Downstream River Systems Research Articles

The ecological operation of ChiTan hydropower station based on hydrological alteration using the PCA method

ABSTRACT Water infrastructure affects the quantity, quality, and environment of water. During reservoir construction, only the social-economic benefits were considered and the health of the downstream river was neglected. Due to the severe downstream river system, the operators and managers faced the challenge of the downstream ecosystem. Many scholars are concerned with the reservoir's optimum functioning, which considers social and economic advantages. Numerous significant opinions and research findings are being presented on ecological scheduling at the moment, both in China and overseas, which surely deepen the connotations of ecological scheduling and serve as the research basis and critical reference of this work. The influence of hydropower development on hydrological conditions, the categorization and calculation of environmental runoff, and the building and solution of ecological scheduling models are all discussed and researched in this work. The study examined how the ChiTan hydropower project influenced the Jinxi River's flow. The total hydrologic alteration calculated using RVA is 50.53%. The component analysis is also utilized to eliminate the statistical redundancy in the hydrologic indicators. The indicators’ monthly flow for July and August's mean minimum flow for 7 days are relevant indicators for the Jinxi River basin's hydrologic modification regime.

Quantifying landscape change following catastrophic dam failures in Edenville and Sanford, Michigan, USA

AbstractDam failures due to changing hydroclimate and ageing infrastructure pose a significant threat to downstream river systems and communities. The detailed geomorphic effects of catastrophic dam failures are not well known because of a lack of high‐resolution topographic data before and after failures. On 19 May 2020, the 17‐m‐tall Edenville and 11‐m‐tall Sanford dams near Midland, Michigan, USA, failed as a result of significant rainfall over the preceding 2 days. We analysed the geomorphic impacts of these failures using a pre‐failure airborne lidar dataset and three uncrewed aerial vehicle (UAV)‐based lidar surveys collected 2 weeks, 3 months and 11 months after failure. Our survey following the dam failure revealed 47 100 ± 11 900 m3 of net floodplain erosion and 17 300 ± 4200 m3 of net deposition downstream of the Edenville and Sanford dam breaches, respectively. Over the year following failure, most geomorphic change was confined to new knickpoints migrating slowly through underlying glacial till substrate, with little change to riverbanks or surrounding floodplains. A lack of impounded reservoir sediment and antecedent downstream topography, including valley width and the location of the breaches relative to the river below the dams, contributed to relatively modest geomorphic changes despite the magnitude of water released. We provide insight into how landscapes are shaped by catastrophic floods, which are likely to become more common with ageing dams and a changing hydroclimate.

Methodology for Exploring Water and Hydropower Operating Criteria That Simultaneously Improve Economic and Environmental Considerations

Despite the reliability and flexibility of hydropower, the operation of hydroelectric power plants may have significant impacts on the downstream river system, including its water stage, sediment transport, and water temperature, ultimately affecting the ecology. To address these challenges, there is a need to identify water scheduling patterns that improve both hydropower economics and the environment relative to current operations. This paper presents a new methodology to explore promising operational criteria/rules that can achieve such improvements. Typical environmental impact statements and relicensing processes generally perform detailed site-specific analyses of a few alternatives that focus on reservoir water release operating rules and their associated environmental impacts. In contrast, the methodology presented in this article uses a widely applicable approach that explores a much larger solution space. This large set of potential alternatives can be represented in a multidimensional space for which one axis represents the economic value and the other axis quantify individual environmental impacts (e.g., sediment transport and fish growth), and they are explored via two approaches: a Monte Carlo simulation that identifies “win–win” alternatives and a multi-objective optimization problem that identifies Pareto-optimal alternatives.

