Tributary Confluences Research Articles

The impact of changes in water–sediment relationships at river confluences on the evolution of river bars

The confluence of tributaries and the main stream affects riverbed siltation and alters the upstream water–sediment relationships and flow structure of the main stream by adding additional flow. The relationship between these changes and the evolution of river bars, however, is yet to be fully understood. In this study, the areas of the river bars were extracted from Landsat image and analyzed using soft clustering to identify evolutionary patterns of the bars at the confluence of the Fen River (FR) and Yellow River (YR), and to analyze the hydrodynamic mechanism with hydrodynamic modeling. The results show that the spatial and temporal evolution of river bars in the study area over the past 50 years exhibits an evolutionary pattern, which exhibited evident clustering distributions and significant stage-based characteristics. This pertains to the water–sediment relationships as well as hydrodynamic fluctuations. At the confluence of FR and YR, the intensity of fluvial erosion experiences a mean increment of 9.67 %, while the incoming sediment coefficient witnesses a mean reduction of 5.74 %. The position of the confluence point exhibits a close association with the evolutionary characterized of spatial clustering and temporal phasing of the river bars. The river confluence area showcases a complex turbulence structure, wherein alterations in sediment transport capacity occur due to the influence of secondary flow and the topography of the confluence area. Consequently, this impacts the flushing of river sandbars and brings about modifications in siltation. Overall, this study provides the scientific basis for YR sediment management and channel modification in response to changing runoff and sediment conditions.

Tempo-spatial Variations in Nitrogen and Phosphorus Loads in Jianli-Hankou Reach of the Middle Yangtze River During the Past 20 Years

Ammonia nitrogen （NH4+-N） and total phosphorus （TP） were the major control pollutants in the Yangtze River Basin. Based on measured data from 2003 to 2020, the temporal and spatial variations in concentrations and fluxes of NH4+-N and TP in the Jianli to Hankou （JL-HK） reach of the Middle Yangtze River were studied, and the impacts of flow-sediment factors, tributary inflows, and others on variations in NH4+-N and TP fluxes were discussed. The results showed that： ① In recent years, NH4+-N and TP concentrations in the mainstream have declined significantly, with annual NH4+-N and TP concentrations at each monitoring station in 2020 averagely decreasing by 41% and 34% compared to those in 2003, respectively. Spatially, NH4+-N and TP concentrations decreased and then increased along the mainstream. NH4+-N and TP concentrations of tributary inflows, which include the Dongting Lake and Han River, were generally lower than that of the mainstream. The multi-year average values of NH4+-N and TP concentrations were both averaged at 0.12 mg·L-1 in the mainstream and were averaged at 0.11 mg·L-1 and 0.09 mg·L-1 in the tributary inflows. ② The flux differences between the upper and lower sections net of tributary confluences showed that NH4+-N and TP fluxes were lost in the Jianli to Luoshan （JL-LS） sub-reach and increased in the Luoshan to Hankou （LS-HK） sub-reach in most years. NH4+-N and TP fluxes decreased in the JL-LS sub-reach, which was related to the lower NH4+-N and TP concentrations in lateral inflows, such as Dongting Lake, and thus lowered the NH4+-N and TP concentrations in the mainstream. The LS-HK sub-reach showed the opposite trends, and the water and sediment loads increased in this sub-reach. Across the whole JL-HK reach, TP flux as well as water and sediment loads were recharged along the reach, whereas NH4+-N flux was reduced greatly, which could be attributed to the pollution abatement conducted in the Yangtze River Basin, which mainly focused on NH4+-N. ③ The correlation analysis results showed that NH4+-N fluxes had the strongest correlation with NH4+-N concentrations but not significantly correlated with discharges and sediment transport rates, indicating that NH4+-N was mainly controlled by point source pollution in the study reach. TP fluxes had higher correlations with discharges and sediment transport rates in high flow level periods, and the correlations between TP fluxes and TP concentrations were better in low flow level periods, reflecting that point source pollution contributed more to TP in dry seasons compared to flood seasons.

