Multi-timescale evolution characteristics of water and sediment fluxes in the lower reaches of the Yellow River under the dominance of human activities

ABSTRACT The graphical summary adopts a left to right flowchart structure, systematically demonstrating the research logic and core contributions of this study. On the left is the "Research Background and Problems" module, highlighting three core elements: "high-intensity human activities (such as reservoir scheduling)", "water and sediment flux in the lower Yellow River", and "multi time scale evolution characteristics", pointing to the research goal of "aiming to reveal". The middle section is the "Multi Method Analysis Framework" module, which lists three methods in sequence: "Regression and Interpolation", "M-K Test and Seasonal Decomposition", and "Wavelet Analysis", and indicates "Applied to 2016-2022 data". On the right is the "Core Empirical Discoveries" module, which presents three key findings: a step like increase in water and sediment flux in 2018, a continuous and amplitude enhanced annual cycle (12 months), and a stronger nonlinear response of sediment transport compared to runoff. On the far right is the "Research Implications" module, which points out that this study provides empirical evidence for the impact of reservoir operation and provides methodological references for the management of regulated rivers. The arrows run through each module, clearly presenting a complete research narrative of "background method discovery revelation". Reservoir operation fundamentally alters natural fluvial processes. This study investigates water–sediment flux evolution in the highly regulated lower Yellow River. Analyzing 2016–2022 high-frequency hydrological data via cubic spline interpolation, Mann–Kendall tests, seasonal decomposition, and wavelet analysis reveals: (1) a nonlinear water–sediment relationship with sediment flux highly sensitive to flow; (2) a step-change increase in interannual flux after 2018, aligning with Xiaolangdi Reservoir's ‘long-term, low-level sediment discharge’ operations; (3) a stable 12.5-month primary periodicity with significantly amplified post-2018 oscillations, indicating intensified human-induced variability; and (4) phase-synchronized but amplitude-decoupled water–sediment dynamics, where sediment exhibits stronger nonlinear responses. By fusing multiple temporal-scale diagnostics, this work quantifies the dominant reshaping role of targeted reservoir scheduling on natural hydrological cycles, offering a mechanistic framework for managing sediment in anthropogenically stressed river systems.