Research and management challenges following soil and landscape decontamination at the onset of the reopening of the Difficult-to-Return Zone, Fukushima (Japan)

Abstract. Twelve years after the nuclear accident that occurred at the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) in March 2011, radiocesium contamination (with a large dominance of 137Cs, with a 30-year half-life) remains a major concern in various municipalities of north-eastern Japan. The Japanese authorities completed an unprecedented soil decontamination programme in residential and cultivated areas affected by the main radioactive plume (8953 km2). They implemented a complex remediation programme scheme to remediate soils that are fundamental to life on Earth, relying on different decision rules depending on the waste type, its contamination level and its region of origin, after delineating different zones exposed to contrasted radiation rates. The central objective was not to expose local inhabitants to radioactive doses exceeding 1 mSv yr−1 in addition to the natural levels. At the onset of the full reopening of the Difficult-to-Return Zone (DTRZ) in spring 2023, the current review provides an update of a previous synthesis published in 2019 (Evrard et al., 2019). Although this ambitious soil remediation and reconstruction programme has almost been completed in the 12 municipalities of Fukushima Prefecture in which an evacuation order was imposed in at least one neighbourhood in 2011, from the 147 443 inhabitants who lived there before the accident, only 29.9 % of them had returned by 2020. Waste generated by decontamination and tsunami cleaning/demolition work is planned to have been fully transported to (interim) storage facilities by the end of 2023. The cost of the operations conducted between 2011 and 2020 for the so-called “nuclear recovery” operations (including decontamination) was estimated by the Board of Audit of Japan in 2023 as JPY 6122.3 billion (∼ EUR 44 billion). Decontamination of cropland was shown to have impacted soil fertility, and potassium fertilisation is recommended to limit the transfer of residual radiocesium to new crops. In forests that cover 71 % of the surface area of Fukushima Prefecture and that were not targeted by remediation, radiocesium is now found in the upper mineral layer of the soil in a quasi-equilibrium state. Nevertheless, 137Cs concentrations in forest products (including wood for heating and construction, wild plants, wildlife game, mushrooms) often keep exceeding the threshold values authorised in Japan, which prohibits their exploitation in the area affected by the main plume. Radionuclides from forests were shown to be exported in dissolved and particle-bound forms to downstream river systems and floodplains, although multiple monitoring records showed the continuous decrease in radiocesium concentrations in both river water and sediment across the main plume between 2011 and 2021. Fish contamination is now generally found to be below the threshold limits although reputational damage remains a major concern for local fishing communities. The remobilisation of radiocesium from sediment accumulated in reservoirs of the region is also of potential concern as it may lead to secondary contamination of fish or irrigation waters supplied to decontaminated fields. Overall, this synthesis demonstrates the need to continue monitoring post-accidental radiocesium transfer in these environments and to keep sharing data in order to refine our predictive understanding of radiocesium mobility and consolidate the tools available to model contaminant transfer in ecosystems. In forests in particular, novel countermeasures and wood uses remain to be developed and tested. Furthermore, the hydrologic connectivity between soils under different ecosystems greatly influences long-term radiocesium transport. The consequences of extreme phenomena (e.g. typhoons, forest fires) that may become more frequent in the future as a result of global change in these contaminated environments should be further anticipated.

Conserving the unique aquatic ecosystem of the Jiuzhai National Heritage Site after the 2017 earthquake: Achievements and challenges

Water resources are the key link within the unique landscape of the Jiuzhaigou World Natural Heritage Site. However, the earthquake on 8 August 2017 induced serious damage to the ecosystem of the Jiuzhaigou region. Water resource quantity was threatened by the direct destruction of the connectivity between the upstream and downstream river systems, whereas water quality and the original aquatic ecosystem were worsened indirectly by secondary disasters, such as landslides and debris flows triggered by earthquakes. It is urgent to protect water resources to maintain a healthy aquatic ecosystem for the Jiuzhaigou World Natural Heritage Site. Therefore, water resource protection strategies are developed by collaboratively considering water quantity, quality and aquatic ecology, including 1) studying the relationship between upstream and downstream in terms of water supply and connectivity, 2) developing emergency plans for extreme precipitation disasters and ecological water regulation schemes for extreme drought, 3) clarifying the impact of vegetation management measures on water conservation, nutrient cycling and water quality, 4) separating the contributions of earthquakes from related disasters, hydrodynamic changes, and lake bank vegetation succession to lake swamping, 5) identifying the potential water pollution risk caused by ecological restoration projects, analyzing the sources of pollutants such as nitrogen, phosphorus and atmospheric acid deposits and developing control measures, and 6) systematically evaluating aquatic ecological health and determining water ecological protection and restoration measures. This review may provide critical viewpoints for conserving aquatic ecosystems, not only in the Jiuzhai World Natural Heritage Site but also in other global conserved aquatic parks.

