Annual Suspended Sediment Research Articles

Projecting sediment export from two highly glacierized alpine catchments under climate change: exploring non-parametric regression as an analysis tool

Abstract. Future changes in suspended sediment export from deglaciating high-alpine catchments affect downstream hydropower reservoirs, flood hazard, ecosystems and water quality. Yet, quantitative projections of future sediment export have so far been hindered by the lack of process-based models that can take into account all relevant processes within the complex systems determining sediment dynamics at the catchment scale. As a promising alternative, machine-learning (ML) approaches have recently been successfully applied to modeling suspended sediment yields (SSYs). This study is the first, to our knowledge, exploring a machine-learning approach to derive sediment export projections until the year 2100. We employ quantile regression forest (QRF), which proved to be a powerful method to model past SSYs in previous studies, for two nested glaciated high-alpine catchments in the Ötztal, Austria, above gauge Vent (98.1 km2) and gauge Vernagt (11.4 km2). As predictors, we use temperature and precipitation projections (EURO-CORDEX) and discharge projections (AMUNDSEN physically based hydroclimatological and snow model) for the two gauges. We address uncertainties associated with the known limitation of QRF that underestimates can be expected if values in the projection period exceed the range represented in the training data (out-of-observation-range days, OOOR). For this, we assess the frequency and extent of these exceedances and the sensitivity of the resulting mean annual suspended sediment concentration (SSC) estimates. We examine the resulting SSY projections for trends, the estimated timing of peak sediment and changes in the seasonal distribution. Our results show that the uncertainties associated with the OOOR data points are small before 2070 (max. 3 % change in estimated mean annual SSC). Results after 2070 have to be treated more cautiously as OOOR data points occur more frequently, and glaciers are projected to have (nearly) vanished by then in some projections, which likely substantially alters sediment dynamics in the area. The resulting projections suggest decreasing sediment export at both gauges in the coming decades, regardless of the emission scenario, which implies that peak sediment has already passed or is underway. This is linked to substantial decreases in discharge volumes, especially during the glacier melt phase in late summer, as a result of increasing temperatures and thus shrinking glaciers. Nevertheless, high(er) annual yields can occur in response to heavy summer precipitation, and both developments would need to be considered in managing sediments, as well as e.g., flood hazard. While we chose the predictors to act as proxies for sediment-relevant processes, future studies are encouraged to try and include geomorphological changes more explicitly, e.g., changes in connectivity, landsliding, rockfalls or vegetation colonization, as these could improve the reliability of the projections.

Uncertainties of Annual Suspended Sediment Transport Estimates Driven by Temporal Variability

AbstractThe majority of sediment transported from rivers to the global oceans is moved in suspension as fine particles. Thereby, the transported sediment shapes the physical environment regarding erosive and accumulative processes. Temporal variations in sediment supply and transport lead to unquantified uncertainties in annual load estimates, requiring high‐resolution data sets and a sound understanding of site‐specific catchment characteristics. We investigate the temporal variability of suspended sediment transport in four catchments in Germany with highly different discharge regimes and catchment sizes (<1,000 km2 to >100,000 km2). The data set consists of high‐resolution 15‐min turbidity measurements with daily discharge and frequent manual sampling. Utilizing a bootstrap approach based on the 15‐min time series, we assessed the impact of the sampling interval on annual load estimates for less frequent data sets. We use the sediment load exceedance time (Ts80%) as a measure of variability and relate it to uncertainties in annual load estimates. Since low‐frequency data sets rely on sediment rating curves, we performed a sensitivity analysis of the rating parameters a, b, and ε. Our results indicate a negative exponential relationship between Ts80% and uncertainties in annual load estimates. Based on the Ts80%, we can derive the shortest sampling frequency necessary to obtain annual load estimates with an error of <20% over varying discharge regimes. Additionally, Ts80% is linked to rating exponent b, with low b‐values indicating high Ts80%‐values and lower variability, and high b‐values indicating higher variability.

