The impact of runoff flux and reclamation on the spatiotemporal evolution of the Yellow River estuarine wetlands

The Yellow River Delta is located in the intertwining zone of marine, terrestrial and river ecosystems, and its evolution is not only restricted by hydrological conditions, but also affected by human activities, which leads to serious degradation of estuarine wetlands. Here, long-term data from a hydrological monitoring station and remotely sensed satellite images were used to explore the effects from runoff, sediment load, and human activities on the evolution of wetlands in the Yellow River Delta from 1976 to 2018. Our results showed that the delta area fluctuated between 233.25 × 103 and 260.40 × 103 ha during the period of 1976–2018. The evolution of the old estuary was mainly affected by erosion, while the current estuary was mainly regulated by sedimentation during 1976–1995 and erosion during 1995–2018. Specifically, during the past four decades, natural wetlands, such as tidal flats and marshes, decreased dramatically by 55.0%, and these wetlands were mainly transformed into aquacultural ponds, salt pans, oil wells and dry land. In addition, the evolution of natural wetlands in different regions in the YRD was influenced to different degrees by natural factors and human activities. In the estuarine zone, the reduction in natural wetlands was mainly driven by natural factors such as coastal erosion, runoff and sediment load. In the old estuary, coastal erosion explained 91.2% of the changes in natural wetlands. The evolution of natural wetlands in the current estuary was mainly controlled by the sediment flux of the Yellow River and human activities, which accounted for 62.7% and 19.0% of the variation in the current estuary, respectively. In the central zone, human activities, such as reclamation and urban construction, have converted 82.7% of natural wetlands into human-made wetlands and non-wetlands. Our study indicates that quantitative quantification of the impact of the runoff and sediment, ocean dynamics and human activities on wetland evolution is conducive to the restoration of estuarine wetlands and the healthy and sustainable development of ecosystems.