Reshuffling of the sediment suspension regime in the starving middle Yangtze River downstream from cascade mega-reservoirs

The impoundment of cascade mega-reservoirs in the upper Yangtze River (uYR) has exerted profound and accumulative impacts on the lower reaches-not only a drastic drop in the incoming sediment load but also overturning of the runoff-sediment relationship. Based on a 1990-2021 dataset, we examined fundamental changes in the sediment suspension regime in the Jingjiang reach, approximately 100 km below the Three Gorges Dam (TGD). The cascade mega-reservoir impacts are elucidated by three parameters: inflow water discharge and sediment concentration released to the Three Gorges Reservoir (TGR) and operating water levels in front of the TGD. The wash-load concentrations in the reach in the post-TGR period illustrate strong relationships with the flow capacity index, they, however, were actually determined by the above three parameters. The relative importance of the three parameters in determining the wash-load concentrations was evaluated by a random forest model. After upstream reservoir impoundment starting in 2003, the unsaturation degrees in the reach tended to increase, and the thresholds distinguishing wash-load and bed-material-load have risen to -0.125 mm and -0.25 mm at SS/JL and ZC after TGD impoundment, respectively. In addition, the flow capacity coefficients, i.e., m and k, in the starving reach became significantly lower than their equilibrium counterparts. The study aims to facilitate effective management of the uYR cascade mega-reservoirs and the experience could be drawn on for other large rivers facing hydropower development.

Automated summary

The impoundment of cascade mega-reservoirs in the upper Yangtze River has had profound and accumulative impacts on the lower reaches, leading to a significant decrease in sediment load and a change in the runoff-sediment relationship. This study analyzed data from 1990 to 2021 to examine the changes in sediment suspension in the Jingjiang reach, approximately 100 km below the Three Gorges Dam. The impacts of cascade mega-reservoirs were evaluated using three parameters: inflow water discharge, sediment concentration released to the Three Gorges Reservoir, and operating water levels in front of the dam. The relative importance of these parameters in determining wash-load concentrations was assessed using a random forest model.