The combined use of wash load and bed material load rating curves for the calculation of total load: An example from the Burdekin River, Australia

In hydraulic response terms, the total clastic load of a river is comprised of wash load and bed material load components. Bed material load includes sediment moving as actual bed load and as suspended material originating from the bed. Classical suspended sediment rating curves do not differentiate between wash load and suspended bed material and their indiscriminant use can therefore lead to ambiguous results. Wash load must be delineated from the total suspension when using a rating curve. This is particularly important for rivers draining tropical catchments as these transport a large proportion of fine sediment. For rivers in which a relationship exists between wash load concentration and discharge, an estimate of fine sediment delivery may be obtained from a wash load rating curve. The remaining bed material load may then be calculated from physical principles. This technique is applied to the Burdekin River of northeast Australia.The Burdekin River drains a tropical semi-arid hinterland and transports a predominantly silt and clay wash load. A wash load rating curve is established by applying correction factors for suspended bed material under different flow rates to a classical suspended sediment rating curve. The resulting estimate of mean annual wash load is 3.0 million tonnes of silt and clay. The use of a theoretical bed load equation (ACKERS & WHITE 1973) together with hydraulic parameters on the coastal plain yields an estimate of mean annual bed material load of 0.45 million tonnes of sand.     

Rapid shrinkage of Kushiro Mire,the largest mire in Japan,due to increased sedimentation associated with land-use development in the catchment

The suspended sediment discharge in the Kuchoro River, a tributary of the Kushiro River, showed that wash load was about 90% of the yearly total suspended load carried into the wetland from the entire watershed. Seasonal floods associated with typhoons and snowmelt carried a large volume of wash load, 44% and 37%, respectively, of the yearly total wash load. The deposition of suspended sediment in the channelized section has aggraded the riverbed by 2 m in the past two decades, which has reduced the cross-sectional area of the channel, so that turbid water spills over and carries wash load and suspended sediment deep into the wetlands. Flooding of turbid water, in association with aggradation of the riverbed, was detected by using a Water Turbidity Index. The flooding and turbidity have significantly increased between 1984 and 1994. Similarly, a Normalized Difference Vegetation Index map showed that forest establishment has continued from the wetland margins and in areas adjacent to the river channel. The suspended sediment carried and deposited by floods and sediment-associated nutrients should alter the edaphic environment from wet nutrient-poor soil to dry nutrient-rich soil providing regeneration habitats for tree species. The vegetation in turn adds resistance and friction against flows and contributes to additional sedimentation. However, flooding and the associated high water table causes stress for trees and may lead to premature decay. Environmental variables, such as water level, water content, organic content and grain size, varied significantly along cross-sectional distance, and are likely regulated by deposition of fine sediments transported by floods. Electrical conductivity and total nitrogen in groundwater varied significantly along the longitudinal distance. Basal areas of willow and alder stands correlated with variables related to spread of turbid water, which indicates that eutrophication of groundwater indirectly affects marsh forest expansion.     

Particle Size Selectivity Considerations in Suspended Sediment Budget Investigations

The delivery of suspended sediment front drainage basins has frequently been quantified in mass terms by use of the sediment budget approach, which identifies sources. Storage and output of mobilised sediment. An attempt is presented here to define the main components of a generalised suspended sediment budget for a drainage basin in Devon, U.K. in teens of particle size characteristics and grain size selectivity, rather than total amounts of sediment. Samples of sediment mobilised from the hillslopes, fluvial suspended sediment and suspended sediment deposited on the river bed were all collected for particle size characterisation. These samples were then treated to remove organic matter and their chemically dispersed (absolute) particle size composition was measured using a Coulter LS130 laser granulometer. Where possible, measurements of the naturalin-situ particle size distribution (effective particle size) were also undertaken. Samples were collected at different times of the year so that temporal variation of hydromcteorolgical and ground conditions was represented. Comparison of the results for the different components of the delivery process shows that significant particle size selectivity occurs in the mobilisation and transfer of sediment from the hillslopes to the basin outlet. This reflects the particle size selectivity of detachment, transport and deposition processes, which is in turn influenced by the aggregation or flocculation (effective particle size) of the sediment.     