Spatiotemporal variability in surface velocity of large tributary-bearing glaciers of the Indian Himalayas inferred from SAR polarimetry

ABSTRACT Glacier pattern changes directly indicate variability in local climatic circumstances. The present study dealt with the proposition and estimation of spatiotemporal glacier movement using a synthetic aperture radar-derived polarimetric tracking method which revealed improved estimates over four selected large tributary-bearing glaciers (Siachen, Bara Shigri, Gangotri, and Zemu) in the Indian Himalayas. The average estimated velocities in 2021–2022 for the four glaciers were 0.13, 0.06, 0.08 and 0.09 m/day. The thick debris cover hindered the movement temporally in the middle and lower reaches of the glaciers. The effect of meltwater influx along the tributary confluence zone was also witnessed over the glaciers. Comparison and validation with the US National Aeronautics and Space Administration (NASA) Inter-mission Time Series of Land Ice Velocity and Elevation (ITS_LIVE) products has shown the utility of the proposed method in studying the contributions of active feeding tributaries of the glaciers in the ablation zone.

Internal flow field analysis of a dendritic pore scaffold for bone tissue engineering

The effective reconstruction of osteochondral biomimetic structures is a key factor in guiding the regeneration of full-thickness osteochondral defects. Due to the avascular nature of hyaline cartilage, the greatest challenge in constructing this scaffold lies in both utilizing the biomimetic structure to promote vascular differentiation for nutrient delivery to hyaline cartilage, thereby enhancing the efficiency of osteochondral reconstruction, and effectively blocking vascular ingrowth into the cartilage layer to prevent cartilage mineralization. However, the intrinsic relationship between the planning of the microporous pipe network and the flow resistance in the biomimetic structure, and the mechanism of promoting cell adhesion to achieve vascular differentiation and inhibiting cell adhesion to block the growth of blood vessels are still unclear. Inspired by the structure of tree trunks, this study designed a biomimetic tree-like tubular network structure for osteochondral scaffolds based on Murray’s law. Utilizing computational fluid dynamics, the study investigated the influence of the branching angle of micro-pores on the flow velocity, pressure distribution, and scaffold permeability within the scaffold. The results indicate that when the differentiation angle exceeds 50 degrees, the highest flow velocity occurs at the confluence of tributaries at the ninth fractal position, forming a barrier layer. This structure effectively guides vascular growth, enhances nutrient transport capacity, increases flow velocity to promote cell adhesion, and inhibits cell infiltration into the cartilage layer.

Estimation of river discharge using Monte Carlo simulations and a 1D hydraulic model based on the artificial multi-segmented rating curves at the confluence of two rivers

During extreme floods caused by climate change, reliable flow discharge data are essential for successful reservoir operation to mitigate downstream flood damage. Generally, the flow discharge is computed using the rating curve (RC) established from the relationship between the flow rate and water stage level. Determining the parameters of rating curves is subject to uncertainties related to the difficulties and limitations of flow monitoring in covering a wide range of flow variations. Especially at river confluences, the uncertainties are pronounced when floods occur owing to several factors such as roughness change, backwaters, and levee overflow. The Seomjin River Basin in Korea suffered from flood inundation that occurred at the tributary confluence during an extreme flood in 2020. To identify a reliable flow rate of the main stream and tributary, this study proposes an indirect flow assessment scheme using a 1D hydrodynamic simulation model to find the best simulated water level in an iterative manner based on Monte Carlo (MC) simulations. With a large amount of discharge data generated from random-number combinations, it is possible to obtain the best results automatically by specifying the reliability limitation considering the uncertainty of the predetermined RC parameters associated with the roughness coefficient. Nash Sutcliffe Efficiency (NSE) was incorporated to evaluate the reproduced water level to meet the threshold specified for NSE ≥ 0.75. The simulated flowrates computed from the revised RC and roughness coefficients revealed an error range of 8%–36.6% compared with the design flood. The approach proposed in this study is applicable for determining the valid parameters necessary to create a revised RC at an existing water level gauge station, where the uncertainties of the RC are pronounced, particularly in the vicinity of the channel confluence.

