That's how the story started. We had always been told that 3 nuns were in a boat that overturned and they drowned there. The boulders sprung up where that occurred. There are other stories too, but 3 women always drown. People give it some kind of spiritual creepy tone.
People imagine there is a lot going on there.
Melville used the ocean as a 'presence' in Moby Dick . Well, yes, I think so. I imagine that the Potomac has a personality. I think that rivers inspire that in the people who live nearby. I grew up in Washington and we used to take auto rides in the summer to Hanes Point, on the banks of the Potomac.