The Potomac River flows into the Chesapeake Bay, located along the mid-Atlantic coast of the United States. The river is approximately 413 statute miles (665 km) long, with a drainage area of about 14,700 square miles (38,000 km²). In terms of area, this makes the Potomac River the fourth largest river along the Atlantic coast of the USA and the 21st largest in the USA.
The river forms part of the borders between Maryland and Washington, DC on the left bank and West Virginia and Virginia on the river's right bank. The entire lower Potomac River is considered part of Maryland, with the exception of a small tidal portion within the District of Columbia. Except for a small portion of its headwaters in West Virginia, the North Branch Potomac River is considered part of Maryland to the low water mark on the opposite bank. The South Branch Potomac River lies completely within the state of West Virginia except for its headwaters, which lie in Virginia.