Water-sediment Regulation Research Articles

Effect of water-sediment regulation operation on sediment grain size and nutrient content in the lower Yellow River

The Yellow River (YR) is famous for its high sediment concentration. The Xiaolangdi dam (XLD), the most downstream dam on the mainstream of the YR, plays an integral role in siltation mitigation, flood defence, agriculture, and hydropower generation. To alleviate siltation of the Xiaolangdi Reservoir (XLDR) and riverbed uplift of the lower reaches of the YR and strengthen flood control safety downstream, the XLDR began to implement water-sediment regulation in 2002. Water-sediment regulation (WSR) has significantly changed the hydrological processes, erosion and deposition, and sediment transport in the downstream channel, and the downstream water environmental conditions have also changed. In this study, 10 sections in the Lower Yellow River (LYR, from the XLDR to the Lijin section) and 4 sections in the soft floodplain area were sampled in the field to analyse their surface sediment grain size composition and nutrient content. At the same time, data on the average annual median particle sizes (D50) of channel sediments and suspended particles in 7 typical sections in the LYR from 2004 to 2015 were statistically analysed. The results show that the channel sediment grain size in the LYR decreased along the river course, and the D50 ranged from 0.20 mm in the HYK section to 0.08 mm in the LJ section. Moreover the D50 of the suspended sediment was less than 0.05 mm. The D50 of the channel and suspended sediment were both ranked as follows: non-flood season > flood season > WSR period. After WSR, fine silt particles with diameters of 0.01–0.05 mm were deposited on the soft floodplain. The channel sediment size was mainly coarse particles with diameters of 0.05–0.20 mm due to channel erosion. The suspension of fine silt particles due to flood scouring led to the total organic carbon (TOC) and total nitrogen (TN) absorbed in the particles to also be resuspended in the water. As a result, the TOC and TN contents in the channel sediments decreased by approximately 40% after WSR.

Reducing the likelihood of carbon loss from wetlands by improving the spatial connections between high carbon patches

With stores that are more than 50 times greater than in terrestrial ecosystems, riverine and coastal ecosystems are the most important carbon (C) stocks worldwide, with most C stored in wetlands. Wetlands are, however, among the most threatened ecosystems worldwide, and face pressures from hydropower development, flow regime alteration, and land use/cover change (LUCC). Despite great efforts to protect and restore wetlands, anthropogenic drivers (e.g., land reclamation) still result in more C loss than gain in most cases. Of these, LUCC is driven by social and economic processes, and is a self-organizing, path-dependent phenomenon. Therefore, we proposed a land use management framework with which to analyze the spatial patterns in the drivers of LUCC, and then verified the strategy using two study areas in the lower reaches and delta of the Yellow River. The first step of the process was to analyze LUCC from historic land use/cover maps (from 1995 to 2015 for the lower river reaches and from 1970 to 2015 for the delta), and identified a suitable land use (waterbody) for the connections. We calibrated and validated the DINAMICA model using data for changes in C. We then set up different scenarios and created connections between high C patches. The simulation results showed that even slight modifications in the connections with water could trigger noticeable changes in the spatial patterns of C gain and loss, and that original hotspots of C loss could be converted to areas of C gain in some cases. Our findings highlight the need to consider both spatial patterns and drivers of LUCC when protecting wetlands and show that water-sediment regulation in the Yellow River should be coordinated with dynamic changes in the landscape in the lower reaches and delta.

