Cutting through the green, snowcapped Andes in southern Chile like a satin ribbon, the “Fu” is nirvana for paddlers. Along with mind-bending Class V rapids, the river has two unique features: On its 120-mile, 8,000-foot descent to the Pacific, the Fu’s melt-water stops in several lakes, which simultaneously warm it – at 61 degrees, it is considerably more temperate than most glacially fed rivers – and filter out almost all the silt.