Suspended Sediment Flux Research Articles

Variability of suspended sediment yields within the Loire river basin (France)

Summary Suspended sediment fluxes and their variability in time and space have received much attention over the past decades. Large databases compiling suspended sediment load ( SL ) data are often used to serve these purposes. Analyses of these databases have highlighted the following two major limitations: (i) the role of lowland areas in sediment production and transfer has been minimised, and studies on small-scale catchments (with a drainage area of ⩽ 10 2 km 2 ) are practically non-existent in the literature; and (ii) inhomogeneous data and calculation methods are used to estimate and compare the SL values. In this context, the present study aims to complete the existing studies by providing a reliable comparison of SL values for various catchments within lowland river basins. Therefore, we focused on the Loire and Brittany river basins ( France ). 111 small to large catchments covering 78% of this area and representative of the basins landscape diversity were chosen. We first present a large database of area-specific suspended sediment yields ( SY ) calculated from the suspended sediment concentration and flow discharge data over 7–40 yr of measurements at gauging stations. Two calculation methods are used, and the calculated loads are confined within a factor of 0.60–1.65 of the real values. Second, we analyse the temporal and spatial variability of the calculated SY values. Finally, using a nested catchment approach, we provide insight into sediment transport from upstream to downstream gauging stations and into the role of small- and medium- scale catchments in sediment production and transfers. The SL values at the outlet of the catchments range from 2.5 * 10 2 to 8.6 * 10 5 t yr −1 , and the SY values range from 2.9 to 32.4 t km −2 yr −1 . A comparison with the limited values available in the literature for this region corroborates our estimations. Sediment exports from the Loire and Brittany river basins are very low compared with mountainous regions and European exports. However, a strong spatial variability within this territory exists. The expected results on the SY spatial pattern distribution and the correlation between SY values and basin sizes are not observed. An analysis of the SY values at different time steps shows a strong effect of the seasonal availability of detached particles to be transported with a high concentration of suspended sediments during the winter and lower values during the summer and autumn. Annual variations are also observed, with export values varying by a factor 2 to 10 between years for one catchment and the amplitude of the annual variations varying between catchments. The influence of rainfall in the sediment exports is predominant, but investigations on physical characteristics of each catchment (e.g., lithology, slope, land use) are required to better understand the production and transfer processes within a drainage basin. These annual variations imply that long-term data are required to provide mean SY values representative of the catchment functioning. From our calculations, 18 complete years of data are required to obtain a mean SY value with less than 10% of variation on average around the mean. From our results on nested catchments over a long-time scale (40 yr), it appears that most of the suspended sediment load entering the water system is transported downstream. Covariations of the annual- SY values are generally observed for two gauging stations located on the same river. The nested catchment approach is an interesting tool for the identification of active sediment sources within a large catchment and for the construction of detailed sediment budgets.

Estimation of the annual discharge of suspended matter by the rivers of North Siberia and the Far East

New estimates of the suspended sediment fluxes from the medium and large rivers of Siberia and the Russian Far East are presented on the basis of the calculations over the period of the instrumental surveys. The method of calculations accounting for the intra-annual discharge variability is considered and its hydrogeological substantiation is given. The obtained estimates are compared with the results of previous studies. The role of the geological evolution and glaciation history concerning the contemporary suspended sediment fluxes is shown. The prevalence of the valley (coastal and slope) generation of the suspended flux over the watershed generation on the territory of the Russian drainage basin is assumed.

An improved method for determining dredging source terms

The determination of sediment loss rates from dredging activities is important for quantifying the potential impact on the surrounding physical and biological environment. This paper describes an improved method for measuring and analysing data to determine measurable dredging ‘source terms’ using Sediview software and data from two vessel-mounted acoustic Doppler current profiler devices, one of which is mounted with a vertical beam. The methodology is described in detail and an example data set is presented based on measurements undertaken around a working back hoe dredger. The technique described in this paper offers an improvement over the traditional application of the software used to measure dredger source terms (i.e. the use of one vessel-mounted acoustic Doppler current profiler in a Janus configuration). The use of a vertically orientated acoustic beam allows for a greater range (depth) of acoustic backscatter measurements to be acquired, as the data are not contaminated by side-lobe reflections. Although partially dependent upon the dredging methodology, this improvement is important as high suspended sediment concentrations tend to occur near to the bed. Subject to robust calibration, an improved source term can be collected using two acoustic Doppler current profiler instruments and the software allows the determination of suspended sediment flux (kg/s) across an entire survey transect, by using discharge data from one instrument and calibrated concentration data from the other, rather than the use of a series of water column turbidity profiles and subsequent spatial interpolation (an alternative method to the use of the software).