近70年长江干流寸滩站以上流域水沙关系变化及其驱动因素

以长江干流寸滩水文站以上流域为研究对象,基于1953-2018年的实测水沙资料,采用线性回归、Mann-Kendall趋势检验和水沙关系曲线分析径流、输沙的时间趋势及其关系变化,并采用双累积曲线法分析水沙变化的驱动因素。结果表明:寸滩站以上流域多年平均径流量为3425亿m^3,多年平均输沙量为3.61亿t;多年平均月径流量为294.8亿m^3,多年平均月输沙量为3013万t,且主要分布在6-10月,分别占年径流泥沙总量的70%和95%以上。趋势分析显示,流域年降雨量和径流量变化趋势不明显,年输沙量呈极显著减小趋势;月径流量在1-4月显著增加,其他月份变化不显著,而各月输沙量均呈显著减少趋势。水沙关系曲线均可用幂函数拟合,拟合参数受时间尺度影响,统计检验显示悬移质输沙量实测值与水沙关系曲线的估算值差异不显著。双累积曲线分析发现水利工程建设和水土保持等人类活动是寸滩水文站输沙减少的主要原因,其作用占69%~93%,其中2000年以来人类活动的贡献超过90%。     

Dynamics of total suspended matter and phytoplankton loads in the river Elbe

Purpose

Identifying sources and fluxes of suspended matter within the catchment is vitally important for the water quality of rivers and for establishing sediment management plans. Constituents of suspended particles are of abiotic and biotic origin. In the Elbe, the biotic fraction of suspended particles is mainly composed of phytoplankton biomass. In this study, total seston and phytoplankton are analyzed for their seasonality, their interdependence and temporal trends over three or five decades, respectively.

Materials and methods

The biotic load was separated from the total suspended matter load, and time series of total suspended substances (seston) (1964 to 2015) and chlorophyll a values (1985 to 2015) were analyzed. Our analyses focused on the seasonal dynamics, long-term trends, and the correlation to hydrological events.

Results and discussion

The mean share of phytoplankton in total seston accounted for 24% in summer months (April–September), with a negative correlation between discharge and total seston, and 11% in winter months (October–March), with a weak positive correlation between discharge and total seston. The long-term trend of seston load was decreasing, while phytoplankton load did not show a significant trend.

Conclusions

Autochthonous biogenic portions should not be neglected in the budget of total suspended matter loads in the Elbe catchment. Our results indicate that land-use and industrial changes subsequent to the German reunification mainly caused the observed trend. Phytoplankton growth superimposes the seasonal dynamics of seston in summer, whereas in the long term, decreasing mineral fraction dominates the significantly decreasing trend.

     

泥沙来源“指纹”示踪技术研究综述

泥沙来源"指纹"示踪技术是综合研究流域土壤侵蚀和泥沙输移的新方法。泥沙来源"指纹"示踪技术基于流域侵蚀产沙过程划分潜在物源类型,根据物源特性筛选具有诊断能力的"指纹"性质,通过定量转换模型建立流域出口泥沙与内部潜在物源间的"指纹"联系,定量描述各潜在物源对流域出口产沙的相对贡献;结合悬移质或沉积泥沙通量监测,定量分析各潜在物源对流域产沙的绝对贡献量及流域侵蚀产沙时、空变化特征。通过综述泥沙来源"指纹"示踪技术的理论基础及实施框架,流域尺度潜在物源类型,泥沙"指纹"因子类别、分布特点及诊断能力,泥沙来源复合"指纹"示踪技术的研究进展,指出泥沙来源"指纹"示踪技术的局限性,并对泥沙来源"指纹"示踪技术进行展望。     

