Mega-reservoir regulation: A comparative study on downstream responses of the Yangtze and Yellow rivers

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.