The Role of Infragravity Waves in Near-Bed Cross-Shore Sediment Flux in the Breaker Zone

Results from a series of field experiments, conducted to investigate the influence of infragravity waves (from wave groups), ripple type and location relative to the breaker line on cross-shore suspended sediment flux close to the sea bed in nearshore environments, are presented. The field data were collected from Cable Beach (Broome) and Mullaloo Beach in Western Australia and Chilaw in Sri Lanka. These beaches experience different incident wave, tidal and morphological conditions, with Cable Beach having a 10-m spring tidal range, whilst the other two beaches have tidal ranges <1.0 m. Measurements included simultaneous records of surface elevation, two-dimensional horizontal current velocities and suspended sediment concentrations, together with half-hourly observations of the seabed topography. Although most of the data sets were obtained just outside of the surf zone, a few results from inside of the surf zone were also included. A significant correlation between wave groups and suspended sediment concentration was found at all of the measurement sites, either with or without bed ripples. The direction and magnitude of cross-shore suspended sediment flux varied with location with respect to the breaker line; however, other parameters, such as bed ripples and velocity skewness, could have influenced this result. In Broome, where the measurement location with respect to the breaker line varied with the tidal cycle, the cross-shore sediment flux due to swell waves was shoreward inside and just outside of the surf zone and seaward farther offshore of the breaker line. Further, sediment flux due to swell waves was onshore when the seabed was flat and offshore over post-vortex ripples. Sediment flux due to swell waves was onshore when the normalised velocity skewness towards the shore was high (positive); the flux was offshore when the skewness was lower, but positive, suggesting the influence of other parameters, such as ripples and grain size. The net cross-shore sediment flux was onshore when the Dean number was less than 1.67 and offshore when the Dean number was greater than 1.67. Nevertheless, the Dean number did not account for the influence of ripples or velocity skewness. The cross-shore sediment flux at the infragravity frequency was mainly offshore outside of the surf zone, whereas it varied between onshore and offshore inside of the surf zone.

Estimating Sediment Budget at a River Basin Scale Using a Process-Based Distributed Modelling Approach

The paper presents a process-based distributed modelling approach for estimating sediment budget at a river basin scale with partitions of suspended and bed loads by simulating sediment loads and their interactions. In this approach, a river basin is represented by hillslopes and a network of channels. Hillslopes are divided into an array of homogeneous grid cells for modelling surface runoff and suspended sediments. Channels are defined by incorporating flow hydraulic properties into the respective hillslope grids as sub-grid attributes for modelling both suspended and bed loads. Suspended sediment transport is modelled using one dimensional kinematic wave approximation of Saint-Venant’s principles of conservation of mass and momentum. Transport capacity of runoff or streamflow is used to set the limit of suspended sediment transport rate. Bed load in channels is estimated based on the instantaneous water and hydraulic parameters. Fractional interchange between suspended load and bed load is then back calculated. The performance of the model was evaluated through a case study application in a large river basin in Japan. The model satisfactorily calculated the sediment transport and total sediment budget in the basin. The simulated bed load was found to be reasonable and consistent with the water flow and suspended sediment flux. The results showed the bed load can be expressed as a linear function of the suspended load. The fractions of different sediment loads also exhibit linear relationships with water discharge for the rising and recession limbs of the flood hydrographs. The case study has demonstrated that the process-based distributed modelling approach can efficiently describe the basin-scale sediment budgets with due consideration of the suspended and bed loads and their interactions in the hillslopes and channels.