The Signature of Climate in Fluvial Suspended Sediment Records

AbstractArid regions are often characterized by exceptionally high rates of fluvial sediment transport, but the processes responsible for this apparent connection between climate and sediment transport remain unclear. We examined decades of continuous flow records and suspended sediment concentrations from 71 rivers across the United States by comparing the suspended sediment rating curve behavior, quantified using power law coefficients and exponents, to an aridity index. Results indicate that higher aridity correlates with both greater overall suspended sediment concentration and lower sensitivity of concentration to changes in discharge, demonstrating that rivers in arid locations on average have greater suspended sediment transport efficiency across most discharges, and achieve high transport rates at a relatively lower discharge than rivers in temperate climates. Furthermore, based on additional analyses of the Normalized Difference Vegetation Index, specific suspended sediment yield, and a hydrograph flashiness index, we attribute the relationships between sediment transport and climate primarily to differences in vegetation density, precipitation, and runoff, variables that all influence both sediment supply and riverbed grain sorting. Finally, we note that the observed contrasts in sediment transport behavior likely represent climate‐driven differences in the magnitude and frequency of sediment supply rather than annual suspended sediment load, which does not depend significantly on climate. This study highlights a critical connection between multiple interrelated climatic factors and sediment transport, an important finding for future hazard mitigation in a changing climate with rapidly shifting vegetation patterns and hydrology.

Separating impacts of storm characteristics and mitigation activities on suspended sediment: Implications for sampling to evaluate mitigation success

Mitigation activities are often implemented to reduce anthropogenic sediment loads to sensitive ecosystems, including coral reefs, but effectiveness is rarely documented due to a lack of pre-mitigation data and to the difficulty of separating the impacts of mitigation from those of storm size and intra- and inter-storm variability. Here we present a method using stormwise metrics to quantify the effectiveness of a sediment mitigation project in American Samoa, where an aggregate quarry covering <1 % of a small watershed (drainage area 1.8 km2) generated approximately half of the annual suspended sediment load to the coast. Rainfall, runoff, turbidity and suspended sediment concentrations (SSC, mg L–1), yield (SSY, tons km−2 per unit time) and loads (SSL, tons per unit time) were monitored periodically over a five-year period upstream and downstream of the quarry. Mitigation included revegetation and placement of gravel on roads and processing platforms followed by installation of retention ponds. Storms were identified using automated storm separation, and a simple regression model using maximum area-normalized storm discharge (Qmax) estimated SSC, SSY and SSL for each mitigation stage (post-gravel, post-ponds) for a reference storm sequence. Regression parameters, and SSL and SSC for the reference storm sequence were calculated for a range of storm separation parameters. Monte Carlo sampling of the regression errors was used to calculate a mean and standard deviation of SSL and its change following mitigation. SSC was nearly 100x higher during large storms than during small storms, demonstrating the need to account for storm size to detect impacts of mitigation. After mitigation, SSL downstream of the quarry decreased by 63–82 % for all storms and by 88–92 % for small storms, and SSY downstream of the quarry was similar to SSY from the forested watershed upstream. Mitigation of sediment loads can target a small fraction of a watershed area that accounts for the majority of the human impact. Combining stormwise metrics with simple regression modeling and uncertainty analysis is a simple and novel way to detect and quantify the effectiveness of mitigation activities on sediment loads.

Drivers of annual suspended sediment and nutrient yields in tributaries to Lake Erie

Abstract In the early 2000s, harmful algal blooms and hypoxia returned to Lake Erie following a period of ecosystem recovery from the late 1980s through the 1990s. This corresponds to a drastic increase in dissolved reactive phosphorus loads and flow-weighted mean concentrations in the major tributaries to the Western Lake Erie Basin. However, there is substantial variability in suspended sediment and nutrient yields across Lake Erie tributaries. It is well known that agricultural and urban land uses lead to increased watershed sediment and nutrient yields, especially in the absence of proper management practices. Yet attributes such as watershed soil types and slope can also affect yields. We examined the influence of watershed land use, hydrology, soil type, and slope on annual nutrient and sediment yields from tributaries to Lake Erie monitored as part of the Heidelberg Tributary Loading Program. A minimum of one sample and, during storm runoff, up to three samples a day are analyzed for all major nutrients and suspended sediments. The 5-year average annual yields across these watersheds exhibited distinct geographic patterns from west to east, with high suspended sediment but lower nutrient yields from the eastern most tributaries (Cuyahoga, Old Woman Creek, Huron) and the lowest sediment along with lower nutrient yields from the western most tributaries (Raisin, Tiffin, Lost). The Maumee, Portage, Sandusky, along with select subwatersheds tended to have intermediate sediment yields with high nutrient yields. Altogether, the % cultivated crops and poorly drained soil in the watershed increased nutrient yields whereas a higher % slope and lower % pasture increased sediment yields. These patterns highlighted unique regional differences that can help guide management decisions for these watersheds that ultimately would improve the health of Lake Erie.

