The rafters hear it before they see it. The roar of crashing water grows louder as the boat rounds the bend and plunges headfirst into a Class III+ rapid. But the group atop the inflated vessel keeps its cool, paddling in unison as it both fights and follows the churning water. During moments like these, Sierra Whitewater's guides help rafters conquer the American River's natural roller coasters. Their trips launch from the Sierra foothills and wind down miles of California's terrain, encountering Class II, III, and III+ rapids christened with names such as Satan's Cesspool, Fowler's Rock, Son of Satan, and Son of Satan's Ungrateful, Middle-Aged Stepdaughter.