We sampled 9 sites (5 free-flowing and 4 impounded) to investigate effects of lowhead dams on the habitat characteristics and the freshwater mussel assemblage of the Fox River in Illinois. We used 2 habitat indices, the Qualitative Habitat Evaluation Index (QHEI) and the Stream Habitat Assessment Protocol (SHAP), to determine effects of lowhead dams on habitat quality. Free-flowing sites had higher QHEI and SHAP scores than impounded sites, indicating higher quality stream habitat. We calculated 3 variables, catch-per-unit-effort (CPUE), extant species richness, and percent missing species, to establish effects of lowhead dams on freshwater mussels. Free-flowing sites had higher CPUE and extant species richness and lower percent missing species than impounded sites. We also examined literature reviews and museum collection holdings to determine species distributions within the basin. These data suggest that dams limit the upstream distribution of 5 species.