Morfometría de una cuenca de los Andes Centrales, San Juan, Argentina: evaluación de los factores condicionantes del terreno a la ocurrencia de aluviones

The present work aims to determine flash flood occurrence prone areas, in the Jáchal river basin, San Juan Province, Argentina. The Jáchal river basin covers ~29,000 km2 and has a perimeter of ~2,400 km, while the eponymous river extends for nearly 570 km. It is a 7th order basin according to Strahler’s classification, subdivided into four 6th order sub-basins. Various morphometric basin characteristics are analysed and some basic and derived parameters are calculated. These calculations allow predicting the basin behaviour due to heavy rainfall, which translates into large volumes of water with peak flows capable of generating significant flash floods. The basin has a torrential character, so in case of intense rainfall the terrain conditions would contribute to increasing the flow of the main rivers. An inventory of alluvial deposits was carried out and their relationship with the conditioning variables of the terrain was evaluated. At the confluence zones, the river tributaries show more intense overflows than the main collector. This promotes the generation of flash floods. The areas most likely to be affected by flash flood events correspond to low-humidity, east-facing slightly rough terrains, with slopes of up to 15° as well as low-lying flood plain sectors, and at the confluence of tributaries with the main rivers. Populated centres and provincial and national routes are prone to be affected by alluvial events in the study area.

2D numerical modeling of intense bedload-transport processes at confluences of mountain rivers and steep tributaries

2D numerical modeling of intense bedload-transport processes at confluences of mountain rivers and steep tributaries

Cooling wisdom of ‘water towns’: How urban river networks can shape city climate?

‘Water town’, a city design idea featuring buildings sitting along rivers and their associated riparian vegetation, serves as natural refuges for citizens to escape the dual challenges of heatwaves and urban heat islands. We investigated the multi-scale cooling effect of river networks in Shanghai, a typical modern city widely known as an assemblage of water towns. At the neighborhood scale, we conducted a series of field experiments along various rivers with different characteristics during the 2022 heatwave periods in central Shanghai. At the city scale, we first quantified the cooling impact of urban river networks by integrating remotely sensed data and fine-resolution urban river network maps. We found that the width of rivers, coverage ratio, density, and morphology of river networks are the key factors affecting the cooling potential. The confluence or proximity of river tributaries can also bring an enhanced cooling effect than standalone ones. Rivers were found to be all-day cool spots compared to adjacent city streets, with greater daytime cooling intensity (1–3 °C) than at night (0–0.6 °C). Compared to vegetation which has more spatial coverage within the city, rivers are constrained by a linear shape along certain paths but possess the ability to flow, which is beneficial for cooling by carrying cool water from upstream parks and offering wind corridors. The atmospheric cooling capability of 20% of river coverage is equivalent to 30% of tree canopy coverage in a neighborhood. The city-scale cooling effect yields a 36.9% cooling area with a maximum cooling extent of 5.5 °C. The disparities in cooling effect between river networks and standalone rivers primarily manifest in confluence areas and the reinforced cooling within dense river networks. Each additional confluence in local river networks leads to a mean increase of 3% in cooling area and 0.01 °C cooling intensity. Our study implies that urban watercourses cannot be neglected in urban climatic studies and should be incorporated into a new conceptualization of water-included urban local climate zone classifications in the world urban database.

Assessing environmental flow alterations induced by dams and climate change using a distributed hydrological model at catchment scale

Abstract Hydrological alterations by dams and climate change can reduce aquatic biodiversity by disrupting the life cycles of organisms. Here, we aimed to evaluate and compare the hydrological alterations caused by dams and climate change throughout the Omaru River catchment, Japan, using a distributed hydrological model (DHM). First, to assess the impacts of dam and climate change independently, we performed runoff analyses using either dam discharge or future climatic data (two future periods, 2031–2050 and 2081–2100 × three representative concentration pathways). Subsequently, we derived indicators of hydrologic alterations (IHA) to quantify changes in flow alterations by comparing them to IHA under natural conditions (i.e., without dam or climate change data). We found that dams altered IHAs more than climate change. However, on a catchment-scale standpoint, climate change induced wider ranges of flow alterations, such as a further decrease in low flow metrics along the tributaries and uppermost main stem, suggesting a catchment-level shrinkage in important corridors of aquatic organisms. We also observed that the altered flow by water withdrawals was ameliorated by the confluence of tributaries and downstream hydropower outflows. Our approach using a DHM captured the various patterns of flow alterations by dams and climate change.