How the flow and sediment pulse influencing the distribution and functional gene composition of bacterial communities? Case study of the lower Yellow River, China

Discharge containing large amount of sediment will cause a series of ecological effects on downstream river systems. To solve the siltation problem of Yellow River and Xiaolangdi reservoirs, water and sediment regulation has been carried out since 2000. To clarify the impact of the regulation on bacterial communities in the lower Yellow River, the composition of bacterial communities and functional groups in the riverbed and suspended sediment was analyzed by high-throughput sequencing and functional annotation of prokaryotic taxa method. The results showed that the overall diversity index of the Yellow River bacterial community was 3.014–7.045, and at a medium level compared to other large rivers in China. During the water and sediment regulation, the bacterial diversity index slightly decreased compared to the previous stage, while significantly increased at the post-stage of regulation, consideringrelatively small changes in chemical factors among three stages, it was thought that the The variation of bacterial community diversity was more influenced by the fluctuations of discharge, turbulent kinetic energy and sediment concentration. The relationship between suspended and riverbed samples to environmental factors also showed differences at three stages of regulation. During the regulation, increasing discharge and high velocity caused the ammoxidation related groups, such as nitrate and nitrite ammoxidation groups, to displace nitrification, nitrate reduction, and nitrate respiration groups, hence inhibit the water purification capabilities, while at the post-stage, the functional group of most sites recovered to similar level to the pre-stage. In summary, we reveal a characteristic pattern on how the microorganism in the high sediment river respond to the severe disturbance.

A Method for Assessment of Sub‐Daily Flow Alterations Using Wavelet Analysis for Regulated Rivers

AbstractNew tools are needed to evaluate the impacts of short‐term hydropower regulation practices on downstream river systems and to progress towards sustainable river‐flow management. As hydropower is increasingly being used to balance the energy load deficit caused by other less flexible sources, sub‐daily flow conditions across many regulated river (RR) systems are changing. To address this, we used wavelet analyses to quantify the discharge variability in RRs and categorized the level of variability based on the conditions in natural free‐flowing rivers. The presented framework used the definition of fluvial connectivity (Grill et al., 2019) to identify free‐flowing rivers used in the study. We tested the developed framework in 12 different RRs in Finland and found higher overall averaged subdaily variations, with up to 20 times larger variability than natural conditions. A large, highly regulated Finnish river system was found to have the highest sub‐daily variations in winter, while smaller RRs with lower levels of regulation the highest variations in summer. The proposed framework offers a novel tool for sustainable river management and can be easily applied to various rivers and regions globally. It had flexibility to analyze sub‐daily variations in desired seasonal or other ecologically sensitive periods.

Effective Flood Control Method in River Downstream Affected by Tidal Effect Using Optimal Operation of Estuary Barrage

This article introduces an effective flood control method in the downstream of a river through the optimal operation of a barrage that is connected with a sea. A new operating model for flood control of an estuary barrage that can comprehensively evaluate the intensive discharge through the floodgates at the estuary barrage during low tide and the flood propagation process in the downstream river system is proposed. The optimization method applied to the operating model of the barrage combines a one-dimensional hydrodynamic model to analyze the flood propagation process in river channels and a search algorithm based on an iterative method to organize the maximum discharge at the estuary barrage during the ebb tide periods. The proposed method was applied to the flood control of the Taedong River downstream in the Democratic People's Republic of Korea, and its effectiveness was shown and evaluated through modeling results. By imagining the downstream boundary of the river before and after the construction of the West Sea Barrage, the inundation simulation at the Taean gauging station in the downstream of the Taedong River for floods of different average recurrence interval (ARI) was achieved by employing the proposed method.

