River-channel confluences create habitat heterogeneity by introducing multiple zones of hydraulic flow and increase macroinvertebrate densities and diversity via the input of coarse sediment, allochthonous detritus, and nutrients. Current understanding of confluence effects on stream community structure and function is based mostly on research conducted at medium-to-large tributary confluences where nutrient and detrital loads are typically high. Confluences of headwater streams and in streams without the input of these materials are understudied. In this study, we sampled macroinvertebrate communities and measured sediment and flow conditions upstream, within, and downstream of confluences. Our sampling included braided confluences, which lack allochthonous detritus and coarse sediment sources, and headwater-stream confluences. Braided confluences had lower macroinvertebrate densities and richness compared to up-and downstream reaches, whereas headwater-stream confluences had a negligible effect on confluence communities. Our results indicate that confluences are not always areas of increased species richness and density for macroinvertebrates. Rather, severe shear stress conditions created by confluence hydraulics can reduce benthic community density and richness relative to adjacent stream locations, especially when tributary inputs of allochthonous food sources to the confluence are low. In addition, we found that all confluences that we studied created small step changes in community composition. Thus, braided rivers may create new habitat patches every time a channel divides or joins, promoting habitat heterogeneity on a broader scale.
