Spatio-temporal heterogeneity of the estuarine turbidity maximum retreat in the Changjiang river estuary under reduced sediment supply

Against the backdrop of a global decline in fluvial sediment discharge, the response of the Estuarine Turbidity Maximum (ETM) exhibits high non-linearity and spatio-temporal heterogeneity. Its long-term evolutionary trend is significantly modulated by high-frequency hydrodynamic processes such as seasonal cycles and spring-neap tides, leading to potential variations in trends under different seasonal and tidal range conditions. Here, we focus on the Changjiang River Estuary, define the offshore distances of the 150 mg/L and 50 mg/L suspended sediment concentration isolines, and employ MMK, SMK, and WSRS trend tests to quantify the spatio-temporal evolution of the ETM based on GOCI satellite remote sensing imagery. The results indicate that from 2011 to 2021, the ETM in the Changjiang Estuary exhibited a significant overall landward retreat, characterized by a “south-fast, north-slow” spatial differentiation. Trend analysis further reveals that this retreat often manifested as intermittent, “step-wise” shifts rather than a smooth, linear monotonic trend, with some northern areas showing significant non-linear retreat only during spring tides. Seasonal analysis identifies spring as the key “window period” for the systemic retreat of the ETM, with significant retreat trends observed during both spring and neap tides. In contrast, its retreat in summer and winter is dependent on the hydrodynamic conditions of spring and neap tides, respectively, while no significant trend was detected in autumn. This research unveils the non-linear dynamics of ETM retreat in a large estuary under “sediment starvation” providing a new perspective for understanding and predicting the future evolution of similar estuarine systems.