Impact of human intervention on channel shrinkage and restoration in the Huanghe Estuary

Human intervention has become a globally dominant factor in the morphological evolution of estuary channels in recent decades. This paper analyzes seasonal morphological changes in the Huanghe estuarine channel for a 37-year period from 1976 to 2012 as a measure of estuarine shrinkage and restoration. The results show that the channel underwent four stages of development: enlargement (1976–1985), a gradual decreased area (1986–1994), shrinkage (1995–2001), and restoration (2002–2012). Generally, changes in the channel transect geometry during the first stage are regulated by natural fluctuations in water and sediment discharges, but the other three stages are impacted mainly by human intervention. The decreased runoff and sediment discharge caused by dam construction and water and soil conservation are the main reasons for the decrease and shrinkage of the channel transect. During the fourth stage, the increased ratio between water and sediment due to new reservoir construction and the water–sediment regulation scheme (WSRS) progressively affects runoff discharge, producing remarkable impacts on channel morphology during restoration. Human intervention has varying influences on different channel transects. The upstream channel section is more stable than the downstream channel, and human intervention in the downstream unstable channel has a 50% greater effect than that in the upstream stable channel. The WSRS has greater effects on the downstream unstable channel than on the upstream stable channel; however, a higher water–sediment ratio during the restoration stage results in greater impacts on the upstream stable channel than on the downstream unstable channel. Seasonal changes in the channel geometry indicate that seasonal adjustment is important for channel self-adjustment and that human intervention enhances variations in seasonal transect areas and impacts channel morphology.

Water conservancy project on the Yellow River modifies the seasonal variation of Chlorophyll-a in the Bohai Sea

The Water Sediment Regulation Scheme (WSRS) is a unique engineering measure that has been regularly performed to reduce reservoir sedimentation and increase the flood capacity of the Yellow River in China since 2002. As a side effect, the WSRS greatly increases the monthly input flux of nutrients to the Bohai Sea (BHS) in summer, potentially exacerbating eutrophication levels therein and subsequently affecting the growth of phytoplankton. However, its influence on the Chlorophyll-a (Chl-a) dynamics over the BHS is still poorly understood. In this study, two approaches were adopted to investigate it: 1) long-term in-situ observations and satellite-derived data of surface Chl-a were used to study its seasonal variations before and since 2002, and 2) one 1D physical-biological coupled model was developed to evaluate the impact of WSRS on seasonal Chl-a. The results showed that the surface Chl-a exhibited two peaks in spring and autumn until 2002, but has exhibited only one peak in spring-summer since 2002. Satellite-derived Chl-a concentrations in spring-summer since 2002 have increased by 56% compared to those until 2002. The simulated results showed that the change in Yellow River discharge induced by the WSRS has resulted in the appearance of high concentrations of Chl-a in summer over the Central Bohai Sea since 2002. The WSRS increased the ratio of added Chl-a owing to the riverine nutrients to total Chl-a by 19% compared to that until 2002. Overall, WSRS greatly affects the seasonal cycling of Chl-a in the Bohai Sea, and the side effect needs to be considered.

Adaptation of Suaeda salsa to water/sediment conditions and nitrogen input in tidal flat wetlands in the Yellow River Delta, China

The application of Water-Sediment Regulation Project provides abundant freshwater for the Yellow River Delta, changes water and sediment condition, as well as brings lots of exogenous substances. Using orthogonal test with three factors and four levels, we examined the effects of water condition, sediment burial depth and exogenous nitrogen input on the growth of wetland plant, Suaeda salsa. The results showed that sediment burial had great effect on protein content and SOD activity. Nitrogen input had great effect on POD activity. CAT activity was not affected by sediment burial, nitrogen input and water depth. The water depth manipulation had significant effect on leaf, stem and total dry weight. With the increases of water depth, leaf, stem and total dry weight showed a decreasing trend, with the maximum values (25.70, 40.86, 69.73 g) at the 2 cm water depth. There was no effect of nitrogen input and sediment burial on dry weight. The results of range analysis showed that the effect of water depth on leaf, stem, root and total dry weight was great, and followed by nitrogen input and sediment burial, with an optimal combination of 2 cm water depth +12 cm sediment burial + 9 g·m-2 nitrogen input. These findings suggested that water condition played a decisive role in affecting the growth of S. salsa. Consequently, more attention should be paid to the control of water depth in the process of water and sediment regulation.

