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.

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Despite abundant studies on gravel‐sand transitions (GSTs), GST origin and migration in large alluvial channels characterized by significant variations in width, numerous tributary (or distributary) streams, and frequent human disturbances remain difficult to explain. Here, we take the Yangtze River as an example and use field observations and numerical modeling to make progress toward addressing GST origin and migration. The results show that the Yangtze River exhibits a non‐abrupt GST approximately 60 km long. Within the GST and the adjacent area, there are two dramatic decreases in shear stress; the upstream one induces the settlement of sand from suspension, explaining the emergence of the Yangtze River GST, whereas the downstream one induces general cessation of gravel movement. However, the deposited gravel was mostly overwhelmed by sand. We argue that these changes in shear stress in the Yangtze River depended primarily on the combined effects of width variability and distributary systems. Since 1970, the Jingjiang Meander Cut‐off, the Gezhouba Dam, and the Three Gorges Dam (TGD) have been completed in succession, repeatedly altering the flow‐sediment regime within the GST. However, the GST position remained unchanged until the TGD began operation in 2003. The stability of the GST was primarily ascribed to the sufficient upstream sand supply (grain size <0.5 mm), the resistant lithologies upstream of Zhicheng, and the Jingjiang Great Levees constraints.