Dear Martin and all other Corbicula lovers/haters,
 
I had noticed two varieties of Corbicula here in the Rio Grande of New
Mexico, as well.  Yes, we, too, used to have a unionid fauna, though not
so diverse as yours.  I stumbled across a paper a couple years ago:
 
Hillis, David M., and John C. Patton. 1982. Morphological and
electrophoretic evidence for two species of Corbicula (Bivalvia:
Corbiculidae) in North America.  American Midland Naturalist 108:74-80.
 
They describe a "white-nacre" form and a "purple-nacre" form.  The forms
also differ in shell shape, habitat, and genes.  The authors suggest
that they might correspond to C. fluminea and C. fluminalis, as has
already been suggested, but "conflicting morphological, reproductive and
distributional data prohibit assigning either of these names (or ny
others) to the two introduced species until revisionary wrk is carried
out on the genus Corbicula throughout its native range."
 
Bruce Neville
Albuquerque, NM