On the cumulative dam impact in the upper Changjiang River: Streamflow and sediment load changes

Climate change and anthropogenic activities such as dam construction alter basin-scale hydrological regime of a river. The upper Changjiang River (uCR) stands out as one of the most heavily dammed rivers in the world after the construction of the Three Gorges Dam (TGD) and other large dams in its mainstem. Quantification of the cumulative dam impact is prerequisite for better river management. In this work, we provide a rigorous appraisal of the changes in streamflow, sediment load, and sediment composition at multiple time scales throughout the uCR based on data in 1950-2017. We observed that a decreasing trend in annual streamflow has emerged since 2015 at Yichang, the outlet of the uCR basin, although the changes were statistically insignificant for the first 65 years. The annual sediment load has decreased progressively and substantially, e.g., by 97% in 2010s compared to 1950s at Yichang. The Three Gorges Dam and the new large dams in the upstream mainstem accelerated the sediment load reduction in 2003 and 2014, respectively. As a result, the suspended sediment became finer, with a decrease in mean diameter from 17 mu m in the 1960s to 8 mu m in the 2010s at Yichang. We established a reservoir storage capacity index, which is the ratio of the total reservoir storage capacity to annual streamflow, and identified a threshold of 4% larger than which the cumulative dam impact will induce profound sediment load reduction. We concluded that climate change and anthropogenic activities, in particular the large dams in the mainstem, have transformed the uCR system from a turbulent and muddy river to a placid one, which can affect fluvial processes as well as aquatic ecosystems by altering sediment and nutrient concentrations and ratios. These hydro-morphological changes merit the urgent attention of concerned authorities.