Intertidal flat development in response to controlled embankment retreat: Freiston Shore, The Wash, UK

A soft engineering method of removing existed flood defences to restore salt marsh has been widely used to protect the coastal region in European countries. This study investigates the response of the intertidal zone at Freiston Shore to a controlled embankment retreat (CER) completed in August 2002, and interprets changes in sediment dynamics and bed level across the intertidal zone based on an analysis of temporal and spatial variations in the suspended sediment flux over semi-diurnal tidal cycles. In addition, a numerical simulation (Delft3D) was performed to analyze the long-term development of the intertidal zone. The current velocity turned further seaward during the ebb 25 months after the de-embankment, with an apparent simultaneous decline of suspended sediment concentration (SSC). Between April 2003 and October 2004, the middle intertidal flat shifted from an erosional to an accretionary state, whereas the reverse was true on the lower intertidal flat. The spatial pattern of suspended sediment flux suggests that sediments eroded from the mid-intertidal zone after 9 months of CER have been re-deposited in the lower intertidal zone, and the process was reversed 16 months after. Tidally residual suspended sediment flux varied with the changing sediment dynamics. The modeling results indicate a similar trend in bed level change to the field observations at Freiston Shore, which was in an early phase of intertidal flat readjustment to the CER. Tidal creek system played an important role in transient geomorphological shift between the middle and lower intertidal flat. Furthermore, the model simulation revealed the time variation of bed level changes at different elevations over the intertidal flat, indicating that the flat would essentially revert to its original pre-CER morphology within decades, which depended highly upon the sediment supply condition. Finally, a conceptual model is developed, based on field survey and numerical simulations, to summarize the developmental processes operating within the intertidal zone in response to CER.

Sediment dynamics in an offshore tidal channel in the southern Yellow Sea

The geomorphology of the southern Yellow Sea (SYS) is characterized by offshore radial sand ridges (RSR). An offshore tidal channel (KSY Channel) is located perpendicular to the coast, comprised of a main and a tributary channel separated by a submarine sand ridge (KSY Sand Ridge) extending seaward. In order to investigate the interactions among water flow, sediment transport, and topography, current velocity and suspended sediment concentration (SSC) were observed at 11 anchor stations along KSY Channel in RSR during a spring tide cycle. High resolution bottom topography was also surveyed. Residual currents and tidally averaged suspended sediment fluxes were calculated and analyzed by using the decomposition method. Results suggested that the water currents became stronger landward but with asymmetrical current speed and temporal duration of flood and ebb tides. Residual currents showed landward water transport in the nearshore channel and a clockwise circulation around the KSY Sand Ridge. Tidally-averaged SSC also increased landward along the channel. The main mechanisms controlling SSC variations were resuspension and horizontal advection, with spatial and temporal variations in the channel, which also contributed to sediment redistribution between channels and sand ridges. Residual flow transport and the tidal pumping effect dominated the suspended sediment flux in the KSY Channel. The KSY Sand Ridge had a potential southward migration due to the interaction between water flow, sediment transport, and topography.

Modelling particle residence times in agricultural river basins using a sediment budget model and fallout radionuclide tracers

ABSTRACTContemporary patterns in river basin sediment dynamics have been widely investigated but the timescales associated with current sediment delivery processes have received much less attention. Furthermore, no studies have quantified the effect of recent land use change on the residence or travel times of sediment transported through river basins. Such information is crucial for understanding contemporary river basin function and responses to natural and anthropogenic disturbances or management interventions. To address this need, we adopt a process‐based modelling approach to quantify changes in spatial patterns and residence times of suspended sediment in response to recent agricultural land cover change. The sediment budget model SedNet was coupled with a mass balance model of particle residence times based on atmospheric and fluvial fluxes of three fallout radionuclide tracers (7Be, excess 210Pb and 137Cs). Mean annual fluxes of suspended sediment were simulated in seven river basins (38–920 km2) in south‐west England for three land cover surveys (1990, 2000 and 2007). Suspended sediment flux increased across the basins from 0.5–15 to 1.4–37 kt y‐1 in response to increasing arable land area between consecutive surveys. The residence time model divided basins into slow (upper surface soil) and rapid (river channel and connected hillslope sediment source area) transport compartments. Estimated theoretical residence times in the slow compartment decreased from 13–48 to 5.6–14 ky with the increase in basin sediment exports. In contrast, the short residence times for the rapid compartment increased from 185–256 to 260–368 d as the modelled connected source area expanded with increasing sediment supply from more arable land. The increase in sediment residence time was considered to correspond to longer sediment travel distances linked to larger connected source areas. This novel coupled modelling approach provides unique insight into river basin responses to recent environmental change not otherwise available from conventional measurement techniques. © 2014 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

