Multi-Scale Streamflow–Sediment Relationships and Regime Shifts in the Yellow River Mainstream

Understanding streamflow–sediment relationships in large rivers is essential for effective ecosystem conservation and sediment management. However, the multi-scale regimes of streamflow–sediment load in the Yellow River, shaped by the combined influences of climate variability and intensive human activities, remain inadequately understood. This study investigates streamflow–sediment coupling patterns at annual, monthly, and flood-event scales using long-term observations (1950s–2022) from six mainstem stations along the Yellow River. The results reveal pronounced spatiotemporal variability in streamflow–sediment relationships and hysteresis behaviors along the river mainstem. At the annual scale, streamflow and sediment load generally exhibit a linear relationship. At both the monthly and annual maximum flood-event scales, power-law sediment rating curves effectively describe the relationships between discharge (Q) and sediment concentration (SC), expressed as SC = aQ^b. Notably, the goodness of fit (R²) of the annual and monthly streamflow–sediment relationships exhibits a declining trend over time, indicating increasing nonstationarity. Despite this overall weakening, a robust linear coupling between ln(a) and b persists in the sediment rating curves at both monthly and flood-event scales, with a remarkably consistent regression slope (approximately −0.14) across different periods. In addition, pronounced hysteresis patterns are observed at both intra-annual and flood-event scales. These hysteresis loops evolve from simple clockwise forms, indicative of sediment supply limitation, toward more complex figure-eight patterns, reflecting enhanced sediment resuspension and altered sediment delivery dynamics. Overall, the results highlight the scale-dependent and dynamically evolving nature of sediment transport processes in the Yellow River, underscoring the need for adaptive, multi-scale sediment management strategies under changing climatic and anthropogenic influences.