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Biologists have long noted higher levels of species diversity in the longitudinal middle-courses of river systems and have proposed many explanations. As a new explanation for this widespread pattern, we suggest that many middle-course peaks in richness may be, at least in part, a consequence of geometric constraints on the location of species’ ranges along river courses, considering river headwaters and mouths as boundaries for the taxa considered. We demonstrate this extension of the mid-domain effect (MDE) to river systems for riparian plants along two rivers in Sweden, where a previous study found a middle-course peak in richness of natural (non-ruderal) species. We compare patterns of empirical richness of these species to null model predictions of species richness along the two river systems and to spatial patterns for six environmental variables (channel width, substrate fineness, substrate heterogeneity, ice scour, bank height, and bank area). In addition, we examine the independent prediction of mid-domain effects models that species with large ranges, because the location of their ranges is more constrained, are more likely to produce a mid-domain peak in richness than are species with small ranges. Species richness patterns of riparian plants were best predicted by models including both null model predictions and environmental variables. When species were divided into large-ranged and small-ranged groups, the mid-domain effect was more prominent and the null model predictions were a better fit to the empirical richness patterns of large-ranged species than those of small-ranged species. Our results suggest that the peak in riparian plant species richness in the middle courses of the rivers studied can be explained by an underlying mid-domain effect (driven by geometric constraints on large-ranged species), together with environmental effects on richness patterns (particularly on small-ranged species). We suggest that the mid-domain effect may help to explain similar middle-course richness peaks along other rivers.