Impact of Water-Sediment Regulation on Variations of Amino Acids in the Middle-Lower Yellow River, China

In order to examine the impacts of water-sediment regulation on regional carbon cycling, we collected water, particulate and sediment samples from the middle-lower Yellow River in late June and early July, 2015 and analyzed their specific amino acids (AA), DOC, POC, and bacteria abundance. Summarized by 14 specific AA, the total hydrolysable AA (THAA), particulate AA (PAA), and sediment AA (SAA) varied in ranges of 2.29–9.05 µmol L−1, 5.22–22.96 µmol L−1, and 81.7–137.19 µg g−1 dry weight. After the regulation, dissolved free AA (DFAA) decreased by 29% while DCAA increased by 72%. These variations suggested that DFAA were further degraded, while DCAA molecules were further activated. Meanwhile, PAA increased almost 4 times as many as those before regulation, and SAA increased as well. After regulation, the amounts of bioactive amino acids (Asp, Glu and Gly) increased in THAA but decreased in PAA, with little changes in SAA. The ratios of Asp/Gly in different phases increased after regulation, indicating the AA contributions were promoted by calcareous organisms rather than by siliceous organisms. Multiple correlation analysis showed that PAA was primary representatives of AA and organic carbon, followed by DCAA and POC. Moreover, bacterial reproduction played a key role in shaping the AA compositions and properties, followed by the redox condition and acid-base balance. The results of this study provided a clear evidence for the effects of water-sediment regulation on regional biogeochemistry of organic carbon in the middle-lower Yellow River.

Spatiotemporal dynamics of suspended particulate matter in the Yellow River Estuary, China during the past two decades based on time-series Landsat and Sentinel-2 data

Twenty-two years of suspended particulate matter (SPM) concentrations in the Yellow River estuary and adjacent sea, China were derived from 532 Landsat and Sentinel 2A/B satellite images. Optimal SPM retrieval model was selected by comparing five state-of-art models using 79 in-situ datasets and recalibrated to ensure consistency among multiple-sensor-derived SPM concentrations. SPM in the estuary, in South Bohai Bay, and Laizhou Bay exhibited distinct temporal variations. 73% and 52% of the interannual and monthly SPM variations near the river mouth were explained by riverine water and sediment discharge, showing impact of the operation of the Xiaolangdi Reservoir and Water-Sediment Regulation Scheme. Land area accretion and erosion in river delta are associated with SPM variation. Riverine impacts on SPM rapidly declined off-shore because of the rapid deposition of the coarse-grain sediment. Ocean current and wind-wave forces explained high concentrations and intra-annual variations of SPM in the South Bohai Bay and Laizhou Bay.

Optimize multi-objective transformation rules of water-sediment regulation for cascade reservoirs in the Upper Yellow River of China

Frequent occurrence of flood and ice disasters caused by river channel sedimentation in the Upper Yellow River of China has seriously threatened the lives and property of downstream residents, which has become the most challenging issues in harnessing the Yellow River. In this paper, we aim at optimizing water-sediment regulation that considers two cascade reservoirs (Longyangxia and Liujiaxia) as the regulatory bodies using one multi-objective model (sediment transport and hydropower generation) and two single-objective models (sediment transport only, and hydropower generation only). We propose an innovative approach (FSS-MOPSO) that hybrids the Feasible Search Space (FSS) with the Multi-Objective Particle Swarm Optimization (MOPSO) algorithm to search the optimal solutions for the multi-objective joint operation of cascade reservoirs. We analyze the transformation rules of water-sediment regulation among five objectives (hydropower generation, sediment transport, water supply, flood control and ice control) under various optimization schemes. The results indicate that the conflict between the hydropower generation objective and the sediment transport objective is prominent. An extreme case indicates that an increase in hydropower output by 2.31 billion kW·h (17.6% increase) would greatly reduce the amount of sediment transport (73.5% decrease) while only makes little effects on the other three objectives. The results demonstrate that the optimal water-sediment regulation not only can ensure to meet water demands in the future (2030) but can provide an important guideline to safely operate cascade reservoirs during ice and flood periods. The research findings contribute to the identification of the relationship among objectives and strategy recommendations on water-sediment regulation for efficient cascade reservoirs operation in the Upper Yellow River.

