The St. Johns River is born in marshes, then winds northward for 310 languid miles before flowing through Jacksonville. Florida's greatest waterway, a haven for manatees and mecca for bass fishing, is under siege by a burgeoning human population. Its challenges are many, but there is great hope. This is the story of a river returning. ​ Flying inflatable boats, cave diving, kayaking and a storm-drain spelunking expedition up the great St. Johns River bring us closer to understanding the environmental challenges facing America's waterways.