We examined patterns of spatial heterogeneity in the Spring River basin fish community along with environmental correlates to assess the relative importance of geographic distances and habitat differences among sites in explaining community structure. Spatial patterns of the fish community and environmental correlates, as indicated by results of Mantel tests and correlograms, were consistent with the hypothesis that environmental factors were the primary factors organizing the community at the basinwide scale. However, a linkage between mainstream and tributary communites, along with spatial autocorrelation in species composition, suggested contagious biotic processes were important in maintaining community structure, particularly at the interface between the mainstream and its tributaries. Space-constrained cluster analysis and principal coordinates analysis revealed three primary groups of sites, reflecting relatively distinct fish faunas within the Ozark Highlands, Central Plains and mainstream regions of the basin. Results of partial Mantel tests indicated that whereas environmental differences were significantly correlated to species differences, variation in community data could alternately be explained by contagious biotic processes, a factor often neglected in community analyses of stream drainage networks.