Unionoids are important in aquatic ecosystems, but despite their continued decline in diversity, few multifactorial studies have been done to identify determinants of their distribution and diversity. We studied the effects of multiple environmental factors on species richness and abundance at 65 sites in 6 major watersheds in the Pine Hills region, Louisiana. We surveyed in-stream habitat variables, land use/cover, and the co-occurring fish assemblages in 2^nd- through 6^th-order streams. A structural equation model suggested that 2 major latent variables were important: 1) reduced habitat disturbance, influenced by lower current velocity, substratum composed mostly of fine sediments, and increasing stream order, and 2) agricultural land use in riparian corridors and associated reduced water quality. These 2 latent variables explained 84% of mussel species richness and 48% of total mussel abundance. Mussel, but not host-fish species richness and abundance increased with stream order. Sites in the lower river basins had increased amounts of fine sediments and lower current velocities. We suggest that the lower river basins have extensive riparian wetlands that ameliorate the effects of frequent floods, thus mediating hydrologic disturbance and increasing mussel diversity.