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• Human interventions affect the dynamics of meandering channels. • Unusual inner bank erosion and outer bank deposition occurred due to upstream damming. • Altered flow-sediment regimes and channel boundary drive the unusual meander dynamics. Human activities, such as reservoir construction and bank revetment, significantly alter downstream flow and sediment regimes and river channel changes, impacting river ecosystem services. However, our understanding of how intense human activities affect morphodynamics of meandering channels in big river systems remains incomplete due to the lack of high-resolution in-situ data. Here, we use comprehensive data encompassing hydrology and sediment, historical channel maps, and high-resolution river bathymetry to investigate the evolution of meandering bends in the Middle Yangtze River (MYR) across three distinct phases: the natural period (1490–1975), the bank revetment period (1975–2002), and the post-dam period (2003–2018). The study revealed that, in the absence of human intervention, the meandering channels of the MYR naturally experienced inner bank deposition and outer bank erosion, largely governed by curvature-induced secondary flow patterns in flow and sediment transport mode. In 1975–2002, outer-bank revetments reduced erodibility, stabilizing outer banks, while inner banks continued to experience lateral and vertical deposition, causing a narrower and deeper channel hydraulic geometry. Conversely, meandering bends had exhibited an unusual pattern of 'outer bank deposition and inner bank erosion' in 2003–2018. Channel morphological changes characterized by channel incision and widening were driven by changes from erodible bank boundaries to solidified bank boundaries, substantial reduction in suspended sediment concentration, and an altered and extended effective flow range (10,000–25,000 m3/s). Our findings highlight that human interventions such as damming and bank revetments can alter the conventional erosion–deposition morphodynamics of meandering bends in large river systems, carrying vital implications for sustainable river management in the Anthropocene. [ABSTRACT FROM AUTHOR]

Large reservoirs can considerably alter the water-sediment dynamics and morphology of alluvial rivers. Here we review the effects of two mega reservoirs operated with different regulation modes. The Three Gorges Reservoir (TGR) on the Yangtze River has a typical anti-seasonal regulation mode, while the Xiaolangdi Reservoir (XLDR) on the Yellow River undergoes a swift drawdown process shortly before the flood season through the Water and Sediment Regulation Scheme (WSRS). We examine the influence of these regulation schemes on downstream water-sediment dynamics and find that vast sedimentation occurred in both the TGR (128.4 Mt./yr) and XLDR (210.2 Mt./yr). The two rivers have experienced different changes in downstream sediment transport capacity, with coincident flood and sediment peaks in the Yangtze River but with sediment peaks lagging behind flood flow peaks in the Yellow River. On the Yangtze River, highly unsaturated flows from the TGR led to widespread incision downstream, while on the Yellow River, such flows did not induce significant erosion in the first two years following impoundment. The low annual runoff and high sediment yield of the Yellow River mean that riverbed erosion occurred mainly when the water discharge and sediment transport capacity were enhanced by the WSRS. In both rivers, the riverbed eroded and coarsened downstream of the mega dams, lowering the dry season water level. The increase in channel roughness maintained or even raised flood season water levels, potentially increasing flood risk. Sediment budgets reveal that the river segments downstream of the dams switched from sediment sinks to sources due to riverbed incision. Despite new supply from downstream reaches, sediment deficits arising from dam interception and other human activities within the drainage basins have posed significant challenges to the sustainability of the Yangtze and Yellow river deltas, resulting in lower accumulation rates or even transition from progradation to degradation in subaqueous areas. In contrast to the anti-seasonal regulation mode of the TGR, the WSRS of the XLDR has proven very effective at mitigating reservoir sedimentation and has boosted the quantity of sediment reaching the sea, facilitating delta stability and coastal sediment replenishment. [ABSTRACT FROM AUTHOR]

• • Thorough analysis of WSRS-induced impacts on the river channel and delta. • • The natural hydrological cycle process has been altered by the artificial floods. • • Reservoirs regulation scheme altered the erosion–deposition regime of the riverbed. • • New light is shed on saving the global sinking deltas. The water and sediment regulation schemes (WSRS) orientated by dams have profoundly adjusted the hydrological process and geomorphological evolution of the lower Yellow River and delta. Two decades since the implementation of WSRS, the observed multidisciplinary datasets allowed us to systematically evaluate the impacts of this unprecedented engineering effort on the river-coastal system. The sediment fluxes transported to the sea showed a notable rebounded and coarsened process due to the erosion of the riverbed during the WSRS periods. The average median grain size of suspended sediment increased from 17.7 μm in the 1990 s to 26 μm in 2002–2009. The natural rhythm of hydrological process is fundamentally altered by WSRS, about 50% and 30% of annual sediments and water delivered to the sea for only 20 days. The water discharge system is more temporally dynamic and complex with higher dimensionality after the operation of WSRS. The erosion pattern has been a dominant feature in all the channel segments downstream the Xiaolangdi Reservoir, with an average total erosion volume of 1.02 × 108 m3/a after the WSRS. However, the erosion efficiency of the lower river showed a decreased trend, due to the riverbed had formed a coarse armored substrate for the continued scouring since 2002. The YRD has changed from the pattern of erosion to progradation for the increase of sediment supply, and the average growth rate of subaerial delta was about 6.89 km2/a during 2003–2014. The lessons learned from the operation of WSRS may provide a valuable reference for the management of global large river systems. [ABSTRACT FROM AUTHOR]