Tributary mitigates river discontinuity by dam depending on the distance from dam to the tributary confluence

AbstractRiver ecosystems host a continuum of various discontinuities, such as dams and tributary inflows, which shape the habitats of aquatic organisms; this makes it important to evaluate the interactions among discontinuities and effects of a single discontinuity point. Herein, we focused on tributaries in dammed rivers to elucidate a manner to mediate discontinuities and hypothesized that the effects of the tributary inflow on the downstream river were mediated by the (1) distance between the dam and tributary inflow, (2) size ratio between the dam and tributary, and (3) ability of the tributary to transport fine sediments, such as sand and gravel. We conducted field investigations to measure 24 environmental factors, including the water column, riverbed, and river topography, along eight dammed rivers to test the three hypotheses. Only five factors differed significantly between the upstream and downstream sites of the tributary confluence, whereas 12 factors differed significantly between the tributary and mainstem. We also classified the variation pattern using principal component analysis (PCA). The correlation between each PCA axis and environmental factors suggests that the modifications of the water quality factors by tributary inflow were significantly correlated with the distance between the dam and the tributary inflow, and that the flux of various riverbed factors, that is, was significantly correlated with the ability to resupply of fine riverbed sediments by the tributary. Overall, tributary‐inflow downstream of a dam has the potential to be applicable in environmental restoration, especially in terms of resupplying fine sediment.

.

Quantitative analysis of topographic-digital data is a fundamental step in structural geomorphological (modern tectonic) and dynamic geomorphological (mass movement) investigations. Initially, there were a limited number of standard algorithms for analyzing topographic data, including the widely used "Stream Profiler". Meanwhile, the digitization of many of the topographic analysis methods democratized access to such algorithms through the development of freely available open-source code. Flexible tools, such as TopoToolBox (TTB) and the Topographic Analysis Kit (TAK). TTB contains few products in its command set, however, it has unique functions that allow complex analysis on a regional scale. Already TAK allows the evaluation of a large amount of data in a short period. The objective of this study is to identify anomalous break points distributed in the drainage network. It is also proposed to investigate the drivers responsible for maintaining the spatial distribution pattern, such as lithological controls and incision pulse propagation. The main result obtained in this study is that there are several continuous and discrete breaks in the longitudinal profiles. Another fact about the spatial distribution of the breaks is that most of them occur in the proximity between tributary confluences and the entrenched valley of the collecting rivers.

Long-distance recovery of nonuniform suspended load in the Middle Yangtze River owing to upstream damming

The amount of suspended sediment entering the Middle Yangtze River (MYR) was greatly reduced owing to the operation of the Three Gorges Dam (TGD) and other upstream dams, which has caused the process of sediment transport in the MYR at a severe non-equilibrium state. A detailed investigation was conducted into the transport of nonuniform suspended load in the MYR using about 30-year observed data: (i) firstly, the spatiotemporal variations in fractional sediment fluxes were investigated during the pre- and post-TGD periods. The total sediment flux decreased after the TGD operation, and the magnitude for the coarse fraction (d > 0.125 mm) was smaller than those for the fine (d < 0.031 mm) and medium (d = 0.031–0.125 mm) fractions; moreover, the sediment flux for each fraction continuously recovered along the 955-km-long reach, with the recovery distance for the coarse fraction being much shorter than the other two fractions; it was mainly attributed to the fact that the riverbed could supply abundant coarse sediment to the flow, and this fraction experienced a considerably higher recovery degree than the other two fractions; (ii) secondly, the sediment budgets for the fine, medium and coarse fractions were calculated to assess the contributions of different sediment sources to the recovery of graded sediments in the MYR. About 50 %, 29 % and 14 % of the increased fractional sediment amounts for fine, medium and coarse fractions were originated from the tributary confluence, while the remaining sediment was supplied from the riverbed during the process of channel erosion. Therefore, the recovery of suspended load was largely attributed to the supply of sediment from the riverbed, especially for the coarse fraction; (iii) finally, the impacts of the altered flow-sediment regime and the bed-material composition on sediment recovery efficiency were analysed. The sediment recovery efficiency greatly increased after the TGD operation, which was caused by the reduced sediment concentration; and the sediment recovery efficiency for the coarse fraction was much larger than those for the fine and medium sediment fractions. It is attributed to the fact that the channel can provide abundant coarse sediment to the flows in the MYR, while the sediment recovery efficiencies of fine and medium fractions were restricted by the low degree of sediment supply from the riverbed.

Modelling of phosphorus and nonuniform sediment transport in the Middle Yangtze River with the effects of channel erosion , tributary confluence and anthropogenic emission.

