Spatiotemporal variations in characteristic discharge in the Yangtze River downstream of the Three Gorges Dam

Abstract

Large dams on rivers have substantial impacts on the flow process, sediment transport, and river morphology. The flow-sediment regimes and river morphology in the Yangtze River downstream of the Three Gorges Dam (TGD) have undergone significant adjustments owing to the impoundment of the TGD. Different methods have been adopted to assess the spatiotemporal variations in characteristic discharge in the reaches downstream of the TGD, which can quantitatively assess the channel-forming capacity of flow-sediment regimes. However, agreement on the spatiotemporal variations in characteristic discharge in a long-distance reach downstream of the TGD does not exist thus far. Therefore, in this study, the effective discharge, dominant discharge, and bankfull discharge were calculated to assess the spatiotemporal variations in the characteristic discharge from Yichang to Datong. It was found that, after the impoundment of the TGD, the attenuation of the flow process, bedload coarsening, and changes in the water surface slope together led to a decrease in dominant discharge, which was consistent with the adjustment of the main deformation area from the bankfull channel to the medium-flow channel. This indicates that the dominant discharge is most representative of the characteristic discharges in the reaches downstream of the TGD. Results show that the post-dam characteristic discharge at each station from Yichang to Datong was reduced by 700–5700 m3/s. Spatially, owing to the tributaries along the main stem, the characteristic discharge decreased from Yichang to Jianli and then increased from Jianli to Datong in the pre- and post-dam periods. This study serves as a valuable reference for quantitatively assessing the channel-forming capacity of flow-sediment regimes for other rivers worldwide. Moreover, it facilitates the prediction of the evolution of river morphology.