Dispersal plays a central role in maintaining biodiversity across the landscape by linking local communities within the regional 'metacommunity'. While previous research on dispersal in aquatic systems has centered largely on lentic systems, identifying the effects of dispersal on lotic microbial diversity remains a key challenge. Because stream microbes rely heavily on surface water flow as a dispersal vector, the distribution of individuals across the stream network may be tightly constrained by network structure. Stream networks are often 'dendritic'––that is, they are hierarchical and branching––and their flow is directed downstream, suggesting that the importance of dispersal vs. the local environment may change from the headwaters to the mainstem. In order to examine metacommunity structure across a stream network, I characterized the bacterial diversity of surface water and sediment communities across the HJ Andrews catchment by sequencing the 16S rRNA gene. Variance partitioning, network analysis, and multivariate statistical approaches separate the spatial component from the local environment in explaining the variance in biodiversity across each site.