Sediment and phosphorus contributions from eroding banks in a large intensively managed watershed in western Iowa, United States

AbstractIn this study, a new remote sensing tool was used in conjunction with sampling of river bank sediments to map channel migration patterns and estimate the net contribution of bank erosion to the sediment and phosphorus (P) budget of the Nishnabotna River in southwestern Iowa. Between the years 2009 and 2018, we found that at least 1.81 ± 0.57 × 107 Mg of sediment and 8.26 ± 2.5 × 103 Mg of P entered the Nishnabotna River due to channel migration. This equates to 0.87 Mg of sediment per meter of channel per year and 0.40 kg of P per meter of channel per year. Barring additional deposition elsewhere in the river corridor, these values represent as much as 77% of annual suspended sediment and 46% of the annual P export from the watershed. Our results also indicate that the contribution of net sediment and P volume loss by stream order increases sharply from third to sixth order, even though the total channel length is much smaller in the higher orders. These results suggest that bank erosion is an important source of sediment and P within the watershed and that future attempts to decrease riparian exports of sediment and P should focus on high‐order reaches.

Hydraulic Design of Sediment-Trapping Basin in Wadis Using Empirical Equations and Deposition Processes

Reservoirs overflow during flood season because of sedimentation cycles, which severely affects their effectiveness. Siltation is a major problem in dams constructed in waterways in arid and semi-arid areas. Therefore, the reservoirs in wadis lose their capacity due to sedimentation. This study determines an optimal design of the trapping basin on steep slope areas for Wadi Bishah in the Asir region of southwestern Saudi Arabia. The empirical design criteria of the sediment-trapping basin is used to mitigate the effects of sedimentation in the King Fahd Dam. The empirical design of the trapping basin constructed upstream of the dam located in the wadi is presented. Moreover, the annual suspended and bed sediment load (Qs and Qb) techniques for estimating the volume of sediments are used, and the relationship between the sediment trapping efficiency and size is determined. The sediment trapping in Wadi Bishah upstream sediment-trapping basins is selected to reduce the amount of sediment. One of the important results of this study tries to create a new concept to trap sediment in wadis, which are located in arid and semi-arid areas. The results obtained were evaluated using theoretical and empirical equations to determine the appropriate size of the basin. The results demonstrate that the optimal dimensions for the sediment confinement basin are Lb × Wb × hs = 3500 × 500 × 1.5 m. Also, for these dimensions, the basin efficiency was assumed to be in the range of 60–70%. The trap basin should be constructed at open check dams upstream (U/S) of the proposed basin to enhance its efficiency. Further investigation is required to understand the transport and deposition of sediments, particularly fine sediments in the basin. Additionally, the effects of sediment traps in Wadi Bishah should be assessed during the construction of these structures to aid water resource management and mitigate flood disasters.

Reconstructing five decades of sediment export from two glacierized high-alpine catchments in Tyrol, Austria, using nonparametric regression