Phosphorus (P) transport plays a crucial role in the aquatic ecology of natural rivers. However, our understanding still remains unclear that how P transport is affected in a river-lake connected system downstream of a dam. This system usually undergoes both severe channel degradation and complex exchange of flow-sediment-phosphorus between the mainstem and tributaries. In the current study, a method was proposed firstly to determine the individual contribution of different sources to P recover based on the calculation of phosphorus budget; then an integrated model was developed, covering the modules of flow, nonuniform sediment and phosphorus transport. The application of the proposed method in the 955-km-long Middle Yangtze River (MYR) shows that the type of P transportation was predominantly changed from particulate phosphorus to dissolved phosphorus after the operation of the Three Gorges Project (TGP), but a significant longitudinal recovery of total phosphorus (TP) flux was observed. The TP flux exporting from the MYR was mainly from the Upper Yangtze River (44%), and 12%, 18% and 26% of that were originated from channel erosion, tributary confluence and anthropogenic emission. Moreover, the effects were investigated of nonuniform sediment transport and bed-material coarsening on P transport in the MYR, based on the proposed integrated model. Obtained results show that the TP transport process in the MYR was more reasonable simulated using the nonuniform sediment mode, and it is also confirmed that the process of bed-material coarsening after the TGP operation would lead to the decrease of particulate phosphorus flux in the MYR.

Machine‐learning modeling of hourly potential thermal refuge area: A case study from the Sainte‐Marguerite River (Quebec, Canada)

AbstractTo understand the temporal and spatial variability of thermal refuges, this study focused on modeling potential thermal refuge area (PTRA) at a sub‐daily time‐step in two tributary confluences of the Sainte‐Marguerite River (Canada) during the summers of 2020 and 2021. Aquatic ectotherm species, such as Atlantic salmon (Salmo salar), seek these refuges to avoid heat stress during high summer river temperatures. To investigate the temporal variability of these PTRA, we employed inverse weighted distance interpolation to delineate the hourly area available at both confluences. We then analyzed the impact of the atypical low flow conditions of summer 2021 on the diel cycle of PTRA extremes using the coefficient of variation and the generalized additive model (GAM). Finally, we used four supervised machine‐learning regression models and three to five hydrometeorological predictors to estimate hourly PTRA availability: multivariate adaptive splines regression (MARS), GAM, support vector machine regression (SVM), and random forest regression (RF). The results showed that tree‐based and kernel‐based regression models, RF and SVM, outperformed GAM and MARS. RF had the highest accuracy at both sites, with a relative root mean square error and Nash–Sutcliffe efficiency coefficient (Nash) of 13% and 93%, respectively. Our study discovered that under warm conditions in August 2021, small perennial tributary inflows in combination with low mainstem discharge could create high and constant PTRA at confluences, potentially providing vital thermal refuges for cold‐water taxa. These refuges may be especially important at the local level, within a specific stretch or section of the river. Given the decreasing availability of thermal refuges for salmonids, it is crucial to monitor stream temperatures at small spatial and temporal scales using data‐driven techniques in order to understand stream temperature heterogeneity at tributary confluences.

Environmental drivers of spatial and temporal water quality variability in four coastal wetlands of Lake Ontario

Great Lakes coastal wetlands serve as mediation zones between the land and the lake, regulating the fate of materials received from tributaries prior to discharge to the lake nearshore zone. To improve our understanding of water quality processing and nutrient fate in coastal wetlands, we evaluated within- and across-wetland water quality as a function of environmental drivers over a decade (2009–2018) in three drowned river mouth (Carruthers, Duffin’s, and Rouge) and one barrier lagoon (Frenchman’s Bay) wetlands on the north shore of Lake Ontario. Overall, land-use had a weak relative association with most water quality parameters, reflecting no appreciable changes in land-use across the study years. The barrier lagoon wetland Frenchman’s Bay had a distinctly different water quality pattern from the drowned river mouth wetlands, where water quality followed a high to low concentration gradient from near the tributary confluences (high) to the lake-wetland confluence (low) (permutational analysis of variance p-value < 0.001). Notably, we observed significant differences among celled (i.e., natural ponds in wetlands) and non-celled sites in Duffin’s and Rouge marshes, primarily attributed to strong covariation among phosphorus, phosphate, and organic nitrogen concentrations (permutational analysis of variance p-value < 0.001). This water-quality signature seemed to be driven by solar radiation and lake level variability (i.e., seiche inundation), inferring that wetlands may be important sites for the mobilization of legacy phosphorus in sediments under certain climatic conditions, and following seiche events.

