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We studied relations between river size, fish species diversity, and fish species composition along four major rivers in the Great Plains of southwestern South Dakota to assess patterns of species diversity and composition. We expected diversity to increase with river size and fish composition to change via species addition downstream. Previous surveys of 52 sampling stations provided fish assemblage data, and we used the Geographic Information System (GIS) to determine watershed area by station. Watershed area did not predict species richness or species diversity (Fisher's a), so species richness of 12 ± 3.5 SD species and Fisher's a of 2.3 ± 0.87 SD characterized species diversity in the study area. Cluster analysis of faunal similarity (Sorensen's Index) among the 52 sampling stations identified two geographically distinct faunal divisions, so species composition was variable within the study area, but changed via species replacements among faunas rather than species additions downstream. Nonnative species were a minor component of all faunas. Uniform species diversity may be a recent phenomenon caused by impacts of Missouri River dams on native large-river fishes and the unsuitability of rivers in the Great Plains for nonnative species. Variation in faunal composition may also be recent because it was affected by dams.