The Potomac is a river in the south-east of the United States, approximately 450 km in length. The river springs in Maryland and flows into the Chesapeake Bay. The river forms part of the borders between Maryland and West Virginia and Virginia and the District of Columbia. The entire river is considered part of the state of Maryland, which has playing an interesting part throughout history.
Here's a picture of the upper part of the Potomac River. The river widens as it nears the Chesapeake Bay, and in the vicinity of Coles Point, Virginia, is between five and seven miles wide.