International Rivers is fighting to preserve biodiversity against the large companies that want to dam this river:

The Pascua River, in Chilean Patagonia, is one of the most pristine and unknown regions on the planet. Why? For one, it is extremely difficult to even get there. Secondly, once you actually see the river, doing anything other than standing with your mouth open and hands over your ears is virtually impossible. This is a rip-roaring, roller-coaster of a river with rugged, impassable canyons and unsurvivable Class 6+ whitewater.