Many estuaries worldwide have been significantly modified by land reclamation, influencing hydrodynamics and ultimately bed level changes. However, detailed observations documenting how hydrodynamic and morphological changes interact, especially revealing positive feedback mechanisms strengthening the original intervention, remain scarce. This study investigates the morphodynamic response in the macro-tidal, highly turbid Hangzhou Bay (China) over the past three decades to interventions, using unique datasets of annual bathymetry (1991–2019) and synchronous hydrography before and after intensive reclamation (1999, 2019). The analysis reveals severe accretion within the inner bay, with an average bed level rise of 1.21 m (total volume ∼ 2.2 × 10⁹ m³) since 1999, concurrent with a drastic 54.6% reduction in intertidal storage volume. Two key morphological parameters, the ratio of intertidal storage volume to channel volume (Vs/Vc) and the relative tidal amplitude (a/h) systematically decreased by 43% and increased by 6.5%, respectively, thereby enhancing flood dominance. All key hydrodynamic and sediment transport parameters, current velocity, suspended sediment concentration (SSC), tidal volume and sediment flux, showed strong correlations with the tidal range at the bay mouth. Despite a ∼ 30% reduction in mean current velocity and tidal volume from 1999 to 2019, SSC paradoxically increased by 47–133%, and sediment flux rose by ∼44%. The changes are driven by a positive feedback mechanism: the loss of accommodation space and increasing flood dominance promote sediment import and accelerated deposition, which in turn further reduces channel volume and reinforces the tidal asymmetry. This work reveals a key feedback loop accelerating the transition of macro-tidal estuaries towards flood dominant, rapidly infilling and highly turbid systems in response to land reclamation. Such knowledge is key to sustainable sediment management in turbid macrotidal estuaries.

Understanding tidal dynamics in estuaries is essential for tidal predictions and assessments of sediment transport and associated morphological changes. Most studies on river-tide interaction ignored the influences of morphological evolutions under natural conditions such as the seasonal and interannual variations of river discharge. This study analyzes the multiple-timescale tidal dynamics in the Qiantang Estuary, a macro-tidal estuary in China with an extremely active morphological evolution. A large dataset including water levels at representative stations, river discharges and bathymetries since 1980 has been collected. The results of the analysis show that within a spring-neap cycle, the tidal amplification in the upper estuary is stronger during spring tide than during neap tide. This unexpected behavior is due to the high sediment concentration and the unique longitudinal profile of the estuary. On the seasonal and interannual timescales, the low water levels in the upper estuary depend on the local bathymetrical conditions. Tidal ranges in the upper estuary are larger in the high flow season and years, than in the low flow season and years, due to the erosion at high flow, in contrast to estuaries with less active morphological changes. During low flow season and years, the bed is gradually recovered, the low waters are elevated, and the tidal ranges decrease accordingly. A good relationship exists between the tidal ranges and the depth of the upper estuary. In the lower estuary, the flood dominance increases continuously due to embankment. In the upper estuary, the flood dominance is increased during the high flow periods, explaining the fast sediment input and bed recovery in the post high flow periods. A conceptual model of river-tide-morphology interaction of the estuary is proposed, which is also applicable for other shallow systems.