The Potomac River is located along the mid-Atlantic coast of the United States and flows into the Chesapeake Bay. The river is approximately 405 miles (652 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 United States and the 21st largest in the United States. It appears in A Fallout Tale in the ruins of Washington D.C. Its course is generally northwest to southeast, and essentially divides the Wasteland in half. Many portions of the river have dried up all the way to the river bed. To the north and east of the river is Maryland, to the south and west is Virginia, and all of Washington, D.C. is on the east side. It empties into the Tidal Basin along with the Anacostia River.
Its waters are moderately radioactive, making swimming a somewhat hazardous method of crossing (depending on the width of the portion you plan on crossing); it is best to find an intact bridge or narrow or dried up section. There are a few boats in the river both above the surface and resting on the bed of the river, these boats sometimes contain ammunition and first aid boxes. Mirelurks are often found in and around it.
In a way it is Cywren's friend and enemy. Through Project Purity it provides clean fresh water to her and Capital Wasteland resident's but it is also highly irradiated and is home to hundreds of Mirelurks, it also has entrances to tunnels and Metros filled with terrifying creatures and raiders.