近70年长江干流寸滩站以上流域水沙关系

以长江干流寸滩水文站以上流域为研究对象,基于1953-2018年的实测水沙资料,采用线性回归、Mann-Kendall趋势检验和水沙关系曲线分析径流、输沙的时间趋势及其关系变化,并采用双累积曲线法分析水沙变化的驱动因素。结果表明：寸滩站以上流域多年平均径流量为3 425亿m³,多年平均输沙量为3.61亿t;多年平均月径流量为294.8亿m³,多年平均月输沙量为3 013万t,且主要分布在6-10月,分别占年径流泥沙总量的70%和95%以上。趋势分析显示,流域年降雨量和径流量变化趋势不明显,年输沙量呈极显著减小趋势;月径流量在1-4月显著增加,其他月份变化不显著,而各月输沙量均呈显著减少趋势。水沙关系曲线均可用幂函数拟合,拟合参数受时间尺度影响,统计检验显示悬移质输沙量实测值与水沙关系曲线的估算值差异不显著。双累积曲线分析发现水利工程建设和水土保持等人类活动是寸滩水文站输沙减少的主要原因,其作用占69%~93%,其中2000年以来人类活动的贡献超过90%。     

Dominant discharges for suspended sediment transport in a highly active Pyrenean river

Purpose

Dominant discharges and associated sediment dynamics of the River Isábena, a 445-km² catchment in the central Pyrenees of Spain that is punctuated by badlands, are analysed.

Materials and methods

Calculations of suspended sediment loads are based on continuous records of discharge and turbidity obtained at the basin outlet for the period 2005–2010.

Results and discussion

Dominant discharges for sediment load (i.e. effective discharge) present a bimodal distribution, with one peak falling in the range of low flows and the other associated to less frequent but higher magnitude floods (i.e. bankfull). The highly suspended sediment availability in the badlands, together with the high connectivity between the badlands and the stream network and the important in-channel fine sediment storage, causes both large and small events to remobilize fines. Baseflows, despite their low competence, generate resuspension and massive sediment loads. Thus, effective discharge (i.e. the discharge which transports most of the sediment) is not solely associated with bankfull (i.e. the discharge that dominates channel form), but to a wider range of discharges. Consequently, this river channel is not specifically adjusted to convey most of the sediment load during high floods, as in many other rivers, but instead large volumes of sediment are transferred downstream at an almost constant rate.

Conclusions

Results suggest that dominant discharge may play a lesser role in terms of (suspended) sediment load in non-supply-limited fluvial systems and/or in rivers that permanently work close to, or at, full transport capacity, as is the case of the Isábena.     

Temporal Dynamics of Sediment Transport and Transient In‐channel Storage in a Highly Erodible Catchment

Elucidating the links between catchment and channel geomorphic processes helps the understanding of landscape evolution and the geomorphic development of river basins, and then the land degradation processes. In this study, we analyse suspended sediment dynamics and its relationship with rainfall, discharge and in‐channel sediment storage in a highly dynamic Mediterranean montane catchment (the River Isáben, Southern Pyrenees, NE Iberian Peninsula). The aim is to assess hydrological controls on and temporal patterns of the suspended sediment load. High loads in this basin occur as a consequence of intense erosion in badlands located in the middle of the catchment. This study focuses on a reach located downstream from the main badland areas, where rainfall, discharge (Q), sediment transport (SSC) and in‐channel sediment storage were monitored for a 1‐year period. Marked seasonality in water and sediment load was observed; whereas most of the discharge occurred in winter and spring, most of the sediment was transported during summer and autumn. Q‐SSC hysteretic loops revealed the importance of sediment availability (whether stored in the channel or originating from wider catchment source areas) in the river's sedimentary response. Clockwise loops dominated during winter and spring, whereas counter‐clockwise loops occurred mostly in summer and autumn, when in‐channel storage reached its maximum. There were significant correlations between rainfall intensity in the sediment source areas, in‐channel sediment storage and sediment yield. These correlations emphasize the importance of understanding sediment availability when analysing the temporal dynamics of sediment transport, especially in catchments where different source areas (slopes and riverbed) may contribute to the load to differing degrees and at different times of the year. Copyright © 2015 John Wiley & Sons, Ltd.     

