Response of the gravel-sand transition in the Yangtze River to hydrological and sediment regime changes after upstream damming

In the Yangtze River, the Three Gorges Dam (TGD) has altered the flow-sediment regimes, resulting in channel degradation and bed surface coarsening, which have been reported in recent years. However, there has been no systematic study on the impacts of the TGD on the downstream gravel-sand transition (GST). Based on detailed field observations and numerical modelling, this study fills this gap and reveals significant adjustments in the GST and the leading causes. The results show that after operation of the TGD began, the GST migrated 49.5 km downstream during 2003-2010 due to a reduction in the suspended-load supply (grain size < 0.5 mm) and then remained stable from 2010 to 2015 under the control of a braided river morphology. In addition, as channel degradation continued, the steepening bed profile upstream of Chenjiawan and the lowering of the downstream water level caused some gravel within the GST to become more frequently mobile during low-flow periods (discharge <15,000 m(3)/s). Combined with the effects of the TGD-induced prolonged low-flow season and reduced frequency of large floods (discharge >= 40,000 m(3)/s), the GST exhibited a poorly understood phenomenon in which gravel was mainly transported downstream during low-flow periods. However, these processes affected only localized sediment movement, contributing little to overall GST migration in the Yangtze River. The results also show that when a low sediment supply persists long term, additional GST migration after transient downstream migration depends primarily on the river morphology. This article has implications for the channel regulation and evolutionary processes of GSTs below dams.