Abstract. Knowledge on the response of sediment export to recent climate change in glacierized areas in the European Alps is limited, primarily because long-term records of suspended sediment concentrations (SSCs) are scarce. Here we tested the estimation of sediment export of the past five decades using quantile regression forest (QRF), a nonparametric, multivariate regression based on random forest. The regression builds on short-term records of SSCs and long records of the most important hydroclimatic drivers (discharge, precipitation and air temperature – QPT). We trained independent models for two nested and partially glacier-covered catchments, Vent (98 km2) and Vernagt (11.4 km2), in the upper Ötztal in Tyrol, Austria (1891 to 3772 m a.s.l.), where available QPT records start in 1967 and 1975. To assess temporal extrapolation ability, we used two 2-year SSC datasets at gauge Vernagt, which are almost 20 years apart, for a validation. For Vent, we performed a five-fold cross-validation on the 15 years of SSC measurements. Further, we quantified the number of days where predictors exceeded the range represented in the training dataset, as the inability to extrapolate beyond this range is a known limitation of QRF. Finally, we compared QRF performance to sediment rating curves (SRCs). We analyzed the modeled sediment export time series, the predictors and glacier mass balance data for trends (Mann–Kendall test and Sen's slope estimator) and step-like changes (using the widely applied Pettitt test and a complementary Bayesian approach). Our validation at gauge Vernagt demonstrated that QRF performs well in estimating past daily sediment export (Nash–Sutcliffe efficiency (NSE) of 0.73) and satisfactorily for SSCs (NSE of 0.51), despite the small training dataset. The temporal extrapolation ability of QRF was superior to SRCs, especially in periods with high-SSC events, which demonstrated the ability of QRF to model threshold effects. Days with high SSCs tended to be underestimated, but the effect on annual yields was small. Days with predictor exceedances were rare, indicating a good representativity of the training dataset. Finally, the QRF reconstruction models outperformed SRCs by about 20 percent points of the explained variance. Significant positive trends in the reconstructed annual suspended sediment yields were found at both gauges, with distinct step-like increases around 1981. This was linked to increased glacier melt, which became apparent through step-like increases in discharge at both gauges as well as change points in mass balances of the two largest glaciers in the Vent catchment. We identified exceptionally high July temperatures in 1982 and 1983 as a likely cause. In contrast, we did not find coinciding change points in precipitation. Opposing trends at the two gauges after 1981 suggest different timings of “peak sediment”. We conclude that, given large-enough training datasets, the presented QRF approach is a promising tool with the ability to deepen our understanding of the response of high-alpine areas to decadal climate change.

Pristine levels of suspended sediment in large German river channels during the Anthropocene?

Abstract. Suspended sediment is an integral part of riverine transport and functioning that has been strongly altered during the Anthropocene due to the overwhelming human pressure on soils, sediments, and the water cycle. Understanding the controls of changing suspended sediment in rivers is therefore vital for effective management strategies. Here we present results from a trend analysis of suspended sediments covering 62 monitoring stations along the German waterways (catchment sizes range between 2000 and 160 000 km2) with more than 440 000 water samples taken between 1990 and 2010. Based on daily monitoring of suspended sediment concentration (SSC), we found significant declines in mean annual SSC and annual suspended sediment loads (SSLs) at 49 of 62 monitoring stations totaling −0.92 mg L−1 yr−1. At some stations decreases during the 20 years represent up to 50 % of the long-term average SSC. Significant decreases in SSC are associated with declining SSL despite an increase in sheet and rill erosion by almost 150 % derived from modeling due to changes in land use and management as well as rainfall erosivity. The contemporary suspended sediment loads of the Rhine at the German–Dutch border are approaching the natural base level of ∼1 Mt yr−1, which was reached by the Rhine during the mid-Holocene when the suspended sediment load was adjusted to the Holocene climatic conditions and before the onset of increased loads due to human-induced land use changes in the Rhine catchment. At this point we can only speculate regarding potential reasons for a decline in sediment transport in larger rivers despite erosion increase. We argue that increased sediment retention in upstream headwaters is presumably the major reason for declining SSC in the large river channels studied.

