Influence of the Three Gorges Dam on the transport and sorting of coarse and fine sediments downstream of the dam

•The effect of operation of the Three Gorges Reservoir on sediment sorting is answered.•The fine sediment recovered along the downstream, and the recovery degree was less than before the dam operation.•The recovery characteristics of coarse sediment remain unchanged, but the intensity is lower than before the dam operation. River hydrology and geomorphology are largely influenced by human activities. The operation of the Three Gorges Dam (TGD), which is the largest global hydropower project, has caused widespread concern because of its substantial influences on downstream water and sediment fluxes. This study assesses the influences of the Three Gorges Reservoir (TGR) impoundment on sediment sorting and spatial–temporal characteristics of downstream transport. The results indicate that the reservoir prevented the transport of 76 % of the sediments from the upstream basin to the downstream area of the dam, and the discharge of fine (d ≤ 0.125 mm) and coarse particles (d > 0.125 mm) was lower than 25 % (24 % and 12 %, respectively). The recovery of total and fine sediments along the lower reaches of the TGD since 2003 was lower than the respective average values before impoundment (1990–2002). The total volume and discharge ratio of coarse and fine sediments decreased owing to the increase in water level and discharge in the reservoir area. When the inflow was higher than 30,000 m3/s during the flooding season (May–October), the discharge ratio of fine and coarse sediments increased and remained stable, respectively. After the TGR operation commenced, the sediment reduction within 954.4 km downstream of the dam resulted in a long-distance cumulative scouring. However, coarse sediments were not considerably recovered owing to the gradual coarsening of the riverbed. The results of this study help clarify the influences of large dams on the sediment transport in alluvial river systems. This study provides an understanding of the effects of human activities in the evolution of the water–sediment-geomorphology system in rivers.