Although insight into local and regional factors that structure communities is growing, the specific contribution of each of these factors generally remains unknown. Moreover, the extent to which community assembly is intrinsically indeterministic (i.e., the degree to which community composition depends on chance events) remains an open question. The complexity of this issue argues for field studies in a semicontrolled environment. We studied species richness and community composition of a guild of phreatophytes along dendritic networks of small forest streams in western Belgium. The streams, situated at the forest-covered headwaters of six different watersheds and characterized by homogeneous environmental conditions, impose spatial dispersal constraints to the plant species. This allowed us to assess the relative importance of deterministic processes guiding community assembly. More specifically, we focused on the role of species migration and colonization processes relative to local processes (competitive exclusion and passive habitat selection) on species richness and on community composition. Our analyses at three consecutive levels (species number, community composition, and species) revealed that plant community assembly along the stream systems is based on (1) historical and stochastic colonization events of species and (2) subsequent deterministic downstream dispersal processes. Apparently, due to the influence of historical chance events, deterministic processes do not necessarily result in community convergence. In addition we show that continuous downstream dispersal and colonization can compensate for local extinction processes. This is another piece of evidence that, when studying community composition, it is absolutely necessary to place communities in their spatial contexts.