Sedimentation dynamics within a large shallow lake and its role in sediment transport in a continental-scale watershed

A comprehensive understanding of the sedimentation dynamics within Lake Winnipeg (surface area: 23,750 km2) and its role in sediment transport in the downstream river system was achieved by determining the properties of lake bottom sediment and patterns of sediment accumulation rates and by constructing a conceptual total (i.e., organic and inorganic) sediment budget. Net deposition was the governing process in the South and North Basins, whereas transportation dominated in the Narrows. The largest fluvial source of sediments to the lake, the Red River, supplies 35% of the total sediment load. Although accumulation rates in profundal zones progressively decreased northward from this source at the south end of the lake, high accumulation rates with low inventories of fallout radionuclides in the northern margin of the North Basin indicate a second sediment source, which was determined to be erosion of north shore banks, which accounts for up to 50% of the total sediment load to the lake. The nearshore-offshore gradient in bottom sediment properties in the North Basin confirmed that the signature of this source can reach at least 20 km southward into the lake. However, the properties of bottom sediments, sedimentation dynamics, and sediment budget suggested that some of the materials eroded from the north shore are exported without interaction with the lake bottom and this local sediment source is the dominant source for the downstream river system. It was concluded that Lake Winnipeg effectively disconnects the downstream Nelson River from sediment transport processes in its upstream watershed (953,250 km2).

Lake Modeling Reveals Management Opportunities for Improving Water Quality Downstream of Transboundary Tropical Dams

AbstractWater quality in tropical rivers is changing rapidly. The ongoing boom of dam construction for hydropower is one of the drivers for this change. In particular, the stratification in tropical reservoirs induces oxygen deficits in their deep waters and warmer surface water temperatures, which often translate into altered thermal and oxygen regimes of downstream river systems, with cascading consequences for the entire aquatic ecosystem. Operation rules of reservoirs, involving water intakes at different levels, could mitigate the consequences for downstream water quality. However, optimized water management of deep reservoirs relies on predictive models for water quality, but such predictive capability is often lacking for tropical dams. Here we focus on the Zambezi River Basin (southern Africa) to address this gap. Using the one‐dimensional General Lake Model, we reproduced the internal dynamics of the transboundary Lake Kariba, the world’s largest artificial lake by volume, created by damming the Zambezi River at the border between Zambia and Zimbabwe. Through this modeling approach, we assessed and quantified the thermal and oxygen alteration in the Zambezi River downstream of the reservoir. Results suggest that these alterations depend directly on Kariba’s stratification dynamics, its water level and the transboundary policies for water withdrawal from the reservoir. Scenario calculations indicate a large potential for mitigating downstream water quality alterations by implementing a hypothetical selective withdrawal technology. However, we show that a different and cooperative management of the existing infrastructure of Kariba Dam has the potential to mitigate most of the actual water quality alterations.

Aerobic methanotrophs in an urban water cycle system: Community structure and network interaction pattern

Aerobic methane-oxidizing bacteria (MOB) play an important role in reducing methane emissions in nature. Most current researches focus on the natural habitats (e.g., lakes, reservoirs, wetlands, paddy fields, etc.). However, methanotrophs and the methane-oxidizing process remain essentially unclear in artificial habitat, such as the urban water cycle systems. Here, high-throughput sequencing and qPCR were used to analyze the community structure and abundance of MOB. Six different systems were selected from Yunyang City, Chongqing, China, including the raw water system (RW), the water supply pipe network system (SP), the wastewater pipe network system (WP), the hospital wastewater treatment system (HP), the municipal wastewater treatment plant system (WT) and the downstream river system (ST) of a wastewater treatment plant. Results clearly showed that the MOB community structure and network interaction patterns of the urban water cycle system were different from those of natural water bodies. Type I MOB was the dominant clade in HP. Methylocysis in Type II was the most abundant genus among the whole urban water cycle system, indicating that this genus had a high adaptability to the environment. Temperature, dissolved oxygen, pH and concentration significantly affected the MOB communities in the urban water cycle system. The network of MOB in WT was the most complicated, and there were competitive relationships among species in WP. The structure of the network in HP was unstable, and therefore, it was vulnerable to environmental disturbances. Methylocystis (Type II) and Methylomonas (Type I) were the most important keystone species in the entire urban water cycle system. Overall, these findings broaden the understanding of the distribution and interaction patterns of MOB communities in an urban water cycle system and provide valuable clues for ecosystem restoration and environmental management.

Descriptive or normative: How does reservoir operations modeling influence hydrological simulations under climate change?