Climatic and hydrogeomorphic controls on sediment characteristics in the southern Sierra Nevada

The magnitude of sediment yield from headwater catchments is controlled by the interactions among hydrology, geomorphology, and soil disturbance. In montane regions like the Sierra Nevada, snow is one of the main factors controlling the timing and magnitude of hydrological fluxes. However, the role of snow on modulating spatial and temporal variation of sediment yield remains unclear. Using 120-site years of sediment yield data from 10 headwater catchments (drainage area 50–475 ha), we examined the sediment yield characteristics across an elevational gradient (1,777–2,381 m elevation) in the southern Sierra Nevada. Across space and time, we calculated an average annual suspended sediment yield of 62 ± 147 Mg/km2. In contrast, the measured mean annual bedload yield from the study catchments was small, 1.1 ± 2.4 Mg/km2. A linear mixed-effects model showed that maximum annual discharge alone can only explain 24 % (marginal R2 = 0.24) of the variance in sediment yield. Similarly, the hypsometric integral, which is often used as a metric for erosion susceptibility, showed no predictive power (marginal R2 = 0.005). As much as 65 % of the variance in sediment yield can be explained by fixed effects when snow related drivers (i.e., center of flow timing and aspect) were included in the model along with maximum annual discharge, suggesting a strong influence of snowmelt. Moreover, the relationship between area normalized suspended sediment yield (Qs) and unit discharge (Q) was significantly different between rain and snow events (p = 0.001). Both average slope (α) and exponent (β) terms of the Qs=αQβ relationship across the ten catchments were higher for rainfall (α=5.8,β=1.84) than those for snowmelt (α=3.1,β=1.77) events. As the erosion severity and power were higher during rainfall than the snowmelt events, a shift in the precipitation form from snow to rain under a warming climate will likely increase sediment yield. These results provide critical insights on background sediment yield in the southern Sierra Nevada and likely changes under future climate.

Longest sediment flows yet measured show how major rivers connect efficiently to deep sea

Here we show how major rivers can efficiently connect to the deep-sea, by analysing the longest runout sediment flows (of any type) yet measured in action on Earth. These seafloor turbidity currents originated from the Congo River-mouth, with one flow travelling >1,130 km whilst accelerating from 5.2 to 8.0 m/s. In one year, these turbidity currents eroded 1,338-2,675 [>535-1,070] Mt of sediment from one submarine canyon, equivalent to 19–37 [>7–15] % of annual suspended sediment flux from present-day rivers. It was known earthquakes trigger canyon-flushing flows. We show river-floods also generate canyon-flushing flows, primed by rapid sediment-accumulation at the river-mouth, and sometimes triggered by spring tides weeks to months post-flood. It is demonstrated that strongly erosional turbidity currents self-accelerate, thereby travelling much further, validating a long-proposed theory. These observations explain highly-efficient organic carbon transfer, and have important implications for hazards to seabed cables, or deep-sea impacts of terrestrial climate change.

Suspended sediment transport in a tropical river basin exhibiting combinations of land uses/land covers and hydroclimatic conditions: Case study of upper Athi Basin, Kenya

Study regionThe Upper Athi River Basin (Lat: 1°25′17.61°S and Long: 37°15′29.55°E) in Kenya, East Africa. Study focusThe effects of land use/land cover type on suspended sediment concentrations and yield were investigated in the upper Athi river basin in Kenya. The suspended sediment concentrations and yield, river discharges, water levels and flow velocities were monitored in stations established at the outlets of sub-basins and also at the outlet of the main basin in period 2012–2015. New hydrological insightsThe magnitude of river discharge in the upper Athi river basin is not only a function of the magnitude of rainfall, but combination of land uses/land covers determined the magnitude of sediment yield in the sub-basins. The highest sediment yield occurred in a semi-arid sub-basin dominated by savannah grassland and livestock grazing. The lowest occurred in the high rainfall sub-basins dominated by forest with mixed farming and settlements. Significant sediment transport commenced at an effective river discharge of 405 m3s−1 which is a frequent discharge event resulting in significant sediment load transport in most rainy seasons and years. The magnitude of suspended sediment discharge is a function of the magnitude of rainfall such that the greatest sediment transport occurs during the long rainy seasons (54%) as compared to 41% during the short rainy season. The total annual suspended sediment load in the basin ranged 2.11–3.77 × 106 tons.yr−1 and sediment production rate ranged 205–366 tons.km−2.year−1.

