Impacts of human activities on morphological evolution in the Modaomen Estuary, China

The morphology of the Modaomen Estuary (ME) has undergone drastic changes in recent decades, and quantifying the contribution of human activities and natural processes is crucial for estuary management. Using Landsat images, chart data, and hydrological and meteorological data, this study analyzed the evolution of the shoreline and subaqueous topography of the ME and attempted to quantify the extent of the contributions of human activities. The results show that local human activities dominated morphological evolution in some periods. From 1973 to 2003, the shoreline advanced rapidly seaward, resulting in approximately half of the water area being converted into land. Human activity is critical to this process, with the direct contribution of local land reclamation projects reaching more than 85%. After 2003, the shoreline remained relatively stable, probably due to a decrease in land reclamation projects. Regarding the evolution of subaqueous topography, the shoals in the estuary were heavily silted and gradually disappeared during 1983–2003, and the waterways narrowed and deepened. The average siltation rate decreased from 15.43 mm/a to −1.02 mm/a, indicating that the ME changed from sedimentation to slight erosion. By detecting variations of sediment load, we found that upstream human activities reduced river sediment, while downstream human activities significantly increased sediment input to the ME, leaving little change in the actual sediment input to the ME for a relatively long period. In addition, based on the empirical relationship between the sediment input and siltation rate, local human activities influenced the shift in the siltation state more than upstream and downstream human activities did. These findings suggest that more attention should be paid to local human activities to improve the estuarine management in the ME.