Emergence phenology of the giant salmonfly and responses by birds in Idaho river networks

Emergence of adult aquatic insects from rivers is strongly influenced by water temperature, and emergence timing helps to determine the availability of this ephemeral food resource for birds and other terrestrial insectivores. It is poorly understood how spatial heterogeneity in riverine habitat mediates the timing of emergence. Such spatiotemporal variation may have consequences for terrestrial insectivores that rely on aquatic-derived prey resources. We investigated emergence phenology of the giant salmonfly, Pteronarcys californica, at three spatial scales in two Idaho river networks. We examined the influence of tributary confluences on salmonfly emergence timing and associated insectivorous bird responses. Salmonfly emergence timing was highly variable at the basin-scale during the period we sampled (May–June). Within sub-drainage pathways not punctuated by major tributaries, emergence followed a downstream-to-upstream pattern. At the scale of reaches, abrupt changes in thermal regimes created by 10 major tributary confluences created asynchrony in emergence of 1–6 days among the 20 reaches bracketing the confluences. We observed 10 bird species capturing emerged salmonflies, including 5 species typically associated with upland habitats (e.g., American robin, red-tailed hawk, American kestrel) but that likely aggregated along rivers to take advantage of emerging salmonflies. Some birds (e.g., Lewis’s woodpecker, western tanager, American dipper) captured large numbers of salmonflies, and some of these fed salmonflies to nestlings. Emergence asynchrony created by tributaries was associated with shifts in bird abundance and richness which both nearly doubled, on average, during salmonfly emergence. Thermal heterogeneity in river networks created asynchrony in aquatic insect phenology which prolonged the availability of this pulsed prey resource for insectivorous birds during key breeding times. Such interactions between spatial and temporal heterogeneity and organism phenology may be critical to understanding the consequences of fluxes of resources that link water and land. Shifts in phenology or curtailment of life history diversity in organisms like salmonflies may have implications for these organisms, but could also contribute to mismatches or constrain availability of pulsed resources to dependent consumers. These could be unforeseen consequences, for both aquatic and terrestrial organisms, of human-driven alteration and homogenization of riverscapes.

The discontinuum of river networks: the importance of geomorphic boundaries

ContextRivers are heterogeneous landscapes characterised by distinct patches separated by boundaries. The significance of tributaries as dominant geomorphic boundaries in determining the character of the river discontinuum is a prevailing, yet largely unscrutinised, paradigm of river science.ObjectivesThis study examines the spatial organisation and strength of geomorphic boundaries within the river network of 10 drainage basins in the Kimberley region of NW Australia. The possible drivers of the spatial organisation of boundaries throughout the river networks are also identified.MethodsUsing a suite of GIS tools and statistical analyses, distinct rivers zones or functional process zones (FPZs) and the strength of geomorphic boundaries between these FPZs were empirically determined for > 35,700 km of river network. The spatial distribution of boundary strengths throughout the river network was analysed against a set of environmental variables hypothesised to influence the location of boundaries, specifically: lithology, slope, elevation, and tributary confluences.Results1410 boundaries were identified in the river network of the Kimberley region, an average of one boundary every 25 km of river. Only 32% of these occurred at river confluences. Transitions between different FPZs – large scale river patches, present in the river network were the dominant geomorphic boundary. Although a range of boundary strengths occurred, some river confluences represented the strongest geomorphic boundaries. The location of geomorphic boundaries was significantly associated with the boundary between different types of lithologies.ConclusionOur analysis shows that the river network of the Kimberley region is naturally highly fragmented, and that tributary confluences are not the dominant control on discontinuities in the river network. We suggest that the character of river network fragmentation depends not only on dams, waterfalls, and confluences, but also on the strength and spatial organisation of geomorphic boundaries between FPZs.

