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Highlights • Morphological changes in the lower Yellow River channel during 1992–2012 is evaluated. • Reservoirs regulation altered scouring and deposition pattern of the channel. • An armor layer on the riverbed reduces the erosion flux significantly. • Riverbed erosion became an important source of sediments entering the sea. Abstract The lower reaches of the Yellow River (Huanghe), a typical "raised bed river," have experienced many huge flood events in Chinese history. As the main factor controlling the topographic evolution of the channel, the water and sediment-supply conditions of the Yellow River have undergone significant changes since the implementation of the water and sediment regulation scheme (WSRS) in 2002 by the joint operation of large reservoirs. Hence, it is necessary to conduct systematic research and evaluation on how the regulation scheme influences the scouring and deposition of the downstream channel. This study used the elevation data of 79 fixed profiles collected between 1992 and 2012 from the channel downstream from Gaocun in the Yellow River to establish a digital elevation model. The elevation model was used to investigate the spatio-temporal characteristics of channel scouring and deposition in the study area and to determine the main controlling factors. The results show that on an inter-annual scale, the channel scouring and deposition pattern began to change from substantial deposition to rapid erosion after 2002. On a seasonal scale, the overall characteristic was erosion in the flood season and deposition in the dry season; however, the erosion rate in the flood season increased and the deposition rate in the dry season decreased after 2002. From the spatial perspective, the downstream riverbed experienced net deposition before 2002 and net scouring after 2002. The WSRS altered the regimes of water and sediment of the river and therefore dominated scouring and deposition in the downstream channel. Since the implementation of the WSRS, continuous channel scouring in the study area provided about 41% of coarse-grained sediments for the Yellow River, and formed an important source of sediments entering the sea. However, because of the formation of an armor layer of coarse-grained sediments on the downstream riverbed surface due to continuous scouring after the implementation of the WSRS, the erosion flux of the river channel in the lower reaches of the Yellow River has shown a significant decrease since 2005. This study provides a typical case study for the response of the topographic evolution of rivers to water and sediment changes under the influence of intense human activities. [ABSTRACT FROM AUTHOR]

The modern Huanghe (Yellow River) delta sedimentary complex has developed since 1855, when the lower river channel migrated northward from the Yellow Sea to the Bohai Sea. As a result, the river-laden sediment accumulated rapidly on the lowlying plain and formed the mega-delta. Here we present high-resolution sedimentary sequences based on 137 Cs and 210 Pb dating and grain-size parameters of two sediment gravity cores collected near the present river mouth (core A11) and in adjacent Laizhou Bay (core A26). Based on these sedimentary sequences, the different responses of the sediment records to natural and artificial channel shifts are presented. The average sedimentation rates of the two cores A11 (1951–2006) and A26 (1808–2006) were estimated to be 2.25 cm/year and 0.80 cm/year, respectively. The results indicated that the geometry of the delta (e.g. the location of river mouth and changing coastline) and sediment supply from the river played an important role in the sedimentation of the subaqueous delta. The channel shifts in 1976 and 1996 shortened the distances from the river mouth to the location of core A11, resulting in the local accumulations of relatively coarse particles from the river mouth. The sedimentation of core A26 indicated four major channel shifts in 1855, 1904, 1947 and 1976. When the river mouth approached the core location, the accumulated sediment became finer; otherwise, the active resuspension due to strong hydrodynamics resulted in the accumulation of coarser sediment. The sediment records preserved in the two gravity cores illustrated different sedimentary responses to the lower channel shifts of the Huanghe and sediment supply from the river at centennial scales, which is critical to understanding the evolution of the modern Huanghe Delta in the past and to predicting the future trend. [ABSTRACT FROM AUTHOR]

