Confluences act as critical nodes in a river network as they affect flow, sediment transport, water quality, and ecological patterns. A complete knowledge about hydro‐morpho‐sedimentary processes at river confluences is still incompleted and it has been usually accepted that secondary flows are weak because of the significant role of form roughness in large rivers. In this study, two field surveys were conducted on the flow structure, suspended sediment transport and morphology of the confluence between the Yangtze River (the largest river in China) and the Poyang Lake (the largest freshwater lake in China). Dual counter‐rotating cells were observed during high flow conditions and a single secondary cell appeared in low flow conditions. These helical cells restricted the core size of high sediment concentration and downwelling flows acted as a barrier hindering the exchange of sediment between the two rivers. Furthermore, the observed large scour hole was likely related to the downwelling and upwelling flows caused by helical motions. In low flow conditions, the scour hole looked like a deep channel, which was likely related to a long‐surviving helical cell. The scour hole disappeared further downstream, when either the helical motion got weak during low flow conditions, or when a reverse helical cell occurred during high flow conditions. Hydrodynamics, suspended sediment transport and morphological features observed at such a large confluence demonstrated that river planform geometry and discharge ratio affected the flow structure, especially the helical motion. This in turn affected sediment transport as well as the local bed morphology.

The knowledge of the dynamics of urban channel confluences is insufficient as most past studies have focused on natural river confluences. This paper reports experimental observations on bed morphology and hydrodynamics of the urban confluences and their dependence to the discharge ratio. Typical hydrodynamic and morphological features such as the shear layer, helical cells, bars and scour holes are identified. Nevertheless, the presence of a region of low-velocity flow for a small discharge ratio differs from results obtained in previous studies. Two sand ridges associated with the corridors of the eroded sediment from the scour holes merge into a mid-stream bar. Results also show that the intense shear towards the bed by downwelling and upwelling flows involved in helical motions is responsible for the mid-stream scour hole.