Relationships between biotic/abiotic factors and the distribution of unionids in Lake St. Clair were investigated from 1998 to 2000. Native freshwater mussel (Bivalvia, Unionidae) populations in the lower Great Lakes have been nearly eradicated by the invasive zebra mussel. There are only a few known unionid refugia remaining in the lower Great Lakes. One such refugium was found during this study in Lake St. Clair near the St. Clair River delta. Morphological, reproductive, and ecological traits were not found to have any significant relationships with the change in frequency before the zebra mussel invasion (1986) compared to after the invasion (1999). Using a GIS model of Lake St. Clair, distance of survey sites from deep water currents were found to have significant positive correlations (P < 0.001) with unionid density. These findings lead to recommendations to help direct future unionid conservation efforts.