The summer spring–neap variation of the water thermohaline–turbidity structure and its dynamical mechanism in the southern Bohai Strait

During summer cruises across the southern Bohai Strait in the spring tide and the neap tide, respectively, the thermohaline–turbidity variations and the mixing–stratification conditions of the water column were analyzed in detail. The thermohaline–turbidity distribution showed both horizontal and vertical variations across the southern Bohai Strait. The suspended sediments across the surveying section were mostly derived from the resuspension of the bottom sediments. A stronger turbulence mixing in the spring tide led to a more intense resuspension of the bottom sediments than that in the neap tide. Moreover, northerly coarsening of the bottom sediments made the sediments more difficult to remain in their resuspended condition in the northern area than in the southern area. The uneven vertical distribution of the resuspended sediments should be considered, when estimating the suspended sediment flux (SSF) across the southern Bohai Strait, for its obvious impact ratio of about 16%, compared with the previous estimation. In addition, the factor of the spring–neap variation in the estimation of SSF should also be involved for its effect on the SSF showed a difference of about 0.06Mt from the estimation without considering it.

Reappraisal of sediment dynamics in the Lower Mekong River, Cambodia

ABSTRACTThe Mekong Basin in southeast Asia is facing rapid development, impacting its hydrology and sediment dynamics. Although the understanding of the sediment transport rates in the Mekong is gradually growing, the sediment dynamics in the lower Mekong floodplains (downstream from Kratie) are poorly understood. The aim of this study is to conduct an analysis to increase the understanding of the sediment dynamics at the Chaktomuk confluence of the Mekong River, and the Tonle Sap River in the Lower Mekong River in Cambodia. This study is based on the data from a detailed field survey over the three hydrological years (May 2008–April 2011) at the two sites (the Mekong mainstream and the Tonle Sap River) at the Chaktomuk confluence. We further compared the sediment fluxes at Chaktomuk to an upstream station (i.e. Mukdahan) with longer time series. Inflow sediment load towards the lake was lower than that of the outflow, with a ratio on average of 84%. Although annually only a small amount of sediment load from the Tonle Sap contributes to the delta (less than 15%), its share is substantial during the February–April period. The annual sediment load transport from the confluence to the delta in 2009 and 2010 accounted for 54 and 50 Mt, respectively. This was on average only 55% of the sediment fluxes measured at Mukdahan, a more upstream station. Furthermore when compared to sediment loads further downstream at the Cambodia–Vietnam border, we found that the suspended sediment flux continued to decline towards the South China Sea. Our findings thus indicate that the sediment load to the South China Sea is much lower than the previous estimate 150–160 Mt/yr. Copyright © 2014 John Wiley & Sons, Ltd.

A wave-resolving model for nearshore suspended sediment transport

This paper presents a wave-resolving sediment transport model, which is capable of simulating sediment suspension in the field-scale surf zone. The surf zone hydrodynamics is modeled by the non-hydrostatic model NHWAVE (Ma et al., 2012). The turbulent flow and suspended sediment are simulated in a coupled manner. Three effects of suspended sediment on turbulent flow field are considered: (1) baroclinic forcing effect; (2) turbulence damping effect and (3) bottom boundary layer effect. Through the validation with the laboratory measurements of suspended sediment under nonbreaking skewed waves and surfzone breaking waves, we demonstrate that the model can reasonably predict wave-averaged sediment profiles. The model is then utilized to simulate a rip current field experiment (RCEX) and nearshore suspended sediment transport. The offshore sediment transport by rip currents is captured by the model. The effects of suspended sediment on self-suspension are also investigated. The turbulence damping and bottom boundary layer effects are significant on sediment suspension. The suspended sediment creates a stably stratified water column, damping fluid turbulence and reducing turbulent diffusivity. The suspension of sediment also produces a stably stratified bottom boundary layer. Thus, the drag coefficient and bottom shear stress are reduced, causing less sediment pickup from the bottom. The cross-shore suspended sediment flux is analyzed as well. The mean Eulerian suspended sediment flux is shoreward outside the surf zone, while it is seaward in the surf zone.

