Understanding the mechanisms that create spatial and temporal patterns of functional diversity in stream networks is a goal of basic research and has implications for effective conservation of freshwater ecosystems. These patterns are likely to be influenced by the combination of temporally variable environmental conditions, movement constraints imposed by network structure, and the trait composition of local communities. We developed a simplified metacommunity model to investigate complex interactions among these factors under lottery competition for local resources, such as establishment sites. We used this model to examine how local and regional community composition varied in 3 scenarios: a null implementation involving only spatial effects, an implementation that combined network constraints with dispersal-trait variation, and an implementation in which a trade-off between multiple functional traits was paired with varying levels of temporal autocorrelation in the intensity of mortality. These simulations clarified the conditions that allow a single functional strategy to exclude others in a dendritic network and demonstrated 2 distinct modes of regional partitioning that can support the persistence of multiple functional strategies within such networks. The results suggested that the emergence of watershed or headwater–outlet partitioning depends on the functional dispersal differences present in the metacommunity and that autocorrelated mortality levels can collapse these regional divisions when they depend on a trade-off between dispersal ability and mortality resistance. We discuss the need to confront the complexity of interacting controls on community composition in rivers and streams and suggest opportunities to move beyond the basic framework we present.