Abstract: This paper examines the application of the erosion-deposition law to the multilevel bifurcating system of Yangtze River Estuary under dam-induced runoff flattening.The spatiotemporal pattern of erosion-deposition change and associated hydrodynamic mechanism are identified by analyzing water-sediment, terrain and engineering data from 1950 to 2021 in terms of a runoff intensity index D_a (i.e., multiyear average days of runoff discharge ≥60 000 m³/s), tracked positions of stagnation points, and estimates of restored erosion-deposition volume of the north passage according to navigation dredging data.It is found that the ebb partition ratios of the north and south branching channels tend to increase and decrease, respectively, as D_a grows; this aggravates erosion and impedes deposition in the north branching channels while facilitating deposition and restraining erosion in the south branching channels.Meanwhile, depo-centers in north and south branching channels move downstream and upstream.The foregoing changes in erosion-deposition are driven by hydrodynamic inertia and the flow-deflecting effect of nodes along the south bank.More specifically, the ebb flow axis swings either northward or southward as the runoff discharge varies, forming a linkage mechanism for the lateral-longitudinal erosion-deposition pattern between the north and south branching channels.As the runoff process becomes continuously flattened owing to upstream dam regulation activities, the north branching channels and their upper sub-reaches are likely to maintain a trend of deposition aggravation and erosion impediment, whereas their lower sub-reaches maintain a trend of erosion aggravation and deposition impediment.Conversely, the south branching channels are likely to witness an opposite trend in erosion-deposition.The critical runoff discharge for migration of the stagnation point between upper and lower sub-reaches in the north passage is 35 000 m³/s.At such a situation, additional deposition is expected to occur in the upper sub-reach, and timely maintenance adjustments would be required along key segments of the navigational channel.