Last weekend while visiting DC, we walked along the Potomac River at one point. In the mud left behind the previous tide, there were these large snail shells.
I am not very good at identifying freshwater snails, but in this case I am putting my money on the Japanese mystery snail, Cipangopaludina (Bellamya) japonica, a native of Japan, Taiwan and Korea.
I have 3 more even larger shells. But they had a thick coat of algae growing on them and were starting to get smelly. So I put them out in the backyard to get them cleaned by the ants and whomever else may be interested in eating such stuff.
The juvenile shells, also from the same location, have 3 keels along their body whorls.
According to this source, this species has been in North America since at least the mid-1900s. They have since spread widely and appear to be "naturalized" in North America. Once an alien species comes to that point, there really is no way of getting rid of it.
There is also the Chinese mystery snail, Cipangopaludina (Bellamya) chinensis, another introduction from Asia. I don't know why these species are called "mystery snails".