How important and different are tropical rivers? — An overview

Tropical river systems, wherein much of the drainage basin experiences tropical climate are strongly influenced by the annual and inter-annual variations of the Inter-tropical Convergence Zone (ITCZ) and its derivative monsoonal winds. Rivers draining rainforests and those subjected to tropical monsoons typically demonstrate high runoff, but with notable exceptions. High rainfall intensities from burst weather events are common in the tropics. The release of rain-forming aerosols also appears to uniquely increase regional rainfall, but its geomorphic manifestation is hard to detect. Compared to other more temperate river systems, climate-driven tropical rivers do not appear to transport a disproportionate amount of particulate load to the world's oceans, and their warmer, less viscous waters are less competent. Tropical biogeochemical environments do appear to influence the sedimentary environment. Multiple-year hydrographs reveal that seasonality is a dominant feature of most tropical rivers, but the rivers of Papua New Guinea are somewhat unique being less seasonally modulated.Modeled riverine suspended sediment flux through global catchments is used in conjunction with observational data for 35 tropical basins to highlight key basin scaling relationships. A 50year, daily model simulation illuminates how precipitation, relief, lithology and drainage basin area affect sediment load, yield and concentration. Local sediment yield within the Amazon is highest near the Andes, but decreases towards the ocean as the river's discharge is diluted by water influxes from sediment-deprived rainforest tributaries. Bedload is strongly affected by the hydraulic gradient and discharge, and the interplay of these two parameters predicts foci of net bedload deposition or erosion. Rivers of the tropics have comparatively low inter-annual variation in sediment yield.

The effects of sample scheduling and sample numbers on estimates of the annual fluxes of suspended sediment in fluvial systems

Since the 1970s, there has been both continuing and growing interest in developing accurate estimates of the annual fluvial transport (fluxes and loads) of suspended sediment and sediment-associated chemical constituents. This study provides an evaluation of the effects of manual sample numbers (from 4 to 12 year−1) and sample scheduling (random-based, calendar-based and hydrology-based) on the precision, bias and accuracy of annual suspended sediment flux estimates. The evaluation is based on data from selected US Geological Survey daily suspended sediment stations in the USA and covers basins ranging in area from just over 900 km2 to nearly 2 million km2 and annual suspended sediment fluxes ranging from about 4 Kt year−1 to about 200 Mt year−1. The results appear to indicate that there is a scale effect for random-based and calendar-based sampling schemes, with larger sample numbers required as basin size decreases. All the sampling schemes evaluated display some level of positive (overestimates) or negative (underestimates) bias. The study further indicates that hydrology-based sampling schemes are likely to generate the most accurate annual suspended sediment flux estimates with the fewest number of samples, regardless of basin size. This type of scheme seems most appropriate when the determination of suspended sediment concentrations, sediment-associated chemical concentrations, annual suspended sediment and annual suspended sediment-associated chemical fluxes only represent a few of the parameters of interest in multidisciplinary, multiparameter monitoring programmes. The results are just as applicable to the calibration of autosamplers/suspended sediment surrogates currently used to measure/estimate suspended sediment concentrations and ultimately, annual suspended sediment fluxes, because manual samples are required to adjust the sample data/measurements generated by these techniques so that they provide depth-integrated and cross-sectionally representative data. Published 2014. This article is a U.S. Government work and is in the public domain in the USA.

