Abstract: At the river confluences of the upper Yellow River, flows carrying hyperconcentrated sediment merge together, and in flood season the merging usually causes severe deposition and dramatic change in bed morphology. In this study, we investigate the morphological process at hyperconcentrated confluences through laboratory experiment, field observation, and theoretical analysis, and develop a new method for predicting sediment deposition at the confluences. Results show that four basic elements of the morphological pattern can be identified: sandbars in the backwater zone, sandbars in the flow separation zone, a mainstream channel for water and sediment transport, and bars downstream of the separation zone. Because of turbulent diffusion, sediment particles carried by the hyperconcentrated flows enter the backwater zone upstream of the junction, and then form sandbar deposition with a volume proportional to the sediment load. We construct a model for calculation of the sediment deposition at river confluences and reveal its main factors: sediment load and discharge ratio of the incoming flows, water discharge of the main stem, and adaption coefficient of the incoming suspended loads. This model is applicable to prediction of the deposition at asymmetrical river confluences. Scour on the sandbars caused by heperconcentrated tributary flows depends on the discharge and duration of main stem floods. For the confluences on the upper Yellow River, the most efficient flow discharge and duration for flushing the sandbars are about 2000 m3/s and 10 days respectively.