Exploring the changes in the ratio of the sediment concentration to the sediment transport capacity in mountainous rivers: A case study of the Upper Yangtze River

The relative magnitude of the suspended sediment concentration (SSC) to the suspended sediment transport capacity (SSTC) is an important indicator for determining riverbed evolution. In this paper, with the use of an extensive dataset, Ruijin Zhang’s formula is employed to calculate the SSTC at representative hydrological stations in the Upper Yangtze River, thereby examining the fluctuations in the ratio of the SSC to the SSTC (SSC/SSTC) across varying flow rates (Q) for the first time, yielding the following three important insights. First, the results confirm that except for some hydrological stations located within the reservoir area, the remaining hydrological stations are generally characterized by a sub-saturated state of flow with respect to sediment, as evidenced by predominantly lower SSC/SSTC values, which are typically less than 1. Second, the results emphasize the substantial influence of changes in the upstream sediment load on the SSC/SSTC values. Third, a distinctive V-shaped correlation is identified between the Q and SSC/SSTC values at each station, characterized by a clear minimum point. This minimum point defines the “critical flow rate”, which can be attributed to the morphology of river channel.