Diel patterns of suspended sediment flux and the zoogeomorphic agency of invasive crayfish

The role of biotic forcing in fluvial geomorphology is understudied.This paper investigates the suggestion that the activities of signal crayfish(Pacifastacus leniusculus) can increase suspended sediment fluxes in rivers.Previous field work, supported by mesocosm experiments, suggests that crayfish nocturnalism can cause night time increases in turbidity, but field data are limited to a 16-hour period from a single site where suspended sediment time-series are not available. Here, field data collected over a 28-day period on the Brampton Branch of the River Nene, UK, are used to quantify the impact of diel fluctuations in suspended sediment concentration on sediment fluxes. Field observations and laboratory experiments are used to evaluate the likelihood that crayfish, whichare abundant in this river, are responsible for the diel patterns. Turbidity and water stage were measured at 2-minute intervals at a single site. Water was sampled for suspended sediment concentration on a diel cycle and during storm events. A relation between suspended sediment concentration and turbidity, along with a local discharge time-series, permitted calculation of sediment flux and sediment loads. Aquarium experiments with one or two crayfish were used to directly observe the relative impacts of crayfish activity and conspecific interactions on sediment suspension. Over the 28-day period, turbidity and suspended sediment exhibited a strong diel pattern, characterised by night-time increases in the frequency and magnitude of spikes in the turbidity data and by increases in ambient suspended sediment concentrations. Small diel fluctuations in stage were also measured, but the rises in stage were out of phase with turbidity and there was no correlation between stage and turbidity at any frequency. In the absence of a credible hydraulic explanation for the increases in night-time suspended sediment concentration, several lines of evidence, including results from the aquarium experiments, strongly suggest that crayfish are the mostlikely cause. We estimate that crayfish activity contributed at least 20% of the suspended sediment load over the 28-day period (which included two moderate floods) and this proportion was 47% during the days when baseflow conditions prevailed. This work extends and strengthens the argument that crayfish are important zoogeomorphic agents with potential implications for managing fine sediment pressures. It also highlights the need to better understand the link between crayfish activity, sediment suspension and downstream dispersal, particularly the catchment-scale distribution and seasonality of such impacts.

Modelling sediment deposition and phosphorus retention in a river floodplain

Phosphorus (P) is one of the major limiting nutrient in many freshwater ecosystems. During the last decade, attention has been focused on the fluxes of suspended sediment and particulate P through freshwater drainage systems because of severe eutrophication effects in aquatic ecosystems. Hence, the analysis and prediction of phosphorus and sediment dynamics constitute an important element for ecological conservation and restoration of freshwater ecosystems. In that sense, the development of a suitable prediction model is justified, and the present work is devoted to the validation and application of a predictive soluble reactive phosphorus (SRP) uptake and sedimentation models, to a real riparian system of the middle Ebro river floodplain. Both models are coupled to a fully distributed two-dimensional shallow-water flow numerical model. The SRP uptake model is validated using data from three field experiments. The model predictions show a good accuracy for SRP concentration, where the linear regressions between measured and calculated values of the three experiments were significant (r2 ≥ 0.62; p ≤ 0.05), and a Nash–Sutcliffe coefficient (E) that ranged from 0.54 to 0.62. The sedimentation model is validated using field data collected during two real flooding events within the same river reach. The comparison between calculated and measured sediment depositions showed a significant linear regression (p ≤ 0.05; r2 = 0.97) and an E that ranged from 0.63 to 0.78. Subsequently, the complete model that includes flow dynamics, solute transport, SRP uptake and sedimentation is used to simulate and analyse floodplain sediment deposition, river nutrient contribution and SRP uptake. According to this analysis, the main SRP uptake process appears to be the sediment sorption. The analysis also reveals the presence of a lateral gradient of hydrological connectivity that decreases with distance from the river and controls the river matter contribution to the floodplain. Copyright © 2014 John Wiley & Sons, Ltd.

Characteristics of suspended sediment and river discharge during the beginning of snowmelt in volcanically active mountainous environments

