The Potomac River is the major river that passes through the Capital Wasteland.
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, with mirelurk hunters and mirelurk kings found at higher levels.
- If you obtain the Broken Steel add-on and continue the story line after starting the purifier in Jefferson Memorial, "purified" water will start gushing out of the pipes coming from the Jefferson Memorial. Contrary to the bottling of Aqua Pura and to reports all across the wasteland of free purified water from the river, the Potomac River is still irradiated, except the Tidal Basin in front of the Jefferson Memorial (except for some PS3 users, where even the Tidal Basin is still irradiated due to a bug). As Dr. Li told the Lone Wanderer, Project Purity was never intended to purify all of the river, just the Tidal Basin, though the ending slides suggestively contradict this information.
- The river is murky, so low-perception players may not notice mirelurks, who may look like rocks.
- It is possible to bypass a number of main quests by swimming the river to the Jefferson Memorial.