The Potomac River begins at a spring at the base of "The Fairfax Stone" which was the original marker for what would become the border of Maryland and West Virginia. I had time for a medium ride this afternoon and it was only about 140 miles round trip so I set it as my destination. I forgot to take the camera the last time I visited. Got it this time.
For your viewing pleasure here is where the Potomac begins:

The spring itself:

The immediate area with bike in background (note, no other people):

Now, this next part is crude. So you might not want to read it. The spring itself was not flowing this afternoon although the ground all around it was very wet. The first flowing water was down channel about 10 feet from the stone. I had to pee. So I pissed just above the first flowing water. So for a brief minute in time 100% of the liquid flowing in the Potomac river was my urin. Sorry, no picture; you'll just have to take my word for it.