| Biophysically complex, unfragmented river networks are increasingly rare, but their study is necessary to understand the processes by which diverse communities of riverine organisms are sustained. In such a network, we studied the increased biophysical heterogeneity that occurs when multiple, adjacent tributaries enter the mainstem in close proximity, and investigated the effect of such ‘tributary-confluence complexes’ on macroinvertebrate biodiversity and metacommunity characteristics. We combined sampling throughout the network (including four confluence complexes and three mainstem sites not associated with confluences) with a decadal-scale analysis of metacommunity dynamics in one confluence complex. From an aggregation analysis, we found that network complexity supported increasingly diverse macroinvertebrate metacommunities. Additionally, we observed emergent metacommunity phenomena at confluence complexes, and that high inter-annual variation among habitats within a metacommunity contributed disproportionately to the maintenance of diversity at the decadal scale. Metacommunity dynamics resulting from heterogeneous mosaics may be important to biodiversity maintenance in river networks. |