The water-sediment regulation scheme (WSRS), beginning in 2002, is an unprecedented engineering effort to manage the Yellow River with the aims to mitigate the siltation both in the lower river channel and within the Xiaolangdi Reservoir utilizing the dam-regulated flood water. Ten years after its initial implementation, multi-disciplinary indicators allow us to offer a comprehensive review of this human intervention on a river-coastal system. The WSRS generally achieved its objective, including bed erosion in the lower reaches with increasing capacity for flood discharge and the mitigation of reservoir siltation. However, the WSRS presented unexpected disturbances on the delta and coastal system. Increasing grain size of suspended sediment and decreasing suspended sediment concentration at the river mouth resulted in a regime shift of sediment transport patterns that enhanced the disequilibrium of the delta. The WSRS induced an impulse delivery of nutrients and pollutants within a short period (~ 20 days), which together with the altered hydrological cycle, impacted the estuarine and coastal ecosystem. We expect that the sediment yield from the loess region in the future will decrease due to soil-conservation practices, and the lower channel erosion will also decrease as the riverbed armors with coarser sediment. These, in combination with uncertain water discharge concomitant with climate change, increasing water demands and delta subsidence, will put the delta and coastal ocean at high environmental risks. In the context of global change, this work depicts a scenario of human impacts in the river basin that were transferred along the hydrological pathway to the coastal system and remotely transformed the different components of coastal environment. The synthesis review of the WSRS indicates that an integrated management of the river-coast continuum is crucially important for the sustainability of the entire river-delta system. The lessons learned from the WSRS in the Yellow River provide insights to the integrated management of large rivers worldwide. [ABSTRACT FROM AUTHOR]

Deltaic river systems including both active channels and abandoned channels, are of significant importance to understand the deltaic morphology, as well as to the coastal management and ecosystem maintenance. However, the healing process of the abandoned channel has not yet been well understood. Based on the sedimentary record and hydrographic surveys, the deposit architecture and sediment dynamics of a tide-dominated abandoned channel, the abandoned Qingshuigou channel of the Yellow River, are investigated. The abandoned Qingshuigou channel has been fully disconnected from the main river channel since an artificial diversion completed in 1996. Twenty years after the abandonment, sedimentation with an average thickness of >20 cm was identified in the abandoned channel, illustrating a prominent seaward thickening trend from middle to lower channel. At the present river mouth of the abandoned channel, sedimentation layer with a thickness of 38 cm well matched the sedimentation rates derived from both chronological framework (1.85 cm/yr derived from 210Pb) and sedimentary facies (1.90 cm/yr). These deposits are composed by fine-grained sediment and enriched with organic matters, differing largely from the river-dominated deposits before abandonment. Tidal delivery of the sediment resuspended in the shallow water nearby Qingshuigou river mouth dominated the refilling process of the abandoned channel. Meanwhile, severe erosion near the abandoned river mouth shortened the channel (23.5 km in 1996 versus 14.5 km in 2018) and induced a lateral migration of the tidal inlet. Due to seasonally varying hydrodynamics, the tide-dominated abandoned Qingshuigou channel converts from a sediment sink in summer seasons to both a source and a sink in winter seasons. • Since abandonment, Qingshuigou channel has received over 20 cm of sedimentation. • Abandoned channel deposits are finer with more organic matter than underlying bed. • Waves gradually shorten abandoned channel and cause estuary to migrate laterally. • The tide-dominated abandoned channel is both a source and a sink for adjacent sea. [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]

The Yellow River has changed into a highly dam-controlled system over the recent decades. Based on gauging records of water-sediment discharge and particulate organic carbon (POC) datasets, this paper investigates the dam impacts on spatio-temporal variability of hydrology and POC transport. With a large part of suspended sediment and POC normally preserved within the Xiaolangdi (XLD) reservoir, erosion of riverbed downstream the reservoir has now become a primary source of sediment and POC to coastal ocean (172 Mt/yr for sediment and 0.56 Mt/yr for POC, respectively). Sediment accumulation and POC burial within the XLD reservoir are 246 Mt/yr and 0.60 Mt/yr, respectively, globally accounting for ∼10% and ∼1% of the retentions in reservoirs. Downstream the XLD reservoir, the averaged Δ14C values of POC was −451 ± 25‰, corresponding to a 14C age of 4763 ± 366 yr BP, indicating that the POC in the lower reaches of the Yellow River was ∼2400 yr younger than that in the Loess Plateau region where most of the sediment and POC are sourced. Petrogenic POC fluxes upstream and downstream the XLD dam were 0.12 Mt/yr and 0.06 Mt/yr, respectively, clearly indicative of large-scale trapping of petrogenic POC by the XLD dam. At present, the XLD dam regulation intercepts this petrogenic POC from the Loess Plateau and enhances the downstream input of biospheric POC (Fm = 0.85) to the coast. In contrast to the sediment and POC delivery of the rivers worldwide that are primarily dominated by tectonic, climatic and geographic settings, human activities, especially the dam regulation in the Yellow River, have significantly altered river hydrologic regime and overwhelmed the sediment and POC delivery within the river-coast continuum, which has profound impacts on carbon cycling in the coastal margin. [Display omitted] • The XLD reservoir acts as a sink of POC coupled with periodic rapid sediment accumulation. • Downstream riverbed erosion is a dominant source of POC to the coastal margin. • Dam regulation has substantially transformed the POC delivery during the WSRS. [ABSTRACT FROM AUTHOR]