Factors controlling hydro-sedimentary response during runoff events in a rural catchment in the humid Spanish zone

In Galicia (NW Spain) studies on the suspended sediment (SS) transported in river catchments are scarce. Using a dataset comprising three hydrological years (2004/05–2006/07) in the Corbeira catchment (NW Spain), the relationships between rainfall, runoff and SS are analysed at event scale. The catchment was instrumented to measure rainfall, water level and SS in the stream. Several event (rainfall, discharge and SS concentration and load) and pre-event (rainfall and discharge) variables were calculated for all the events analysed. Correlation matrices were developed using these data. Discharge characteristics and rainfall depth showed good correlations with SS concentration and load. Suspended sediment and discharge relationships were also examined. In most of the events, the SS peak preceded peak discharge (clockwise hysteresis), implying that zones near the stream are the main sediment source areas.     

Evaluation of suspended load changes using AnnAGNPS and SWAT semi-empirical erosion models

Simulation models are frequently used tools for suspended sediment load quantification in mesoscale catchment areas. To improve the simulation results, it is important to compare and verify different models using measured data. In this study, we tested two continuous simulation, semi-distributed erosion and sediment transport models AnnAGNPS and SWAT for the period of 1995–2004 in the Blšanka river basin. The catchment (374 km²) is an agricultural hop-growing region in the north-western part of the Czech Republic. Both models were calibrated using the first 5 years of data, and then validated using the second 5 years of daily discharge and suspended sediment yield values. For the long-term continuous simulation, the results of SWAT model simulation showed a better agreement to the measured data, while for short-term rainfall-runoff event simulation, especially short duration intensive rainfall events, suspended load was more accurately predicted by AnnAGNPS. Land use changed significantly in parts of the river basin during the observation period. Many hop-gardens were dismantled and arable land was partially converted to grassland and pasture. This enabled us to test the applicability of AnnAGNPS and SWAT models under changing land use conditions. According to both models, suspended sediment load was reduced after the land use changes by 10%–30%, which was in agreement with decreasing sediment discharge observed at the watershed outlet.     

Characteristics of sediment discharge in the subarctic Yukon River, Alaska

The characteristics of sediment discharge in the Yukon River, Alaska were investigated by monitoring water discharge, water turbidity and water temperature. The river-transported sediment, 90 wt.% or more, consists of silt and clay (grain size62.5 μm), which probably originated in the glacier-covered mountains mostly in the Alaska Range. For early June to late August 1999, we continuously measured water turbidity and temperature near the estuary and in the middle of Yukon River by using self-recording turbidimeters and temperature data loggers. The water turbidity (ppm) was converted to suspended sediment concentration (SSC; mg/l) of river water, using a relation between simultaneous turbidity and SSC at each of the two sites, and then, the suspended sediment discharge, approximately equal to water discharge times SSC, was numerically obtained every 1 or 2 h. It should be noted that the sediment discharge in the Yukon River is controlled by SSC rather than water discharge. As a result, a peak sediment discharge occurred in mid or late August by local sediment runoffs due to glacier-melt (or glacier-melt plus rainfall), while a peak water discharge was produced by snowmelt in late June or early July. Application of the “extended Shields diagram” indicates that almost all the river-transported sediments are under complete suspension.     

Fingerprinting the sources of suspended sediment delivery to a large municipal drinking water reservoir: Falls Lake, Neuse River, North Carolina, USA

Purpose

We employ a geochemical-fingerprinting approach to estimate the source of suspended sediments collected from tributaries entering Falls Lake, a 50-km² drinking water reservoir on the Neuse River, North Carolina, USA. Many of the major tributaries to the lake are on North Carolina’s 303(d) list for impaired streams, and in 2008, the lake was added to that list because of high values of turbidity, likely sourced from tributary streams.

