A diffluence‐confluence unit is an elementary component within a river system and presents a complex yet linked pattern of both flow and sediment transport in between. This study deals, by means of field investigations and numerical modelling, with morpho‐dynamics of such a unit on the lower Yangtze River reaches. The unit comprises, looking downstream, a secondary (left) course and a main (right) course. Field surveys are performed for measurements of flow discharge, sediment loads at selected locations and river bathymetry at certain intervals. The field data show that the reach is mainly composed of suspended load, whose amount exhibits a declining trend with the elapse of time. Simulations in 3D are made to complement the field data and clarify the basic features of the unit, especially the partitioning of flow and suspended sediment in the diffluence and their subsequent reciprocal adjustment in the confluence. The results indicate that approach flow variations have a bearing on the diffluence flow partition. To augment flow discharge in the left branch, a training wall is devised in the diffluence to modify the intake flow. Secondary flow structures are found to be more influenced by the thalweg curvature than the flow division. The “inlet step” or differential topography contributes to the unequal flow division. In the confluence, a two‐cell flow structure coexists, which may diminish along with the dynamical adjustment of the two waters. The classical bed discordance is also observed. With the typical flow and sediment features, the main course is prone to slight erosion, while the secondary branch faces up with gradual siltation. These findings contribute to the understanding of the alluvial behaviours of such units, and provide reference for studies in similar situations and river management.