Long‐term suspended sediment and particulate organic carbon yields from the Reynolds Creek Experimental Watershed and Critical Zone Observatory

AbstractLong‐term (&gt;20 y) suspended sediment (SS) and particulate organic carbon (POC) records are relatively rare and yet are necessary for understanding linkages between climate, erosion and carbon export. We estimated long‐term (&gt;23 y) SS and POC yields from four nested catchments that ranged from &lt;1 to 54 km2 in area across the Reynolds Creek Experimental Watershed and Critical Zone Observatory (RCEW‐CZO) in southwestern Idaho, USA. We found strong relationships between log10SS and log10POC (R2 = 0.38–0.86) that varied across catchments but remained robust across years, one dry and one of the wettest water years on record. Mean annual SS yields varied from 18 to 89 g SS m−2 y−1 and POC from 0.6 to 11.0 g C m−2 y−1 across the four catchments. Water yield explained much of the temporal variation (72%–85%) in SS and POC yields except in a small, snow‐dominated headwater catchment where it explained 15%–51%. The largest five water years accounted for 69%–84% of the total SS and POC yields in catchments with 24 y records. All catchments had positive slopes (&gt;0) for SS and POC concentration‐discharge (C‐Q) relationships, with large catchments exhibiting greater slopes (0.66–0.97) than smaller ones (0.14–0.16). In addition, most catchments were dominated (80%) by clockwise hysteretic curves. Lack of seasonal exhaustion in the SS‐POC relationships, positive C‐Q and clockwise relations indicated that these systems were transport‐rather than supply limited, and that sediment and POC appeared to be sourced from channel/bank erosion and remobilization. POC yields represent 1%–10% of mean water year net ecosystem exchange depending on elevation; lower elevation catchments may shift from being carbon sinks to sources after accounting for fluvial POC export associated with changes in climate.

Runoff and sediment response to deforestation in a large Southeast Asian monsoon watershed

Deforestation has been reported to increase annual streamflow, baseflow, and sediment load in many parts of the world. However, studies about the hydrological effects of deforestation are scant in the tropical monsoon region. In this study, these effects were examined in the Upper Chao Phraya River basin (UCPRB) which is a typical tropical monsoon basin and is an important area of water resources in Thailand. A significant breakpoint for annual streamflow and annual precipitation in the entire UCPRB is identified as 1993, and this breakpoint is also identified for annual baseflow in most upstream sub-basins during the past decades (1981–2015). Significant increasing trend is detected in the annual suspended sediment load of the Wang River sub-basin. Land use maps generated by a CA-Markov model suggest that continuous deforestation occurred during the study period. The contributions of climate change and deforestation to the hydrological and sedimentary changes are then attributed within the Budyko Framework. Precipitation increment dominates the annual streamflow increment in the entire UCPRB and the annual baseflow increment in the Ping River sub-basin, and deforestation dominates the annual streamflow increment in the upstream sub-basins, the annual baseflow increment in most of the upstream sub-basins, and the increasing suspended sediment load in the Wang River sub-basin. The hydrological and sedimentary response to deforestation is then quantified. In the entire UCPRB, one percent reduction of the forest cover can increase the annual streamflow by 1.9%. In the upstream sub-basins, one percent reduction of the forest cover can increase the annual streamflow and annual baseflow by 2.5%–5.4% and 2.6%–6.7%, respectively. One percent reduction of the forest cover can increase the annual suspended sediment load by 8.7%. These findings will help develop better understanding of land use management in the UCPRB for water-soil conservation.