Long-term water-sediment multi-objectives regulation of cascade reservoirs: A case study in the Upper Yellow River, China

Water-sediment regulation (WSR) is an effective non-engineering measure to alleviate the problem of suspended river and bring benefit to flood control security in sediment-laden river. However, WSR may decrease the socio-economic benefit of reservoirs, for example, reduction of hydropower production and water supply. In order to satisfy the practical requirement of WSR and other utilization objectives, this paper presents a multi-objective operation model for a cascade reservoirs simultaneously considering the maximization water volume for WSR and power generation and water supply, as well as various complex constraints. Then, the Non-dominated Sorting Genetic Algorithm (NSGA-II) is improved to solve the aforementioned model and key control indicators of WSR are analyzed. Meanwhile, a sediment transport model has been introduced to quantify the effect of WSR. The models are applied to the cascade reservoirs in the Upper Yellow River. The following conclusion can be drawn from results (1) Pareto fronts of the model solution demonstrate a strong competition between WSR and water supply, water supply and power generation, a low sensitivity between WSR and power generation; (2) the ability of WSR in Upper Yellow River is 6 times in 24 years, which means the frequency of WSR is four years averagely; (3) 233.77 million tons of sediments are transported by long-term WSR in the Ningxia-Inner Mongolia reaches, account for 19.10% of sediment deposition; (4) the risk-free conditions of LYX and LJX reservoirs’ water volume for WSR are 137.42 × 108 m3 and 41.08 × 108 m3, respectively, which could be used as a reference in actual operation. The research results have an important practical significance and application for sediment control and governance of suspended river, and the multi-objective operation model of WSR proposed in this study can be effectively and suitably used in sediment regulation with similar conditions.

Spatial variations of arsenic and heavy metal pollutants before and after the water-sediment regulation in the wetland sediments of the Yellow River Estuary, China

To investigate the effects of the water-sediment regulation on the spatial variations of metals in the surface sediments of the Yellow River Estuary (YRE), sampling sites were set in 2015 (51 sites in June and 33 in October). The content of Cu, Pb, Cd, Cr, Zn, Ni and Mn was determined using inductively coupled plasma spectrometry and of Hg and As atomic fluorescence spectrometry. The results showed that the concentrations of metals in the sediments of YRE were lower after the regulation, while that of most metals increased in the tidal areas of the current estuary with fine particles. Environmental pollution and ecological risk of metals were mainly from Hg and Cd. Metals carried with the silt of the river should be focused. The findings were expected to update the current status of metal pollution of YRE and be helpful for the delicacy management of the regime and silt-laden rivers.

Whole-processes modeling of flow movement and sediment transport during the period of water-sediment regulation in Xiaolangdi Reservoir

