If you stand at the Tidal Basin in D.C. during cherry blossom season, the Potomac looks like a massive, placid lake. It’s huge. But if you head up to the Fairfax Stone in West Virginia, you’re looking at a tiny spring where you could basically hop across the water without getting your socks wet.
So, let's get to it. How long is the Potomac River?
Most experts, including the Interstate Commission on the Potomac River Basin (ICPRB), clock it at roughly 405 miles.
That sounds simple. It isn't. Measuring a river is a bit like trying to measure a piece of wet string that’s been tossed on the floor in a pile—it depends on where you start the tape measure and how much you care about all those little squiggles and bends. If you include the North Branch, you get that 405-mile figure. If you’re talking about the main stem starting from the confluence of the North and South Branches near Green Spring, West Virginia, the number drops significantly.
The Geography of the Nation's River
The Potomac is the fourth largest river on the Atlantic coast. It drains about 14,670 square miles of land. That’s a lot of territory. We’re talking about parts of West Virginia, Maryland, Virginia, Pennsylvania, and the entirety of the District of Columbia.
It’s a hard-working river.
The "length" of the river is actually divided into two very different personalities. You have the Upper Potomac and the Tidal Potomac. The dividing line is the Fall Line at Little Falls. This is where the river drops from the hard rocks of the Piedmont plateau down to the soft sediments of the Coastal Plain.
It's a dramatic shift.
Above the falls, it’s a freshwater mountain stream that turns into a rocky, winding river. Below the falls, it becomes a massive tidal estuary. This part of the river is actually an arm of the Chesapeake Bay. The tide rises and falls all the way up to the Georgetown waterfront. Because of this, the "length" of the river isn't just a horizontal measurement; it’s a measurement of two completely different ecosystems.
Where does it actually start?
The "official" source is the Fairfax Stone. It’s a historical marker located at the corner of Grant and Tucker counties in West Virginia.
Lord Fairfax owned a massive land grant in the 18th century, and this stone marked the limit of his property. It’s a bit of a trek to get there, and honestly, the "river" there is just a trickle. But that trickle eventually provides drinking water for millions of people in the Washington metro area.
The South Branch is actually longer than the North Branch. This leads to a lot of arguments among geography nerds. If you measured the river from the source of the South Branch, the total length would technically be longer than 405 miles. But by convention, the North Branch is considered the official start for the "total length" calculation.
Why the Length Matters for History
The 405-mile stretch isn't just a stat for a trivia night. It’s the reason D.C. exists where it does. George Washington was obsessed with the Potomac. He thought it was the "Gateway to the West." He spent years trying to make the river navigable through the Patowmack Canal project.
He wanted to connect the Atlantic to the Ohio River Valley.
It didn't really work. The river is too moody. It floods. It dries up. It has jagged rocks. But the length of the river determined the trade routes of the 1800s. The C&O Canal follows the river for 184.5 miles, from Georgetown to Cumberland, Maryland.
If the Potomac were shorter or straighter, the entire economic history of the Mid-Atlantic would look different. We might not have had the same push for the B&O Railroad, which was built specifically because the river was so difficult to navigate for its entire length.
Understanding the "Potomac Low"
One of the weirdest things about the Potomac’s length and boundaries is that Maryland actually owns almost all of it.
Usually, when a river separates two states, the border is in the middle. Not here. Thanks to a charter from King Charles I in 1632, the border between Maryland and Virginia is the high-water mark on the Virginia side.
This has caused centuries of legal bickering.
Virginia watermen and Maryland authorities have fought over oyster rights and water usage since before the United States was even a country. Even today, if you’re standing on a pier in Alexandria, you’re technically in Virginia, but the water beneath you is Maryland.
The Surprising Depth vs. Length
While we’re talking about how long is the Potomac River, we should probably talk about how deep it is, because the two are related.
The river is relatively shallow for most of its 405 miles. However, near Morgantown, Maryland, there’s a spot called "The Hole." It’s over 100 feet deep.
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This contrast is wild.
You have a river that is hundreds of miles long, but in some places, it’s barely a foot deep, and in others, it could hide a ten-story building. This variation is why the Potomac is so dangerous for swimmers. The length of the river creates a massive volume of water that has to squeeze through narrow gaps like Mather Gorge at Great Falls.
When the river is high, the current there is terrifying. It’s one of the most dangerous sections of whitewater in the country.
Current State of the River
The Potomac used to be a mess. In the 1960s, it was famously called a "national disgrace." President Lyndon B. Johnson looked at the river and saw a sewage-filled eyesore.
Things changed.
The Clean Water Act did wonders. Today, the 405-mile stretch is a recreational hub. You’ll see kayakers, stand-up paddleboarders, and bass fishermen. The river is actually considered one of the best urban smallmouth bass fisheries in the United States.
It’s not perfect. Runoff from farms in the Shenandoah Valley and "forever chemicals" (PFAS) are modern problems. But compared to fifty years ago? It's a miracle.
Practical Ways to Experience the Potomac
If you want to see the river's full 405-mile personality, you can't stay in the city. You have to move.
- Visit Great Falls Park: This is the most visceral way to see the river's power. It's only 15 miles from D.C., but it feels like a different world. The water drops 76 feet in less than a mile.
- The C&O Canal Towpath: You can bike or hike 184.5 miles of the river's length on a flat, dirt path. It’s the best way to see the transition from the Piedmont to the mountains.
- Mallows Bay: Head down to the tidal section in Charles County, Maryland. You can kayak through the "Ghost Fleet"—hundreds of sunken WWI-era wooden steamships. It’s the largest shipwreck graveyard in the Western Hemisphere.
- The Fairfax Stone: If you're a real completionist, drive into the Monongahela National Forest in West Virginia to see where it all starts.
The Potomac is more than just a line on a map. It’s a 405-mile long living history book. It has been a barrier, a highway, a border, and a playground.
Whether you're looking at the mountain springs of West Virginia or the five-mile-wide mouth where it hits the Chesapeake Bay at Point Lookout, you’re looking at the literal lifeblood of the region.
If you’re planning to explore the Potomac this weekend, check the USGS water gauges first. The river's personality changes entirely based on recent rainfall in the Allegheny Mountains. A "long" river means a lot of space for rain to collect, and a calm day in D.C. can quickly turn into a flood event if it rained 200 miles upstream two days ago. Be smart, wear a life jacket if you're on the water, and respect the current.