Differences within and between regional metacommunities could be driven by differences in abiotic and biotic environmental conditions, dispersal connectivity, and ecological drift. Drift—stochastic variation in population dynamics—has recently received attention but is still much less understood relative to environmental and dispersal effects. This is especially the case in freshwater ecosystems despite evidence of ecological drift’s importance on river diversity patterns. Here, we present a stochastic modeling framework to examine how ecological drift links with abiotic and biotic environmental drivers, intrinsic growth rates, and dispersal connectivity to generate measurable variation in metacommunity assembly. We considered a range of population dynamic parameters underlying demographic stochasticity, forms of environmental stochasticity, dispersal stochasticity, maximum growth rate, and species interactions to represent the breadth of conditions encountered among different stream types, locations, and communities. Models between temperate and tropical communities assume temperate communities have species with lower intrinsic growth rates while tropical communities have species with higher intrinsic growth rates. Model output generated predicted patterns of local (alpha) diversity and changes in diversity over time (beta). We found that increased growth rates and dispersal rates decrease dissimilarity to a point and increase dissimilarity among community populations over time, suggesting a possible reduction in the influence of drift on the dissimilarity between sites at medium states of growth rate and dispersal. Our model indicates that temperate rivers, compared to tropical river habitats, have different magnitudes of ecological processes. This, in turn, changes the influence and importance of drift, connectivity, and changing environmental conditions. However, local dynamics of the mainstem in low growth rates are akin to headwaters of high growth rates under certain species interactions and population dynamic parameters.