A one-dimensional coupled model has been proposed to consider the flow exchanges between main stream and tributaries during the alternative calculations between open-channel flow and turbidity currents, which is capable of simulating the whole process of flow and sediment transport covering these three different physical factors during the period of water-sediment regulation in the Xiaolangdi Reservoir. The modified storage-cell method was adopted for a complex tributary system to calculate the net flow exchanges caused by the water surface variation, and the discharge formula considering the effect of bottom slope of tributaries was applied to calculate the turbidity currents intrusion. Meanwhile, the plunge criterion of turbidity is used to connect the calculations of open-channel flow and turbidity currents, the static water surface assumption was no longer used so as to couple the interaction between the upper clear water and lower turbidity currents. The whole event of water-sediment regulation conducted in the Xiaolangdi Reservoir in 2002 was used for parameter calibration and the event of 2012 was simulated using both the proposed coupled model (Model I) and the model without considering the flow exchanges (Model II). The results show that the reservoir can be divided into three regions: the upper region with no tributary, the middle region with just open-channel flow and the lower region with turbidity currents, where both contain tributaries. Model I presented the results to closely agree with the measured processes in three regions. Among which the upper region is the only region the results calculated by Model II are as precise as Model I, However, with effects of the flow exchanges on the sediment transport and the conservation of water mass and flow momentum being omitted, Model II presented incorrect predictions in the lower two regions where there are tributaries: the calculated water level, discharge and sediment concentration in the middle region are rather high during the occasion of water intrusion and rather small during the occasion of water recharge; in the lower region, the location and occurrence of the plunge point were predicted to be delayed, and these turbidity factors were overestimated, covering interface level, discharge, flow velocity and sediment concentration, correspondingly, the sediment delivery ratio are rather large. Therefore, the effect caused by the tributary system is so enormous that it should not be neglected in the simulation. In addition, a detailed sensitivity analysis of coefficients in the turbidity module was also conducted in order to investigate the effects of these coefficients on the model predictions, with the proper variation range being determined for each coefficient. It indicates that the effects on the calculated factors caused by the drag coefficient at interface, the resistance coefficient of intrusion and the mixing coefficient between clear water and turbidity decreased in turn. Besides, with an increase in drag coefficient, the model response shows a time lag at a certain extent; with the bottom slope of tributaries not being constant, the effect of resistance coefficient requires a further discussion based on a specific slope.

Impact of a water–sediment regulation scheme on the hydrodynamics and sediment conditions in the Sheyang Estuary

Tidal asymmetry (i.e., unequal ebb and flood intensities and durations) is a dominant factor in the dynamic equilibrium of current-related sediment transport in estuaries. The evolution of estuarine tides and sediment transport is of great importance to the development and utilization of channels in estuaries. Based on field hydrological and sedimentological data collected over multiple years, the hydrodynamic characteristics, suspended sediment concentration (SSC) and grain size, and median grain size of seabed sediments were analyzed to investigate the effects of the construction of a water–sediment regulation scheme (WSRS) in the Sheyang Estuary in China. Our results showed that small grain sizes (fine sand, silt, and clay) dominated and that suspension was the dominant transport mechanism in the area where tidal asymmetry was greatest. Tidal distortion occurred due to the construction of the WSRS. In this flood-dominant system (i.e., before and after the construction of phase I of the WSRS), the flood current velocity was significantly greater than the ebb current velocity, while it also had a shorter duration that caused higher SSCs during the flood current. This likely induced net sediment import to the estuary, which was the main cause of estuary infilling. Furthermore, the net transport of sediment into the estuary caused by the high flood current velocities resulted in the gradual deposition of sediments in the shipping channel of Sheyang Port. Ebb-dominant conditions (i.e., after the construction of phase II of the WSRS) induced net sediment export from the estuary, and sedimentation was significantly reduced. Since the ebb–flood current velocity ratio (Rv) and ebb–flood mean SSC ratio (Rs) could reflect tidal asymmetry and the direction of residual sediment transport with and without the WSRSs, they were used as important and sensitive indices to investigate hydraulic and sedimentological conditions. Results from this study can be used to improve the monitoring and management of navigation channels by providing insights into new methods of preventing or reducing sediment deposition in estuarine channels (i.e., by employing Rv and Rs).

Impact of water-sediment regulation on the transport of heavy metals from the Yellow River to the sea in 2015

