The Great Falls of the Potomac River are located upstream (to the northwest) of which US city?

Question

The capital cities of two neighbouring countries in the southern hemisphere are located on the opposite sides of the mouth of which river?

Answer 1

West Virginia, Maryland, Virginia, Washington, D.C.

The Potomac River is located along the mid-Atlantic coast of the United States and flows into the Chesapeake Bay.
Potomac River is 4th largest river along the Atlantic coast of the United States and the 21st largest in the United States.