Zipping across Grand Lake in an outboard motorboat on a warm day in late September, Mike Cassio eases off the throttle as he nears a break in the shoreline and points out one of two tributaries that feed Colorado’s largest and deepest natural body of water. This one is called the North Inlet. It winds about a dozen miles northeast into Rocky Mountain National Park. “If you go up here,” Cassio says, “this river is pristine.