Inside Los Alerces National Park, the Rivadavia is short, achingly clear water connecting Lago Rivadavia to the park's wider lake system — the kind of river where you can watch a trout inspect a fly for several seconds before deciding, which is either maddening or the whole appeal depending on your temperament. Slow, deep glides sit between faster connecting runs, and small CDC emergers or a Foam Ant fished with real stealth take far more fish than anything cast carelessly. Strict National Park rules apply throughout, with access and ecological protections — biosecurity checks on waders and gear are taken seriously here to guard against didymo and other invasive species. It's postcard water, genuinely one of the most beautiful short rivers in the region, and worth building an unhurried morning around rather than trying to fish it quickly between other stops.

• Volcanic glacial