Impacts of Water and Sediment Fluxes into the Sea on Spatiotemporal Evolution of Coastal Zone in the Yellow River Delta

Water and sediment fluxes into the sea are the basis for the stability of the ecological pattern of the Yellow River Delta (YRD). As a Ramsar wetland of international importance, the YRD is facing the huge ecological risk of land degradation due to changes in water–sediment fluxes into the sea. In this study, we investigated the spatiotemporal dynamics of the coastline and subaerial delta using annual remote sensing images and revealed more detailed and clear relationships between water–sediment fluxes into the sea and the YRD evolution, including the whole delta and its subregions (e.g., the Qingshuigou and Diaokou regions) from 1976 to 2022. Our results showed that the mean yearly water and sediment fluxes during the study period amounted to 210.50 × 108 m3 yr−1 and 367.81 Mt yr−1, respectively. There was an abrupt change in water and sediment fluxes into the sea in 1999, and both decreased significantly from 1976 to 1999, whereas the water discharge has significantly increased and the sediment flux has stabilized since around 2000. The delta area evolutions of the whole YRD and the Qingshuigou region can be characterized by three stages: a rapid growth stage (1976–1993), a rapid retreat stage (1993–2002), and a gradual recovery stage (2002–2022). The area in the Diaokou region displayed a continuous decreasing trend from 1976 to 2022. The regression analysis indicated that the relationships between cumulative sediment flux and cumulative land accretion area presented spatiotemporal differentiation. The cumulative land accretion area increased with the cumulative sediment flux in the whole YRD and its subregions from 1976 to 1992, decreased with the cumulative sediment flux in the YRD from 1993 to 2002, except for the northeast of Qingshuigou, and then expanded with the cumulative sediment flux in the YRD from 2003 to 2022, except for the southeast of Qingshuigou.