Diurnal oscillation in dissolved oxygen at sediment-water interface fuels denitrification-driven N removal in Ganga River

Human activities have substantially enhanced riverine nitrogen loads in many parts of the world. The processes such as denitrification remove a large fraction of N added to the system, and thus, are very helpful in mitigating the effects of increasing human N loads in aquatic ecosystems. No systematic studies, so far, are available on current magnitude and factors regulating efficiency of N removal by the Ganga River. Using three sub-sets of studies- main river course, two tributary confluence trajectories and two point source trajectories along 520 km middle stretch, we generated high spatio-temporal resolution data to show that spatially targeted river reaches with higher nitrate concentrations are acting as ‘hotspots’ of N removal in the Ganga River. The static core method was used to measure the rate of denitrification. Intact sediment core samples were incubated in the laboratory condition and efficiency adjusted. Pollution-impacted point sources showed higher rates of denitrification (0.40 mg N/m2/hr to 2.48 mg N/m2/hr) relative to main stream sites (0.32 mg N/m2/hr to 2.09 mg N/m2/hr). Our results indicate that increased level of carbon (C) and phosphorus (P), which often accompany nitrogen (N) inputs from human activities, have enhanced N removal by the river. Periodic oxic-hypoxic cycle at sediment-water interface (DOsw), which might link nitrification with denitrification, has stimulated N removal at study sites in an opportunistic manner. These results suggest that data on this natural process of N removal (denitrification), and factors thereof, especially those accounting for interactive effects at spatially targeted locations, should be taken into account to formulate policies for the management of nutrient and organic pollution.

The nature and controls on downstream change of channel sediment along the Shiyang River, Northwest China

The grain size composition and distribution of river sediments are important for understanding regional geomorphological evolution, source-sink processes and drainage ecology. The Shiyang River basin, an inland river system in northwestern China, provides an environmental context within which to investigate the relationship between sediment particle size and environmental factors. Based on the analysis of the grain size characteristics of modern riverbed sediments, basin geomorphological parameters and lithology, it is found that the median grain size (Md) of river sediments shows a decrease trend from upstream to downstream in the basin. One of the tributaries named the Jinta River shows an obvious downstream fining trend (exponentially decreasing) of Md, which is related to the old geomorphological development stages and relatively homogeneous lithology in the basin. The downstream fining trend of Md along the Xiying River, another tributary, shows complex fluctuations, which might be affected by the tectonically active, young geomorphic development stage, complex lithology of the basin, and the sediment confluence of tributaries. The gravel-sand transition zone occurs in the plain section of the river about 28 km out of the mountain, which is related to the combination of regional geomorphological features, river morphology and hydraulic sorting.

Longitudinal variation of channel evolution along the middle Yangtze River after the operation of the Three Gorges Project

三峡工程运行后,坝下游河道发生持续冲刷。本文研究了长江中游(955 km)不同河段沿程演变差异及其原因。总体而言,河床形态调整幅度自上而下减弱,这是因为在河床持续冲刷过程中,水流含沙量沿程恢复,故越往下游冲刷相对缓慢。平面形态方面,长江中游岸线崩退及洲滩变形的强度均呈沿程减弱趋势,且在荆江河段最为显著。断面形态方面,河床冲深幅度在宜枝下段＞荆江河段＞宜枝上段＞城汉河段＞汉湖河段。理论上距离三峡工程最近的河段冲刷应最为剧烈,但由于宜枝上段床沙粗化显著,限制了冲刷的进一步发展。过流能力方面,宜枝河段由于距洞庭湖较远,并未受到入汇顶托作用,故其平滩流量的调整基本由进口水沙条件控制,并随着河床冲深下切而增大;对于荆江、城汉和汉湖河段,河床冲刷虽显著,但支流或湖泊的入汇顶托对平滩流量产生的影响大于前者,故平滩流量总体随上下游水位差同步波动。;Significant degradation have occurred in the middle Yangtze River since the operation of the Three Gorges Project. In the current study, longitudinal evolution differences of different reaches along the middle Yangtze River were quantitatively compared, and the main reasons were investigated. In terms of planform geometry, the intensity of bank retreat and central-bar deformation showed a decreasing trend along the reach, and it was most significant in the Jingjiang Reach. With respect to cross-sectional geometry, channel degradation developed gradually from the Yizhi Reach to downstream reaches. It is uncommon that the adjustments of bankfull channel geometry in the upper Yizhi Reach were smaller than those in the downstream reaches. The reason is that the significant coarsening of bed-material composition in the upper Yizhi Reach hindered the development of channel degradation. In terms of discharge capacity, the variations of reach-scale bankfull discharge in the Yizhi Reach were mainly controlled by the incoming flow and sediment conditions, and it increased with the process of channel degradation. In the other subreaches, the variations of bankfull discharge were mainly influenced by the backwater effects owing to the confluences of tributaries and lakes.