To better understand instream suspended sediment delivery and transformation processes, we conducted field measurements and laboratory experiments to study the natural function of spatial and temporal variation, sediment particles, stable isotopes, particle size, and aspect ratio from tributary to mainstream flows of the Sukhaya Elizovskaya River catchment at the beginning of and during snowmelt. The Sukhaya Elizovskaya River is located in the Kamchatka Peninsula of Russia and is surrounded by active volcanic territory. The study area has a range of hydrological features that determine the extreme amounts of washed sediments. Sediment transported to the river channels in volcanic mountainous terrain is believed to be strongly influenced by climate conditions, particularly when heavy precipitation and warmer climate trigger mudflows in association with the melting snow. The high porosity of the channel bottom material also leads to interactions with the surface water, causing temporal variability in the daily fluctuations in water and sediment flow. Field measurements revealed that suspended sediment behaviour and fluxes decreased along the mainstream Sukhaya Elizovskaya River from inflows from a tributary catchment located in the volcanic mountain range. In laboratory experiments, water samples collected from tributaries were mixed with those from the mainstream flow of the Sukhaya Elizovskaya River to examine the cause of debris flow and characteristics of suspended sediment in the mainstream. These findings and the geological conditions of the tributary catchments studied led us to conclude that halloysite minerals likely comprise the majority of suspended sediments and play a significant role in phosphate adsorption. The experimental results were upscaled and verified using field measurements. Our results indicate that the characteristics of suspended sediment and river discharge in the Sukhaya Elizovskaya River can be attributed primarily to the beginning of snowmelt in volcanic tributaries of the lahar valley, suggesting a significant hydrological contribution of volcanic catchments to instream suspended sediment transport. Daily fluctuations in discharge caused by snowmelt with debris flow were observed in this measurement period, in which suspended sediment concentration is ~10mg/l during nonflooding periods and ~1400mg/l when flooding occurs. The oxygen and hydrogen isotope measurements, when compared with Japan, indicated that the Kamchatka region water is relatively lightweight, incorporating the effects of topography; and the water from the beginning of the snowmelt is relatively lightweight when compared with water from the end of the snowmelt. The trend line of isotopes from the beginning of the snowmelt was defined by a slope of 6.88 (n=12; r2=0.97), significantly less than that of isotopes from the snowmelt (8.72). The sediment particles collected during the snowmelt were round in shape caused by the extreme flows and high discharge. The shape of the sediment particles collected at the beginning of the snowmelt, assumed to be fresh samples from the hillslope, was sharper caused by the relatively small discharge by moderate snowmelt. Finally, the relationship between river discharge and suspended sediment concentration was indicated. The results are compared with mountainous rivers of Japan and Malaysia. A new diagram is proposed to describe the relationship between suspended sediment concentration and river discharge.

Accelerated export of sediment and carbon from a landscape under intensive agriculture

The export of total organic carbon (particulate and dissolved) from terrestrial to aquatic ecosystems has important implications for water quality and the global carbon cycle. However, most research to date has focused on DOC losses from either forested or peaty catchments, with only limited studies examining the controls and rates of total fluvial carbon losses from agricultural catchments, particularly during storm events. This study examined the controls and fluxes of total suspended sediment (SS), total particulate (TPC) and dissolved organic carbon (DOC) from two adjacent catchments with contrasting intensive agricultural and semi-natural land-use. Data from 35 individual storm events showed that the agricultural catchment exported significantly higher SS concentrations on a storm-by-storm basis than the semi-natural catchment, with peak discharge exerting a greater control over SS, TPC and DOC concentrations. Baseflow DOC concentrations in the agricultural catchment were significantly higher. DOC quality monitored during one simultaneous rainfall event differed between the two study catchments, with more humic, higher molecular weight compounds prevailing in the agricultural catchment and lower molecular weight compounds prevailing in the semi-natural catchment. During an eight month period for which a comparable continuous turbidity record was available, the estimated SS yields from the agricultural catchment were higher than from the semi-natural catchment. Further, the agricultural catchment exported proportionally more TPC and a comparable amount of DOC, despite a lower total soil carbon pool. These results suggest that altered hydrological and biogeochemical processes within the agricultural catchment, including accelerated soil erosion and soil organic matter turnover, contributed to an enhanced fluvial SS and carbon export. Thus, we argue that enhancing semi-natural vegetation within intensively farmed catchments could reduce sediment and carbon losses from these areas and increase their resilience to more extreme hydrological events, anticipated as a result of climate change.

