The Tallahatchie River, Mississippi, southeastern U.S.A., is a medium-sized waterway originating in the North Central Hills and flowing into the Mississippi Alluvial Plain physiographic province. Although the river's molluscan fauna remains relatively poorly studied, a survey by Haag and Warren (2007) of Lower Lake, an impounded, regulated segment of the Little Tallahatchie River high in the drainage, revealed a freshwater mussel assemblage that was surprisingly diverse and healthy given current stream management practices. Mussel assemblages from three prehistoric sites in Leflore County, Mississippi, further downstream on the main Tallahatchie River, yielded 32,303 valves representing 41 taxa, including 24 new river records, one of which, Quadrula fragosa (Conrad, 1835), represents a new state record and a notable range extension for this species. Comparison with modern data shows that about twice as many species existed in the waterway prior to Historic-era impacts, including five currently listed as threatened or endangered. These results highlight the importance of applied zooarchaeology for establishing conservation baselines.