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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]

The presently active Yellow River (Huanghe) delta lobe has been formed since 1976 when the river was artificially diverted. The process and driving forces of morphological evolution of the present delta lobe still remain unclear. Here we examined the stepwise morphological evolution of the active Yellow River delta lobe including both the subaerial and the subaqueous components, and illustrated the critical roles of riverine discharge and sediment grain size in dominating the deltaic evolution. The critical sediment loads for maintaining the delta stability were also calculated from water discharge and sediment load measured at station Lijin, the last gauging station approximately 100 km upstream from the river mouth. The results indicated that the development of active delta lobe including both subaerial and subaqueous components has experienced four sequential stages. During the first stage (1976–1981) after the channel migration, the unchannelized river flow enhanced deposition within the channel and floodplain between Lijin station and the river mouth. Therefore, the critical sediment supply calculated by the river inputs obtained from station Lijin was the highest. However, the actual sediment load at this stage (0.84 Gt/yr) was more than twice of the critical sediment load (~ 0.35 Gt/yr) for sustaining the active subaerial area, which favored a rapid seaward progradation of the Yellow River subaerial delta. During the second stage (1981–1996), the engineering-facilitated channelized river flow and the increase in median grain size of suspended sediment delivered to the sea resulted in the critical sediment load for keeping the delta stability deceasing to 0.29 Gt/yr. The active delta lobe still gradually prograded seaward at an accretion rate of 11.9 km 2 /yr at this stage as the annual sediment load at Lijin station was 0.55 Gt/yr. From 1996 to 2002, the critical sediment load further decreased to 0.15 Gt/yr with the sediment grain size increased to 22.5 μm; however, the delta suffered net erosion because of the insufficient sediment supply (0.11 Gt/yr). In the most recent stage (2002 − 2013), the intensive scouring of the lower river channel induced by the dam regulation provided relatively coarser sediment, which effectively reduced the critical sediment load to 0.06 Gt/yr, much lower than the corresponding sediment load at Lijin station (~ 0.16 Gt/yr). Consequently, the subaerial Yellow River delta transitioned to a slight accretion phase. Overall, the evolution of the active Yellow River delta is highly correlated to riverine water and sediment discharge. The sediment supply for keeping the subaerial delta stability is inconstant and varying with the river channel morphology and sediment grain size. We conclude that the human-impacted riverine sediment discharge and grain-size composition play dominant roles in the stepwise morphological evolution of the active delta lobe. [ABSTRACT FROM AUTHOR]

The temporal variations of sea surface chlorophyll- a concentration off the Yellow River Mouth were investigated using SeaWiFS and MODIS data over the period of 1997–2012. And the complex interplay of different dominant factors (e.g. river inputs and coastal dynamics) on the phytoplankton dynamics in different seasons was discussed. High concentration of nutrients from the Yellow River inputs plays a significant role in sustaining phytoplankton growth. Approximately 59% of the interannual variations of the chlorophyll- a concentration off the Yellow River Mouth were attributed to river inputs (water discharge, sediment load and median grain size of sediment particles). On seasonal time scale, the Yellow River discharge of water and nutrients is a major factor that influences the increased chlorophyll- a concentration from August to October (summer and autumn). From February to May, the stronger winds and enhanced wave actions resuspend and vertically mix the previously deposited riverine sediment and the associated nutrient, thus possibly contributing to higher chlorophyll- a concentration. [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]

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]