The Potomac River () drains the Mid-Atlantic United States, flowing from the Potomac Highlands into Chesapeake Bay. It is 405 miles (652 km) long,[4] with a drainage area of 14,700 square miles (38,000 km2),[5] and is the fourth-largest river along the East Coast of the United States and the 21st-largest in the United States. Over 5 million people live within its watershed. There is a hydroelectric plant along the Potomac river constructed in 2014 by Dominion.
The river forms part of the borders between Maryland and Washington, D.C. on the left descending bank and between West Virginia and Virginia on the right descending bank. 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.