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 the importance of ecological drift on river diversity patterns. Here, we present a stochastic modeling framework where we examine how ecological drift links with abiotic and biotic environmental drivers, intrinsic survival, fecundity, 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 survival rate, and species interactions in order to represent the breadth of conditions encountered among different stream types, locations and communities. Model output generated predicted patterns of local (alpha) diversity, changes in diversity over space (beta), and time (beta and synchrony). We found that increased negative environmental effects on survival rates reduce local population fluctuations over time due to ecological drift. However, the increased negative environmental effect on survival rate reverses the theory-predicted patterns of community size, drift, and temporal beta diversity. Our model predicts that as headwaters become more similar to one another or more variable over time, the metacommunity's average spatial beta decreases, while temporal beta values and their spread increase.