Natural and reservoir-induced channel changes in the Yangtze River Tidal Reach

The Yangtze River Tidal Reach (YRTR) is one of the most important land-sea transition zones in China. The YRTR is 640 km long and the adjacent land area of 220,000 km2 has a population of ca. 150 million. Even though the effect of Three Gorges Dam (TGD) on its downstream has been extensively studied, a comprehensive understanding of natural and dam-induced changes in the YRTR is still a challenging task. Here we use a digital elevation model and a numerical model to identify the causes of channel changes in the YRTR. Our analysis highlighted three main points.1) Changes in the tidal range caused an increase in the ebb maximum discharge and led to an increase in channel width, depth, and cross-section area. However, the channel depth abruptly starts to dwindle at the transition zone where unidirectional ebb flow changes to bidirectional ebb-flood flows. 2) Dam interception has reduced the sediment supply into the YRTR causing a change from deposition to erosion in the river bed. An increase of one billion m3 in reservoir capacity cut down annual suspended sediment supply by 2.26 million tons and scoured 1.54 million tons of riverbed sediment annually from the YRTR into the estuary. 3) Dam storage caused an increase of ca. 3800 m3/s runoff into the YRTR in the spring season during 2001–2013 compared with the same period in 1959–1970. This caused an average increase of ca.10% in the water diversion rate into the branch and an increase in flow velocity in flood plains and branch channels of 2.9 and 2.5 times that in the trunk channels. Such hydrodynamic increases have accelerated erosion of the branches and flood plains in recent decades especially post TGD. The study presents original insights into the tidal channel longitudinal abrupt changes induced by shift of natural flow regime, and riverbed changes from deposition to erosion induced by reservoirs, and indicates the likely response of the YRTR to further decline in sediment load in the future.

Catchment properties controlling suspended sediment transport in wind-water erosion crisscross region

Study regionWuding River catchment, the wind-water erosion crisscross region on the Chinese Loess Plateau. Study focusThe wind-water erosion crisscross region suffers severe soil erosion in the world, and various catchment properties control fluvial processes. This study investigated suspended sediment (SS) transport characteristics in a typical catchment with two geomorphological types (aeolian sand and loess hilly). Lorenz curves and partial least-squares regression were used to quantify the SS transport characteristics and attributions. New hydrological insights for the regionThere were significant differences in the SS transport characteristics between the two geomorphological regions. In the aeolian sand region, the mean annual SS loads were much lower than that in the loess hilly region (P < 0.01) under similar precipitation and runoff conditions. Rivers in this region transported SS all year round. The relatively lower concentration indicated that extreme events did not dominate total SS load. Conversely, SS transport in the loess hilly region was highly concentrated during a few events in rainy season, with the largest 10% of the events transporting exceed 80% of the total SS load. Statistical analysis showed that drainage density, catchment slope, and soil texture were the dominant factors controlling SS load, while the concentration of SS transport events was more related to the perimeter-area fractal. These findings have important implications for fluvial sediment transport in arid and semi-arid environments.

How did the suspended sediment load change in the North Caucasus during the Anthropocene?

AbstractQuantifying and understanding catchment sediment fluxes is crucial both from a scientific and environmental management perspective. To deepen the understanding of landuse impacts and climate change on sediment load, we explore factors controlling the suspended sediment load formation in the Northern Caucasus during the Anthropocene. We examine how sediment flux of various river basins with different land‐use/landcover and glacier cover changes during the 1925–2018 period. Our analysis is based on observed mean annual suspended sediment discharges (SSD, kg s−1) and annual fluxes (SSL, t year−1) from 33 gauging stations of The Federal Service for Hydrometeorology and Environmental Monitoring (Russia).SSLseries have been analysed to detect statistically significant changes during the 1925–2018 period. The occurrence of abrupt change points inSSDwas investigated using cumulative sum (CUSUM) charts. We found thatSSLhas decreased by −1.17% per year on average at most gauges. However, the decline was not linear. Several transition years are expected in the region increasing trends from the 1950s and decreasing trends from 1988 to 1994. Correlation analyses showed that variation inSSLtrend values is mainly explained by gauging station altitude, differences in landuse (i.e. the fraction of cropland), and catchment area. Nonetheless, more accurate quantifications ofSSLtrend values and more refined characterizations of the catchments regarding (historical) landuse, soil types/lithology, weather conditions, and topography may reveal other tendencies.

