A week ago I took a long walk on the towpath along the C&O Canal near Georgetown. At one point I descended into the canal itself to photograph some tracks (I will write about those some other time). These clams were extremely abundant in the mud at the bottom of the canal (there wasn't much water in the canal, obviously).
They are a Corbicula species, probably Corbicula fluminea, an invasive alien native to China.
The other bivalve species I noticed, which was much less abundant than Corbicula, was the following native unionid.
Tim Pearce identified it from the picture as either Elliptio dilatata or E. complanata.
I did not notice any live bivalves. But then again, I wasn't looking for them where there was water.