The Potomac. Historic, polluted, urban, right? Well, sorta. Over 400 miles long and the southern headwaters of Chesapeake Bay, the Potomac is in parts wild, heartbreakingly scenic, and also heartbreakingly industrialized and urban. When two childhood friends decided to bike and raft the length of the river they loved, they discovered a far more dangerous side of the Potomac than they imagined.