The Andes Mountains rise to heights exceeded only by the Himalayas. Here, 5,000-foot canyons seem to dot the countryside like footprints in a sandbox. At the bottom of nearly all of those canyons is a wild, primeval, tempestuous river—a whitewater fanatic's paradise. Peru, with its combination of native cultures and remote rivers feeding into the Amazon rainforest, is ground zero for exploratory raft trips, where you encounter wildlife—parrots, giant river otters, condors—that is much less surprised to see you than the indigenous locals are (you wave, they stare). Eventually the rafts round the bend and drop in on another set of Class IV rapids. It's all very civilized, in a Gods Must Be Crazy sort of way. Global Descents offers multi-day trips on the (relatively) easily accessed but still remote Rio Apurimac, near Cusco ($3,500, including an Inca Trail trek; globaldescents.com). Earth River Expeditions has a new trip this year exploring the practically undiscovered Rio Yavero, where Peru's highlands transform into Amazonian rainforest in a less-than-100-mile stretch ($3,300; earthriver.com). South America's most renowned whitewater classic, though, is Chile's Futaleufú. The Fu, as it is known, has the unique trifecta of being impossibly clear, massively powerful, and refreshingly brisk. While plenty of companies lead trips on the Fu, only Earth River offers multi-day outings that involve wooden hot tubs, five-star meals, and a night spent above the forest floor in a handcrafted treehouse ($3,300; earthriver.com).