The river that flows through Washington, D.C. is the Potomac River. This river runs from the Allegheny Mountains in West Virginia and provides a natural border between Maryland and Virginia. It plays a vital role in the region's ecology and has historical significance, being a key site during the American Revolutionary War and the Civil War. The Potomac is popular for recreational activities such as kayaking and fishing, and its banks are lined with various monuments and memorials, enhancing its cultural importance.