Unraveling the Multi-Decadal Morphological Regime Shift under Synergistic Drivers of Climate and Human Activity in a Hydro-Engineered Estuary

Global river estuaries are increasingly subjected to the compounding pressures of anthropogenic sediment starvation and climate-induced intensified marine hydrodynamics. While morphological degradation is widely reported, systematic and quantitative insights into the timing and mechanisms of non-linear transitions in estuarine evolution remain limited. The Nakdong River Estuary (NRE) in South Korea, an intensively engineered estuarine system controlled by cascading upstream dams and an estuarine barrage, serves as a paradigmatic case study to deconstruct this mechanism. Drawing on a 60-year (1965-2024) archive of high-resolution bathymetric data and Geomorphological Information Entropy (GIE) analysis, this study quantitatively reveals the regime shift of this mega-estuary from a sediment sink to an erosional source.

Our results indicate that the system maintained a state of metastable equilibrium for decades (1985-2017), masking the cumulative stress of artificial regulation. However, this fragile balance shifted post-2017, initiating an estuary-wide morphological transition. In the seven years from 2017 to 2024 alone, the system recorded a net erosion volume of over 100 million m³, with the annual erosion rate increasing to four times the historical average. We attribute this shift to the synergistic drive of the “Hungry Water” effect and extreme hydro-meteorological events: chronic sediment cutoff due to upstream damming, and channelization altered the morphodynamical impact of extreme floods (e.g., in 2020), transforming them from depositional events into high-energy erosive agents that scoured the riverbed and subaqueous delta. Concurrently, the degradation of barrier islands reduced the natural shelter effect, facilitating the intrusion of wave energy into the inner estuary.

This study demonstrates that anthropogenically transformed estuaries may exhibit apparent stability for decades before undergoing a rapid state transition, suggesting that such period may represent a lag phase preceding significant morphodynamical disorder. The observed transformation of the NRE provides a critical reference for understanding the trajectory of coastal systems worldwide, indicating that rigid engineering control may reduce system resilience against climate shocks. We suggest that under current climate trends, passive conservation strategies may be insufficient; a shift towards holistic source-to-sink sediment restoration, aimed at rebalancing sediment supply with hydrodynamic energy, is essential to mitigate long-term degradation in these vital coastal interfaces.