't seen a fly all week. A Parachute Adams or Elk Hair Caddis fished tight to structure works as well here as anywhere in the region, and a dry-dropper rig covers the deeper slots efficiently. Local knowledge of access points and the best beats is worth arranging in advance — this isn't a river you stumble onto from the road, and a guide who fishes it regularly will save a day of blind wading.

• Volcanic glacial