With memories of the U.S. Open at Congressional Country Club still fresh, this would be a good time to reveal one of the Potomac River’s least-known secrets.

It has to do with golf balls. Those of us who poke around the river’s shores know that golf balls constitute a small but ubiquitous share of its trash load. But I’m probably the only one that knows a spot in the river that is virtually covered with golf balls.

I won’t say exactly where this spot is, except that it’s within sight of the American Legion Bridge.

Layers deep. This ultimate rough lies between two rock reefs and measures about 20 feet across. There, golf balls lie in scores, even hundreds, in some places stacked several layers deep. They gleam in the still water, the old ones greenish with algae, new ones shiny white, yellow, and orange. Oddly, this same spot is also a favored resting spot for black walnuts, which look like golf balls imitating shrunken heads.

So here we have two mysteries courtesy of our local trickster, the Potomac River. First, where do all of these golf balls come from? And second, why should they be concentrated in this one spot?

As for the first question, I would presume that they come from golf courses, not only those along the main stem of the Potomac itself, but also ones traversed by streams and creeks that eventually empty into the river itself. In a heavy rain, the runoff picks up trash and debris—plastic drink bottles, old tires, golf balls, everything else—and sweeps it into the nearest watercourse.

As for the second question, the river tends to sort things out and put them where they belong depending on the strength of the current and the properties of the thing being sorted. For example, flood waters deposit bigger, heavier stones closer to their mountain sources and carry the smaller, lighter ones further downstream. Even where the river empties into a bay or the ocean, the sorting continues, with the pebbles deposited first, followed by sand, and finally the tiny grains of silt.

We see this in the Potomac as well. For example, we can find old tires just about anywhere. but in some places they seem to congregate, like otherworldly geological formations.

It seems to be the same with the golf balls. At my spot between the two rocky reefs, flood waters presumably carry the balls over the first rocks. But these same rocks slow the current on their downstream side, thus losing the force to lift the balls up and over the second set of rocks.

And so the balls accumulate, like a giant cup on a putting green with room for all.

In some far off time, some intelligent form of life with a knack for geology might stumble upon this spot and come to the same conclusion I’ve outlined here. But our alien geologist will likely never think that one of those balls might have been hit by Rory McIlroy at this year’s open, perhaps when nobody was looking.