From Keyser to Frederick, it's a problem that affects the river across the four-state region.
"The area that the Potomac drains through is rapidly developing and is already very developed," says Jessica Seipp of the Frederick County Office of Sustainability. "There's a lot of concrete and pavement and cars and people. So the more you have of that, the more impact you have on water quality."
Runoff can carry all kinds pollutants.
"Whatever pollutants like oil, gas, fertilizers, herbicides, whatever is on that surface, runs off with the water into the stream," says Seipp.
Not just endangered, but dangerous. Everything in it flows to the Chesapeake Bay.
The Office of Sustainability says people can help by using less fertilizer near the river and watching what else goes on their lawns, since it can all run into it.