To discuss disaster countermeasures caused by the inflow of sediment and water from tributaries into the mainstream due to large‐scale landslides and debris flows, a quantitative prediction of the impact of water and sediment inflow from tributaries on the flow conditions of the main river is required. In this study, a flume experiment was conducted assuming a 45° confluence angle between the main river and the tributary. Through comparison with the results of previous experimental studies the impact of different confluence angles on the flow conditions of the main river due to water and sediment inflow from the tributary was evaluated and discussed. We have found that the planar distribution position of the boundary between the main river and tributary flows exhibited a positive correlation with the flow rate ratio at each time point, with a higher coefficient of determination when riverbed changes were minimal at the 45° confluence angle.