Sediment load complexity downstream of the Three Gorges Project via the multiscale entropy analysis method

The Three Gorges Project (TGP) plays a pivotal role in flood control, power generation, and water resource management along the Yangtze River. Nevertheless, its construction and operation have profoundly altered the downstream suspended sediment regime, reshaping the complexity of sediment transport processes. However, traditional studies often rely on single-parameter analyses, failing to provide a holistic view of the system’s complexity and the nonlinear, interactive nature of downstream recovery. To quantify these changes, we applied multiscale entropy (MSE) analysis to daily suspended sediment load records (1992–2023) from eight mainstream gauging stations. The results revealed a spatially heterogeneous response: In the near-dam sections (Yichang, Zhicheng, and Shashi), the complexity of the suspended sediment dynamics decreased while the sequence similarity increased, whereas in the middle and lower reaches (Jianli, Luoshan, Hankou, Jiujiang, and Datong), the complexity increased and the uniformity decreased. On the basis of a process understanding and empirical observation of the system, we have established a functional framework HSF = f ((Var(Q), Var(SSC), φtrib; ηprot) to mechanically explain these comparative reactions. Specifically, (1) flow regulation by the TGP compresses daily discharge variability (↓Var(Q)), suppressing extreme events and reducing complexity; (2) sediment trapping narrows the range of suspended sediment concentration (↓Var(SSC)), further enhancing uniformity; (3) tributary and lake inputs (↑ φtrib) introduce temporally variable and compositionally diverse sediments, increasing stochasticity and complexity; and (4) bank and bed protection (↑ ηprot) restricts local sediment supply, contributing to homogenization. The interplay of these drivers produces a dual-response pattern—simplification in near-dam reaches versus diversification in far-dam reaches—highlights the spatial heterogeneity of dam-induced impacts. By explicitly linking entropy metrics to physical drivers, this study provides not only a diagnostic tool for characterizing sediment dynamics but also a mechanistic understanding of how large dams restructure riverine processes. These insights offer a theoretical basis for more targeted sediment management strategies in regulated river systems.