Due to accelerated river-derived sediment deficits, global deltas are presently under an increasing risk of being submerged. To address the fundamental need to save Earth's deltas, boosting riverine sediment supply can be considered a reasonable option, but only if the new supply of sediment is prolonged and greater than the opposing forces: sea level rise, delta subsidence and compaction, and ocean energy. In July 2018, an artificial flood with a peak discharge of 3780 m3/s was released from the Xiaolangdi Reservoir, the last reservoir located on the mainstream of Yellow River, China. From July 03 to July 29, 412 million tons (Mt) of sediment was discharged out of the dam, including 240 Mt. that was previously sequestered within the reservoir. During this event, 140 Mt. of the escaped sediment directly reached the estuary. A total of 245 Mt. of sediment was deposited along the lower river channel bed, of which 83.6 Mt. was eroded once again and transported to the sea in the subsequent five months. This dam-released pulse, coupled with channel diversion and tidal shear fronts, nourished the deltaic area. New land was built, with the submarine offshore gaining up to 2.6 m in seafloor shoaling. Sediment impounded behind the dam, if properly released, could thus become a potential savior for sediment-starved deltas. • 240 Mt. of sediment previously trapped behind the dam was released during one flood. • 140 Mt. of sediment was directly delivered to the estuary during this 27-day event. • Both the subaerial and subaqueous parts of the Yellow River delta were nourished. [ABSTRACT FROM AUTHOR]

As an inevitable result of fluvial-deltaic avulsions, abandoned distributary channels and associated lobes form an important component of coastal marine systems, serving as a sediment source and sink for the adjacent delta region. The distinct differences between normal tidal flats and abandoned channel systems restrict the application of classic intertidal mud flat morphological theory to the study of the evolution of abandoned channels. Herein, a quantitative model based on the concept of mass conservation was proposed to describe the infilling processes for an abandoned channel. Sediment cores and elevation profiles from an abandoned channel of the Yellow River delta (China), along with hydrodynamic observational data, are used to constrain a numerical approach and test model performance. The modeled accumulation thicknesses are satisfyingly consistent with those derived from sediment cores. The model results indicate that the tide-delivered sediment flux and channel water depth are the dominant variables governing the variation in accumulation thickness in the abandoned channel, whereas the channel shortening rate largely impacts the depositional area rather than depositional thickness. By contributing to steady deposition on the mud flat, tide-associated landward sediment deposition formed a steep profile near the estuary, while the final four years of deposition represent only a quarter of the total deposition compared to that from the first five years owing to a decreasing depositional rate. The model-derived sediment volume indicates that shore-parallel flow is significant in shaping mudflat morphology, especially in the late stages, while the shortening of tidal channels leads to shrinking of the mud flat and sediment volumes. Despite its limitation to residual channels, our model presents a new approach for quantitative interpretation of the geomorphic evolution of an abandoned river channel with finite boundary conditions, providing a good reference to apply to other large deltaic systems. • A model was proposed to quantify the in-filling process of an abandoned channel. • Landward flux and channel depth are dominant variables for the in-filling process. • Shore-parallel flow shapes the mudflat morphology at the late stage of abandonment. • The abandoned channel acts as a sediment sink in the deltaic depositional system. [ABSTRACT FROM AUTHOR]