Variations in particulate and dissolved heavy metal (Cu, Co, Ni, Zn, Pb, Cr, Cd and As) concentrations and fluxes as well as their response to the Water-Sediment Regulation Scheme (WSRS) process in 2015 were studied based on a daily water and sediment survey at Lijin gauge. The results showed that the water and sediment flux increased rapidly in the first stage of the WSRS, which was characterized by high water discharge and suspended sediment concentration (SSC). The suspended sediment was coarser than the natural state, while the particulate and dissolved heavy metal contents increased. In the second stage, the SSC decreased rapidly followed by a gradual reduction in water discharge, the suspended sediment became even coarser and both the particulate and dissolved metal contents showed a decreasing trend. The heavy metal flux during the WSRS period accounted for 42%–54% of the total year, and the transport form was dominated by the particulate form. Dissolved metal contents were affected by the release of heavy metals derived from the channel in the lower reaches, while particulate heavy metals mainly came from erosion of the riverbed and their contents were much lower than Xiaolangdi reservoir sediment. Heavy metal transportation was influenced significantly by the WSRS process. Changes in sediment flux resulted in significant differences in the flux of heavy metals and the distribution of metals in different transport forms between the first and second stage of WSRS.

A red clay layer in soils of the Yellow River Delta: Occurrence, properties and implications for elemental budgets and biogeochemical cycles

Clay-enriched layers occurring in deltas are important as they reveal erosion, transportation and deposition of source materials from the basin to the coast. In the Yellow River Delta a red clay layer (RCL) with a thickness of 5–50 cm at 1 m depth in the soil profile occurs in the floodplain area dominated by fluvo-aquic soil derived from Yellow River sediments. The RCL typically has a median grain size of <20 μm, redness (a⁎) of >7, and relatively high contents of illite, calcite and iron oxides. In the Loess-Yellow River sediment-Yellow River Delta soil continuum a regime of weathering, mechanical sorting and dilution by carbonates and silica have been important factors altering the elemental geochemistry. The abundance of secondary minerals and deficiencies of silica and zircon in the RCL suggest that it was separated from a mixed source and was likely derived from highly weathered sediments (such as paleosol) from the basin. RCL samples show substantial accumulation of inorganic carbon with enhancement of the organic carbon content, indicating that increasing organic carbon in alkaline soils may lead to an increase in inorganic carbon in the subsoil. Large variations in elemental ratios have been found in the RCL characterized by excess nitrogen and trace metals and phosphorus deficiency due to adsorption and weathering effects. Implementation of the water-sediment regulation scheme (WSRS) may have altered the sedimentary environment and may have aggravated an elemental imbalance due to the occurrence of the yellow silt layer (YSL)-RCL sequence and thereby impacted biogeochemical cycling in the river-delta-coast continuum.

Radium isotopes–suspended sediment relationships in a muddy river

Radium isotopes are known to be excellent geochemical tracers for study of oceanographic processes. We show here that radium isotopes can also be used to assess adsorption/desorption and transport processes in rivers. The Yellow River (Huanghe), one of the longest, most turbid and heavily regulated rivers in the world, is used as an example. We first investigated the temporal and spatial behavior of radium isotopes (224Ra and 226Ra) in the lower reaches of the river, and found that this zone displayed some of the highest known riverine radium concentrations and fluxes in the world. Suspended particulate matter (SPM) is shown to be the dominant factor controlling radium activities. Laboratory simulation experiments showed that radium desorption from SPM obeys an exponential relationship in fresh water (S = 0). When salinities are >10, the increase in radium concentration follows a linear increase with respect to the amounts of SPM added. Significantly higher radium concentrations (3–5 times), especially for short-lived 224Ra, were observed during the “Water-Sediment Regulation Scheme” (WSRS), an annual management event when ∼15%–55% of the annual water discharge and ∼30%–75% of the annual sediment load are released from a reservoir to control sedimentation in the Yellow River. The radium fluxes during WSRS periods (∼2 weeks long) accounted for more than half of the entire annual load during the periods studied. Sediment erosion and pore water release are also thought to be important processes supplying radium to the river. After a WSRS, Ra desorption from SPM increases and becomes the prevailing process.

