North American freshwater mussels of the Order Unionoida are critically imperiled, primarily due to stream habitat modifications and fragmentation by reservoirs. Whereas many species respond negatively to impoundments, some species benefit by increases in lentic habitat. During winter drawdown of Tuttle Creek Reservoir, KS in 2006–2007, we collected freshwater mussel shells to characterize spatial variation in assemblage structure within the reservoir and compare reservoir assemblages to stream assemblages within the surrounding drainage basin. Of the 22 unionid species that occurred in the basin, six were found in Tuttle Creek Reservoir. Species richness in the reservoir did not differ from that found in both small and large streams. Species composition in streams varied along a gradient from small to large (1st-7th) order streams, and mussel assemblages in the reservoir were most similar to that of large order streams. This study identified the subset of stream-dwelling unionid species that are habitat generalists and capable of persisting in reservoirs.