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.

Half a century ago, Leopold (1956) predicted that dams would someday become so numerous on American rivers that they would be the primary factor controlling the characteristics of river channels. His prediction is becoming true for river systems worldwide, with 𝐴𝐴 𝐴 45,000 large dams (height 𝐴𝐴 𝐴 15 m) distributed over 140 countries (Ma et al., 2022;Syvitski et al., 2022). Dams are often designed to generate hydropower and mitigate floods (Li et al., 2022;Ma et al., 2022), but they also disrupt river continuity and induce drastic temporal and spatial alterations in river flow and sediment regimes (

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Investigating river bars and their vegetation dynamics in response to upstream damming is important for riverine flood management and ecological assessment. However, our mechanical understanding of the damming-induced changes in river bar and vegetation, such as bar area, morphology, and leaf area index (LAI), remains limited for large river systems. Leveraging satellite images and in situ observed hydrogeomorphic data from, we improve a machine learning-based LAI inversion model to quantify variations in river bar morphology, vegetation distribution, and LAI in the Middle Yangtze River (MYR) following the operation of the Three Gorges Dam (TGD). Then we analyze the mechanisms controlling the bar and vegetation dynamics based on high-resolution river cross-sectional profiles as well as daily discharge, water levels, and sediment in both the pre- and post-TGD periods. Our results indicate that the river bar area decreased by approximately 10% from 2003 to 2020, while the vegetation area and average LAI of these bars increased by >50% and >20%, respectively. Moreover, the plant community on most river bars tended to expand from the bar tail to the bar head and from the edge to the center. The main factor driving vegetation expansion in the MYR after the TGD’s operation was the reduction in bar submergence frequency (by 55%), along with a slight bar erosion. Further analysis revealed that the standard deviation of annual discharge decreased by approximately 37%, and the frequency of vegetation-erosive flow decreased by approximately 74%. Our data highlight the potential impact of large dams downstream flow regimes and vegetation encroachement. Such findings further the understanding of the biogeomorphological impacts of large dams on the river bar vegetation and have important implications for riverine plant flux estimatin, flood management and ecological restoration in dammed river systems.