We study the observed geographic distribution of freshwater fish species in the Mississippi-Missouri river system, focusing on the size and spatial distribution of geographic ranges. We use a particular metric of geographic distribution known as environmental resistance, a biogeographic index that quantifies the pointwise average spatial loss of community similarity to identify biogeographic regions of the river basin. Empirical patterns are compared with the results of a neutral metacommunity model in which local fish communities are interconnected through the ecological corridors provided by the river networks. Because neutral theory assumes that all individuals across all the species are functionally equivalent, the comparison is aimed to quantify how much of the geographic range patterns are the result of species' similarity rather than species differences, thus searching for an ecological null model for the analysis of biogeographic range. We also analyze how river network topology affects the spatial arrangement of species. Our results suggest that broad patterns of geographic range of freshwater fish in the Mississippi-Missouri can be explained simply by neutral dynamics engaged in river topology and competition for resources among species without invoking mechanisms that involve asymmetric interspecific interactions.