Effects of Sediment Burial and Exogenous Cd Input on Biomass Allocation and Antioxidative Enzyme Activities of Suaeda salsa in the Coastal Wetland of the Yellow River Delta

The Yellow River Delta has been facing the threat of functional degradation during the recent years. The Water-Sediment Regulation Project not only supplements abundant freshwater, but also alters the sediment burial and heavy metal levels, which affects vegetation growth. Thus, we selected the pioneer species Suaeda salsa, to study the effects of different sediment burial depths (0, 3, 6, 12 cm) and exogenous Cd inputs (0, 0.5, 1.0, 1.5 mg·kg-1) on biomass allocation and activities of antioxidative enzymes in the coastal wetlands of the Yellow River delta. The results showed that a shallow or moderate burial depth had a stimulatory effect on chlorophyll content, while an excessive burial depth inhibited the growth of Suaeda salsa and chlorophyll content. With increasing Cd input, chlorophyll content and dry mass decreased. At a lower Cd input and moderate burial depth, activities of CAT and SOD increased, and at high levels, SOD activities decreased, while activities of CAT at a 12 cm burial depth and 1.0 mg·kg-1, 1.5 mg·kg-1 Cd input were higher than those for the control (62.66% and 58.56%). CAT activities reached high values (15.76 U·mg-1) at a high Cd input (1.5 mg·kg-1) and burial depth (12 cm). Analysis of variance showed that Cd input had a significant effect on protein content, and CAT and SOD activities, and sediment burial depth had a significant effect on the protein content and SOD activities. Interaction between Cd input and sediment burial depth had a significant effect on CAT and SOD activities (P<0.05). These results demonstrated that sediment burial depth and Cd input had a great influence on the growth of Suaeda salsa, and to some extent, Suaeda salsa could change its biomass allocation and antioxidative enzyme activities to adapt to severe environments.

Morphological variability of the active Yellow River mouth under the new regime of riverine delivery

The Yellow River subaqueous delta (YRSD), once the most rapid depo-center among river deltas worldwide, has been under the risks of subsidence and degradation due to the new regime of riverine delivery affected by human interventions. Utilizing hydrologic and bathymetric surveying datasets, we examined the latest regime of river input from the perspective of water-sediment relationship, and the responding morphological evolutionary processes of active YRSD over a period of 20 years between 1996 and 2016. Results show that the new discharge regime is strongly interfered by the Water-Sediment Regulation Scheme (WSRS), characterized by a more drastic decline of sediment load than that of water discharge; more harmonious relationship between water and sediment discharges in the lower reach of the river to the sea; coarser sediment delivery and low suspended sediment concentration (SSC). We identified inverse erosion-accretion trends in the subaqueous region: net accretion of 0.15 m/yr in the active Yellow River mouth (AYRM) and severe erosion of −0.1 m/yr in the Gudong littoral zone (GDLZ). As the primary sink for sediment delivery, AYRM received approximately 68% of sediment delivery during the study period and sedimentation was mainly occurred in the shallower area where water depth was less than 10 m. In addition, recent morphological evolution of AYRM is found to have undergone through four stages, namely: moderate accretion (1996–2002), rapid accretion (2002–2007), reduced accretion (2007–2015) and rapid erosion (2015–2016). The new regime of riverine delivery presents multiple spatiotemporal scales in shaping deltaic morphology. Compared with the previous research, we present the morphological evolution of deltaic system over decadal timescale is strongly influenced by reduction of sediment supply derived from basin-scale human impacts, and the variability of subaqueous portion during the study period is closely related to inter-annual variability of river input. Besides, the building of AYRM is shaped by event-scale WSRS induced-floodwater, and decade-scale change of sediment pathway governed by frequent mouth channel migration. The results show, for the first time, that AYRM has experienced a significant erosion since the implementation of WSRS, with a decline of 99% sediment delivery in 2016 compared to the natural mode during 1950s. The results also indicate that to maintain the erosion-accretion balance of AYRM, an estimation of 41.4–62.3 Mt/yr sediment delivery should be kept. Due to the fluvial regime change from the natural to the highly human-regulated modes, the AYRM, as well as the whole YRSD, is expected to be transforming from the accretion to erosion states.