Human activities have a strong influence on the hydrological cycle altering natural patterns of evapotranspiration, soil infiltration capacity, ice cover, groundwater distribution, and, ultimately, streamflow at different spatio-temporal scales. Yet, modelling human activities and the associated impacts are often given a secondary importance in hydrological models with respect to the high-fidelity characterization of natural processes, especially at the catchment scale. While this has little or no influence on model accuracy when modelling pristine watersheds, it remarkably deteriorates the model performance in river basins with a considerable anthropogenic footprint. Operations of water infrastructures within a watershed, e.g., a dam or a diversion dam, are commonly modelled based on observational data, when available. This reproduces to some extent the historical decisions, but might be inadequate to simulate operations outside of historical climate or socio-economic conditions. In this paper, we compare a descriptive approach traditionally adopted in hydrological models, where reservoir operations are determined by tracking the historical average release, and a normative approach, where the operations are dynamically conditioned upon the reservoir storage and can adapt to the climate and socio-economic conditions influencing the reservoir operations. We contrast these two approaches by assessing the reservoir dynamics and the impacts on the downstream river system across time scales, from daily to seasonal. We first discuss the accuracy of the two approaches in reproducing historical observations. Then, we explore their potential in anticipating the impacts of future reservoir operations when considering climate and socio-economic change scenarios, thus testing the approaches in decision making contexts increasingly altered with respect to the historical one. We critically present the advantages and disadvantages of either approach, thus contributing to clarify the importance of adopting an appropriate approach to model reservoir operations when reconstructing past dynamics or anticipating future dynamics of catchments impacted by human activities.

Do Aquaculture Farmers Have an Incentive to Maintain Good Water Quality? The Case of Small-Scale Shrimp Farming in Indonesia

While shrimp is the most important farmed seafood product worldwide, its production often induces negative externalities as discharge of nutrients to the aquatic environment. If farmers have an incentive to maintain good farm-level water quality, it may also positively affect the surrounding environment, reducing the risk of eutrophication in downstream river systems. If this is done, then the need for public intervention regulating the externalities may be reduced. This study analyzes the incentives for farmers to maintain good water quality by identifying whether technical efficiency increases with improved water quality. Based on interviews with 183 shrimp farmers producing Litopenaeus vannamei and Penaeus monodon in Sidoarjo, Indonesia, technical efficiency was estimated using data envelopment analysis (DEA). A positive correlation between good water quality and technical efficiency was identified using a second stage DEA, but only found in L. vannamei. L. vannamei farmers thus have an incentive to improve farm-level water quality, potentially decreasing the need for public interventions. Moreover, the technical efficiency of farmers of P. monodon was significantly higher than for farmers of L. vannamei. The higher productivity and environmental robustness of P. monodon indicate a possibility for revitalizing the global supply of the species after two decades of stagnation.

Six-year monitoring study of 137Cs discharge from headwater catchments after the Fukushima Dai-ichi Nuclear Power Plant accident

Since headwater catchments are the source areas of 137Cs for downstream river systems, 137Cs discharge from headwater areas needs to be evaluated. Dissolved form (Dissolved), coarse organic matter (Org), and suspended sediments (SS) were sampled and 137Cs concentrations were measured from June 2011 to November 2016 in four headwater catchments in Yamakiya District, located 35 km northwest of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP). The data up to September 2013 (2.5 y after the accident) have been already published (Iwagami et al., 2017a, b). The data up to November 2016 (5.7 y after the accident) are newly reported in the present paper together with data at a new sampling site. The whole data from June 2011 to November 2016 is discussed.The normalized 137Cs concentrations (137Cs concentrations normalized by the average deposition density of each catchment) in Dissolved, Org, and SS were in the order of 10−6 m2/L, 10−2 m2/kg, and 10−1 m2/kg, respectively, before 2013 and declined to around 10−8 m2/L, 10−4 m2/kg, and 10−2 m2/kg, respectively, in 2016. As a result of the decontamination program, the discharge of SS increased, whereas 137Cs concentrations in SS declined significantly and the total flux of 137Cs decreased. Although the clear effect of land use on decline trend in normalized 137Cs concentrations in Dissolved was not found, more data are necessary for elucidating the relation between them.