Global suspended sediment and water discharge dynamics between 1960 and 2010: Continental trends and intra-basin sensitivity

Establishing a quantitative description of global riverine fluxes is one of the main goals of contemporary hydrology and geomorphology. Here we study changes in global riverine water discharge and suspended sediment flux over a 50-year period, 1960–2010, applying a new version of the WBMsed (WBMsed v.2.0) global hydrological water balance model. A new floodplain component is introduced to better represent water and sediment dynamics during periods of overbank discharge. Validated against data from 16 globally distributed stations, WBMsed v.2.0 simulation results show considerable improvement over the original model. Normalized departure from an annual mean is used to quantify spatial and temporal dynamics in both water discharge and sediment flux. Considerable intra-basin variability in both water and sediment discharge is observed for the first time in different regions of the world. Continental-scale analysis shows considerable variability in water and sediment discharge fluctuations both in time and between continents. A correlation analysis between predicted continental suspended sediment and water discharge shows strong correspondence in Australia and Africa (R2 of 0.93 and 0.87 respectively), moderate correlation in North and South America (R2 of 0.64 and 0.73 respectively) and weak correlation in Asia and Europe (R2 of 0.35 and 0.24 respectively). We propose that yearly changes in intra-basin precipitation dynamics explain most of these differences in continental water discharge and suspended sediment correlation. The mechanism proposed and demonstrated here (for the Ganges, Danube and Amazon Rivers) is that regions with high relief and soft lithology will amplify the effect of higher than average precipitation by producing an increase in sediment yield that greatly exceeds increase in water discharge.

PH-levels in intensively drained and peatland-dominated river basin: Paleolimnological approach to detect impacts of past land use

Peatland drainage can have major ecological and biogeochemical impacts on aquatic environments in boreal regions. In the Sanginjoki river catchment in North Ostrobothnia, Finland, peatland drainage is suspected to increase the acidity of recipient waters. Possible reasons are acid sulphate soils (ASS) or black schist deposits, which occur especially in lower parts of the river basin, or increased transport of humic substances. It is not known how long acidification has affected water quality and biota in the river. To study historical pH levels, we examined sediment cores from two ponds in the main channel of the Sanginjoki river and one small lake in the Sanginjoki catchment. Diatom and geochemical analysis and 137Cs-derived chronology were combined to estimate historical pH levels and changes in land use practices. The results suggest that changes in land use in the 1970s have affected runoff acidity. During that period, there was a change in diatom communities and sediment geochemistry, indicating that land use in the catchment area, most likely peatland drainage, had probably changed suspended sediment fluxes and pH conditions. The results also showed that changes in pH levels probably did not originate from ASS or black schist horizons, but rather mainly from humic substances leached from the catchment area. However, ASS and black schist deposits may have had an influence on pH levels in lower parts of the Sanginjoki river during dry/wet periods. Thus, the findings suggest that future changes in land use and land management are likely to cause increased acidity in the Sanginjoki catchment. Therefore actual or potential acidification of river water should be included in risk assessments and future land use practices, and should be avoided by not changing natural humic-rich ecosystems.

Lateral Baroclinic Forcing Enhances Sediment Transport from Shallows to Channel in an Estuary

We investigate the dynamics governing exchange of sediment between estuarine shallows and the channel based on field measurements at eight stations spanning the interface between the channel and the extensive eastern shoals of South San Francisco Bay. The study site is characterized by longitudinally homogeneous bathymetry and a straight channel, with friction more important than the Coriolis forcing. Data were collected for 3 weeks in the winter and 4 weeks in the late summer of 2009, to capture a range of hydrologic and meteorologic conditions. The greatest sediment transport from shallows to channel occurred during a pair of strong, late-summer wind events, with westerly winds exceeding 10 m/s for more than 24 h. A combination of wind-driven barotropic return flow and lateral baroclinic circulation caused the transport. The lateral density gradient was produced by differences in temperature and suspended sediment concentration (SSC). During the wind events, SSC-induced vertical density stratification limited turbulent mixing at slack tides in the shallows, increasing the potential for two-layer exchange. The temperature- and SSC-induced lateral density gradient was comparable in strength to salinity-induced gradients in South Bay produced by seasonal freshwater inflows, but shorter in duration. In the absence of a lateral density gradient, suspended sediment flux at the channel slope was directed towards the shallows, both in winter and during summer sea breeze conditions, indicating the importance of baroclinically driven exchange to supply of sediment from the shallows to the channel in South San Francisco Bay and systems with similar bathymetry.