They'd filter the water, wouldn't they? Which might make up for their shortcomings in the lively-entertainment area! Actually people suggest that you should poke your clams gently each morning: "Hello there! still alive are we?"
After a couple of days, you'd begin to know.
While they are alive, they'll put down a muscular foot and work their way through your gravel like a bulldozer.
I think all the freshwater clams are Unionidae. (Try a
www.google.search for them.) The Unionids have one unpopular habit. Their larvae, if they spawn, attach themselves to fishes' gills. Fish transport the young clams upstream, which is not the easy direction for a clam. Without fishes the clam population would drift slowly farther and farther downstream. The fish won't like this aspect.
(Apparently the world's richest, most endangered freshwater bivalves are right here in the U.S. So the whole conception is interesting.)