Seasonal differences in plankton community structure are more pronounced than spatial patterns in the headpond and downstream portions of a large impounded river

ABSTRACTPlankton community samples from a reservoir were compared to samples from a river downstream of a large hydroelectric generating station on the lower Saint John River, New Brunswick, Canada. The study focused on spatiotemporal variation of the plankton communities and their association with physicochemical parameters in the reservoir–downstream river system after almost 50 years of limnological evolution. The survey revealed 288 phytoplankton and 85 zooplankton taxa, dominated in abundance by rotifers and Cyanobacteria. The taxa richness was significantly greater than reports at the time of the reservoir creation. Spatial variability of the plankton community in the reservoir was statistically homogeneous among sites and the water layers across sites during its stratified period. The river plankton communities followed the same patterns as the reservoir, and river sites were always statistically similar to the reservoir community in terms of richness and abundance. The similarity of the communities across the reservoir and the river was highest during summer. Influential variables explaining seasonal differences in phytoplankton and zooplankton communities included physical, nutrient, and biotic factors. Our study demonstrates how a large, run-of-the-river hydroelectric generating station can regulate the plankton community up to 20 km downstream.

Physical and chemical impacts of a major storm on a temperate lake: a taste of things to come?

Extreme weather can have a substantial influence on lakes and is expected to become more frequent with climate change. We explored the influence of one particular extreme event, Storm Ophelia, on the physical and chemical environment of England’s largest lake, Windermere. We found that the substantial influence of Ophelia on meteorological conditions at Windermere, in particular wind speed, resulted in a 25-fold increase (relative to the study-period average) in the wind energy flux at the lake-air interface. Following Ophelia, there was a short-lived mixing event in which the Schmidt stability decreased by over 100 Jm−2 and the thermocline deepened by over 10 m during a 12-h period. As a result of changes to the strength of stratification, Ophelia also changed the internal seiche regime of Windermere with the dominant seiche period increasing from ~ 17 h pre-storm to ~ 21 h post-storm. Following Ophelia, there was an upwelling of cold and low-oxygenated waters at the southern-end of the lake. This had a substantial influence on the main outflow of Windermere, the River Leven, where dissolved oxygen concentrations decreased by ~ 48%, from 9.3 to 4.8 mg L−1, while at the mid-lake monitoring station in Windermere, it decreased by only ~ 3%. This study illustrates that the response of a lake to extreme weather can cause important effects downstream, the influence of which may not be evident at the lake surface. To understand the impact of future extreme events fully, the whole lake and downstream-river system need to be studied together.

Eco‐hydraulic evaluation of herbaceous ecosystems below headwater dams without a base flow: Observing below dam reaches as new stream sources

AbstractThe roles of headwater streams are usually underestimated and inadequately managed compared with the larger downstream river systems. This two and half‐year study covering several wet and dry seasons evaluated the impacts of small headwater dams without a base flow on downstream herbaceous dynamics. Results showed herb aerial cover and diversity below dams (new stream sources) are unique and different to original stream sources as well as to a comparable unregulated reach. The geomorphological habitats formed subsequent to damming did not show major influence on aerial cover of herbs; nevertheless, diversity showed a clear dependence. Downstream of dams were dominated by Acorus gramineus, which had >40% aerial cover, and this figure increased up to 60% within 2 years (in between a major flood event). Simultaneously, downstream herb diversity decreased by 33%. However, diversity and aerial cover of the unregulated reach and original sources remained more or less the same. Herb pulling tests (simulating an uprooting mechanism during a flood) showed A. gramineus can withstand an eightfold higher force than the other herbs, especially in concrete surfaces and rock/sand pools. The estimated shear force on bed during the major flood was several folds less than the observed. This suggests downstream of dams behave opposite to the conventional understanding that large floods are considered as an agent of restoration.

