Gelare Moradi, Colin Rennie, Romain Cardot, Stuart N Lane 

River confluences influence the formation of secondary circulation, bed morphology, and associated feedbacks. With distance downstream through a drainage network, it becomes likely that the flow momentum of tributaries is lower than that of the main river, creating confluences with very low momentum ratio. However, the tributary may be able to supply significant quantities of sediment, especially in watersheds with high relief. High flow and sediment supply events in the tributary may decouple in time from those in the main river. The capacity of the main river to evacuate tributary-delivered sediment may occasionally be lower than the sediment delivery rate. The result is the formation of confluences with large tributary mouth bars that may influence confluence flow structures even when the tributary discharge has declined. There are no field examples measured to date for such confluences. Here, we report the first field data for three such river confluences, along the upper Rhône River, Switzerland. We combine aDcp measurements with the analysis of the provenance of sediments in the tributaries and main stem. We introduce the angular momentum ratio for confluence classification. The formation of tributary mouth bars and a scour hole was identified for the two junctions with significant tributary sediment supply. These bars were sufficient to introduce secondary circulation even at very low flow momentum ratios. The analysis of sediment provenance suggested that the origin of the bars was not necessarily the tributary but could be a consequence of the effects of the tributary upon main channel sediment routing.