Changes in River Cross-section Morphology and Response to Streamflow and Sediment Processes in Middle Reaches of Yellow River, China

Changes in river cross-section morphology have decisive influences on the flood discharge and sand transport capacity of rivers; thus, these changes strongly reflect the vitality of a river. In this paper, based on the river cross-section and water and sediment data of two different periods(1974-1987 and 2007-2021), the trend analysis, change-point analysis and sediment rating curve method were used to analyze the change process of river cross-section morphology and its response to streamflow and sediment changes in the main river stream of the Yellow River at the Longmen hydrological station. From 1974 to 1987(except in 1977), the riverbed experienced siltation, and the riverbed elevation rose. Conversely, from 2007 to 2021, the riverbed experienced scouring, and the riverbed elevation gradually decreased. The cross-section shape changed from rectangular to U-shaped(deeper on the right side) at the Longmen cross-section. The changes in streamflow and sediment processes significantly impacted the evolution of river cross-section. Streamflow(P < 0.05), sediment discharge(P < 0.01), and the sediment load coefficients(P < 0.01) decreased significantly. The relationship between the water depth and sediment load coefficients followed a power function. The decreasing trend in sediment discharge was significantly stronger than that in streamflow. Suspended sediment particles tended to become finer. The sediment rating curve indicates that the sediment supply from upstream decreased while the erosive power in the river channel increased, leading to a gradual decline in riverbed elevation at the Longmen cross-section from 2007 to 2021. These findings help us better understand the impacts of ecological restoration on changes in river streamflow and sediment during river evolution.