Changes and Driving Forces of the Water-Sediment Relationship in the Middle Reaches of the Hanjiang River

Riverine sediment transport plays an important role in the global geochemical cycle. With a growing interest in global riverine environmental changes, a better understanding of water-sediment relationship dynamics and their driving forces is crucial for basin management, which is particularly associated with cascade dam construction. In this contribution, a simple and effective sediment load reduction factor analysis is used to attribute the changes in river sediment load to different drivers. The Mann–Kendall (MK) trend test and the double-mass curve (DMC) method were combined to reveal the trends and causes of change in the water-sediment relationship from 1965 to 2015 in the middle reaches of the Hanjiang River. We found that sediment load trend decreased significantly, which was caused by a decrease in water yield (5.05%), and the relative contributions of precipitation decrease (1.66%). Furthermore, only one mutation could be identified in 1974 at Huangzhuang station. Evapotranspiration and vegetation coverage had slight decreasing effects on sediment load. The impact of human activities on the water-sediment relationship has intensified over the past 15 years. We therefore propose the establishment of an integrated basin-wide ecosystem and optimized reservoir operation rule for sustainable water use and sediment regulation.

Dramatic source-sink transition of N2O in the water level fluctuation zone of the Three Gorges Reservoir during flooding-drying processes.

Biogeochemical cycling of nitrous oxide (N2O), a significant greenhouse gas (GHG), can influence global climate change. The production and emission of N2O mediated by hydrological regimes is particularly active in water level fluctuation zones (WLFZs). However, the hydrological mechanisms affecting N2O transformation and production across the water-sediment micro-interface remain unclear. In this study, intact sediment cores from the WLFZs of the Three Gorges Reservoir (TGR) were incubated for 24days in a laboratory microcosm to identify the effects of the flooding-drying processes on the yield and emission of N2O. Results showed a source-sink transition of N2O in the first 1.5days during the flooding period, with the water column subsequently acting as a sink relative to the atmosphere in the following experimental period. The source-sink transition was ascribed to changes in oxygen concentration in the water column and sediment regulation of NO3--N transformation, resulting in denitrification and N2O production. Preliminary estimates on the mass budget of N2O in a typical WLFZs of the TGR showed slight emission fluxes, ranging from 13.08 to 43.08μmolm-2 from flooding period to drying process. Although these N2O emissions were relatively low, the emission peak detected during the initial period (first 1.5 days) of the flooding phase provides important knowledge on the mitigation of GHG emissions from hydropower sources, which should be incorporated into future reservoir operations.

Role of Suspended Particulate Matter in Regulating the Behavior of Dissolved Uranium in the Yellow River Estuary

In order to examine the mixing behavior of dissolved uranium (U) in estuaries under different suspended particulate matter (SPM) regimes, three laboratory-based experiments were conducted by mixing seawater with river water containing different concentrations of SPM. Comparing this study with other field and laboratory-based experiments, dissolved U behaved differently depending upon the concentration of SPM. When SPM concentrations are > 0.8 g/L in the Yellow River, desorption/dissolution of U from SPM becomes predominant and dissolved U is enriched relative to the theoretical mixing line. However, when SPM concentrations are < 0.8 g/L, dissolved U behaves conservatively with some extent of removal during estuarine mixing. 234U/238U activity ratios were somewhat constant showing no measurable isotopic fractionation during physical mixing and U sorption/desorption to/from particles. Addition of dissolved 238U desorbed/dissolved from SPM during the annual Yellow River water-sediment regulation scheme (Jun 30th–Jul 14th, 2014) was estimated at 6.4 × 1011 dpm, about 9% of the total riverine flux of dissolved 238U during that same period. This study represents a contribution to studies of dissolved U in muddy rivers and estuaries throughout the world. Results reported here provide not only a perspective to better estimate U flux from rivers to the ocean but also new insights into better understanding its estuarine mixing behavior and controlling factors.