Hyper-concentrated flow response to aeolian and fluvial interactions from a desert watershed upstream of the Yellow River

Aeolian and fluvial interactions (AFIs) are critical earth-surface processes in arid zones, especially in desert watersheds. The response of hyper-concentrated flows to seasonal alternate AFIs shows very high rates of sediment transport and has important environmental and ecological consequences from local to global scales. Here, we present the aeolian processes-induced hyper-concentrated (AHC) flows that occurred in the Sudalaer desert watershed upstream of the Yellow River, which primarily transport non-cohesive coarse aeolian sand (>0.08mm) and show a peak suspended sediment concentration (SSC) of 1.1 to 1.4×106mgl−3. Our field study and theoretical analysis indicate that non-cohesive coarse aeolian sand downstream in the channel can be entrained from the bed and can be suspended in the turbulent flow by the significant runoff generated upstream with a SSC γ0 value of 0.5×106mgl−3. Severe aeolian processes can provide an abundant coarse sediment supply in the channel, which, once entrained, can also trigger and promote the development of AHC flows. We define, for the first time, the ratio of the weight percentage of coarse sediment to fine sediment C>0.08mm/C<0.05mm as the optimal grain size indicator (OGI) in suspended sediment, indicating that, as the fraction of coarse sediment increases, the significant runoff gradually changes to hyper-concentrated flows and reaches the peak SSC when OGI=3.25. Due to the high frequency of sandstorms and the infrequency of rainstorms, most of the significant rainfall-induced runoff with a certain SSC γ0 can develop into AHC flows and can substantially contribute to the total sediment yield, even leading to deleterious effects on the downstream river system and ecology. Compared with other desert watersheds in semiarid regions, we propose that a SSC γ0 of 500kgm−3 is the threshold criteria for the occurrence of AHC flows in the arid desert watersheds. Comprehensive governing of soil erosion in the upstream gully-dissected slopes is an essential and effective measure for controlling AHC flows.

Carbon fate in a large temperate human‐impacted river system: Focus on benthic dynamics

AbstractFluvial networks play an important role in regional and global carbon (C) budgets. The Seine River, from the Paris urban area to the entrance of its estuary (220 km), is studied here as an example of a large human‐impacted river system subject to temperate climatic conditions. We assess organic C (OC) budgets upstream and downstream from one of the world's largest wastewater treatment plants and for different hydrological conditions using a hydrobiogeochemical model. The fine representation of sediment accumulation on the river bed allows for the quantification of pelagic and benthic effects on OC export toward the estuary and on river metabolism (i.e., net CO2 production). OC export is significantly affected by benthic dynamics during the driest periods, when 25% of the inputs to the system is transformed or stored in the sediment layer. Benthic processes also substantially affect river metabolism under any hydrological condition. On average, benthic respiration accounts for one third of the total river respiration along the studied stretch (0.27 out of 0.86 g C m−2 d−1). Even though the importance of benthic processes was already acknowledged by the scientific community for headwater streams, these results stress the major influence of benthic dynamics, and thus of physical processes such as sedimentation and resuspension, on C cycling in downstream river systems. It opens the door to new developments in the quantification of C emissions by global models, whereby biogeochemical processing and benthic dynamics should be taken into account.

Speciation and hydrological transport of metals in non-acidic river systems of the Lake Baikal basin: Field data and model predictions

The speciation of metals in aqueous systems is central to understanding their mobility, bioavailability, toxicity and fate. Although several geochemical speciation models exist for metals, the equilibrium conditions assumed by many of them may not prevail in field-scale hydrological systems with flowing water. Furthermore, the dominant processes and/or process rates in non-acidic systems might differ from well-studied acidic systems. We here aim to increase knowledge on geochemical processes controlling speciation and transport of metals under non-acidic river conditions. Specifically, we evaluate the predictive capacity of a speciation model to novel measurements of multiple metals and their partitioning, under high-pH conditions in mining zones within the Lake Baikal basin. The mining zones are potential hotspots for increasing metal loads to downstream river systems. Metals released from such upstream regions may be transported all the way to Lake Baikal, where increasing metal contamination of sediments and biota has been reported. Our results show clear agreement between speciation predictions and field measurements of Fe, V, Pb and Zn, suggesting that the partitioning of these metals mainly was governed by equilibrium geochemistry under the studied conditions. Systematic over-predictions of dissolved Cr, Cu and Mo by the model were observed, which might be corrected by improving the adsorption database for hydroxyapatite because that mineral likely controls the solubility of these metals. Additionally, metal complexation by dissolved organic matter is a key parameter that needs continued monitoring in the Lake Baikal basin because dependable predictions could not be made without considering its variability. Finally, our investigation indicates that further model development is needed for accurate As speciation predictions under non-acidic conditions, which is crucial for improved health risk assessments on this contaminant.