SOME QUESTIONS OF ESTIMATING SUSPENDED SEDIMENT IN WATERCOURSES OF NORTH-WEST RUSSIA

In the practice of water management for economic purposes, calculations of suspended sediment for various time scales – from per-second to annual, are important. The estimation of annual suspended sediment transport, as well as of the statistical parameters of turbidity and the reliability of the estimates, is extremely difficult because of irregular and insufficient light conditions in different phases of the water regime. At the same time, owing to the knowledge of the processes that shape the quality of natural waters and a sufficient toolkit of calculation methods, the main indicators of water quality with can be reproduced with acceptable accuracy. The capabilities of statistical analysis of water quality indicators can be expanded by an integrated approach to estimating the indicators by means of well-proven deterministic and stochastic calculation algorithms with arguments observed regularly and for a long time. In this case, a composite method can be used to estimate the parameters of the suspended sediment distribution or water turbidity, which permits finding the parameters of the distribution curve of the function from the parameters of the distribution curve of its arguments. The paper presents a deterministic-stochastic modeling system “weather-runoff-suspended sediments”, based on a stochastic weather model, a model of runoff formation in a catchment area, and a model of annual suspended sediment transport. The system can estimate the distribution parameters of daily suspended sediment load and turbidity values in the situation of observed data shortage or changes in runoff formation in the catchment resulting from natural causes or economic activities.

Contemporary Trends in River Flow, Suspended Sediment Load, and Soil/Gully Erosion in the South of the Boreal Forest Zone of European Russia: The Vyatka River Basin

Recent decades in the north of the East European Plain have been characterized by significant changes in climate and land use/cover, especially after the collapse of the USSR in 1991. At the same time, the hydrological consequences of these changes, especially changes in erosion processes and river sediment load, have been studied insufficiently. This paper partially covers this existing knowledge gap using the example of the Vyatka River basin. Draining an area of 129,000 km2, the Vyatka River is among the largest rivers in the boreal forest zone of European Russia. Cultivated land occupies about one-fifth of the river basin area; about three-fourths is covered by taiga forest vegetation. The results of state long-term hydrometeorological monitoring and information on land use/cover made it possible to reveal contemporary (since the 1960s) hydrological and erosion-intensity trends and their drivers within the greater (96%) part of the river basin. There has been a statistically insignificant increase in water discharge in the Vyatka River basin during recent decades. This is due to a statistically insignificant increase (for the entire basin studied) in the spring snowmelt-induced floodwater flow and a statistically significant rise in the discharge in the year’s warm and cold seasons. The main reason for the detected trends is increased precipitation, including heavy rainfall during the warm season. In contrast to this, the total annual suspended sediment load of the river (especially that which was snowmelt-induced) and, consequently, soil/gully erosion intensity have experienced a significant decrease in recent decades (up to 58% between 1960–1980 and 2010–2018). Land-use/-cover changes (a reduction of cultivated land area and agricultural machinery, a decline of livestock in pastures) following the collapse of the Soviet Union are considered the main reasons for this decrease. The most noticeable changes in water discharge, suspended sediment load, and erosion intensity were observed in the most agriculturally developed southwest and south parts of the Vyatka River basin. All the above trends may be considered with a high probability to be representative for the south sector of the taiga zone of the East European Plain.

Sediment transport at the network scale and its link to channel morphology in the braided Vjosa River system

AbstractIn this article we apply the CASCADE network‐scale sediment connectivity model to the Vjosa River in Albania. The Vjosa is one of the last unimpaired braided rivers in Europe and, at the same time, a data scarce environment, which limits our ability to model how this pristine river might respond to future human disturbance. To initialize the model, we use remotely sensed data and modeled hydrology from a regional model. We perform a reach‐by‐reach optimization of surface grain size distribution (GSD) and bedload transport capacity to ensure equilibrium conditions throughout the network. In order to account for the various sources of uncertainty in the calculation of transport capacity, we performed a global sensitivity analysis. The modeled GSD distributions generated by the sensitivity analysis generally match the six GSDs measured at different locations within the network. The modeled bedload sediment fluxes increase systematically downstream, and annual fluxes at the outlet of the Vjosa are well within an order of magnitude of fluxes derived from previous estimates of the annual suspended sediment load.We then use the modeled sediment fluxes as input to a set of theoretically derived functions that successfully discriminate between multi‐thread and single‐thread channel patterns. This finding provides additional validation of the model results by showing a clear connection between modeled sediment concentrations and observed river morphology. Finally, we observe that a reduction in sediment flux of about 50% (e.g., due to dams) would likely cause existing braided reaches to shift toward single thread morphology. The proposed method is widely applicable and opens a new avenue for application of network‐scale sediment models that aid in the exploration of river stability to changes in water and sediment fluxes.