Coarse bedload routing and dispersion through tributary confluences

Abstract. Sediment routing fundamentally influences channel morphology and the propagation of disturbances such as debris flows. The transport and storage of bedload particles across headwater channel confluences, which may be significant nodes of the channel network in terms of sediment routing, morphology, and habitat, are poorly understood, however. We investigated patterns and processes of sediment routing through headwater confluences by comparing them to published results from lower-gradient confluences and by comparing the dispersive behavior of coarse bedload particles between headwater confluence and non-confluence reaches. We addressed these questions with a field tracer experiment using passive-integrated transponder and radio-frequency identification technology in the East Fork Bitterroot River basin, Montana, USA. Within the confluence zone, tracers tended to be deposited towards scour-hole and channel margins, suggesting narrow, efficient transport corridors that mirror those observed in prior studies, many of which are from finer-grained systems. Coarse particles in some confluence reaches experienced reduced depositional probabilities within the confluence relative to upstream and downstream of the confluence. Analysis of particle transport data suggests that variation in the spatial distribution of coarse-sediment particles may be enhanced by passing through confluences, though further study is needed to evaluate confluence effects on dispersive regimes and sediment routing on broader spatial and temporal scales.