Materials and methods

Suspended sediments were collected from four streams with a time-integrated sampler during high-flow events. In addition, composite sediment samples representing potential sources were collected from stream banks, forests, pastures, construction sites, dirt and paved roads, and road cuts within tributary basins. Radiocarbon dating and magnetic susceptibility measurements were used to determine the origin of stream bank alluvial deposits. Sediment samples were analyzed for the concentrations of 55 elements and two radionuclides in order to identify tracers capable of distinguishing between potential sediment sources. The relative sediment source contributions were determined by applying a Monte Carlo simulation that parameterized the geochemical tracer data in a mixing model.

Results and discussion

Radiocarbon and magnetic susceptibility measurements confirmed the presence of “legacy” sediment in the Ellerbe and New Light Creek valley bottoms. Mixing model results demonstrate that stream bank erosion is the largest contributor to the suspended sediment load in New Light Creek (62%), Ellerbe Creek (58%), and Little Lick Creek (33%), and is the second largest contributor in Lick Creek (27%) behind construction sites (43%).

Conclusions

We find that stream bank erosion is the largest nonpoint source contributor to the suspended sediment load in three of the four catchments and is therefore a significant source of turbidity in Falls Lake. The presence of legacy sediment appears to coincide with increased contributions from stream bank erosion in Ellerbe and New Light creeks. Active construction sites and timber harvesting were also significant sources of suspended sediment. Water quality mitigation efforts need to consider nonpoint-source contributions from stream bank erosion of valley bottom sediments aggraded after European settlement.     

Factors controlling suspended sediment yield during runoff events in small headwater catchments of the Basque Country

For the first time in the Basque Country, turbidity (NTU), discharge (l/s) and precipitation (mm) have been continuously monitored in the gauging stations located at the outlet of three catchments (Aixola, Barrendiola and Añarbe) since October 2003. In this study, several data sets derived from flood events were used to develop turbidity and suspended sediment relationships for the three catchments separately, and so to estimate continuous suspended sediment concentration (SSC). Linear relationships are found in Barrendiola and Añarbe, and two curvilinear relationships for Aixola owing to changing sediment sources in the catchment. Several event (discharge, precipitation and suspended sediment concentration) and pre-event (discharge and precipitation) factors are calculated for all the events registered. With them correlation matrixes were developed for each catchment. Although some differences are found between catchments good correlation between precipitation (P), discharge (Q) and suspended sediment (SS) variables is found in general. Pre-event conditions are also well correlated with Q and SS variables in Barrendiola (higher regulation capacity) and Añarbe (larger area) but not in Aixola (“flash floods”). SSC-discharge evolutions through the events were also analysed. For Aixola four different types of hysteretic loops were observed: single lined, clockwise, counter-clockwise and eight-shaped; while for Barrendiola and Añarbe just clockwise loops were observed.     

Implication of relationships among suspended sediment size, water discharge and suspended sediment concentration: the Yellow River basin, China

According to the yearly maximum suspended sediment concentrations (SSC) in the Yellow River and its tributaries, the rivers are divided into three types of more than 300, 20–300, and less than 20 kg/m³. The middle Yellow River basin is located in the transitional zone from subhumid to semiarid climates, and covered by a thick loess mantle. Neighboring on the desert areas to the northwest, the surface material of the Loess Plateau exhibits some marked areal differentiation in grain size and forms three zones covered by sandy loess, (typical) loess and clayey loess from northwest to southeast. Controlled by these physico-geographical conditions, the grain size of river-transported sediment shows some particular characteristics; at small water discharge or SSC, the grain size of suspended sediment abruptly decreases to a minimum with increasing water discharge or SSC. At water discharge of more than ca. 40 m³/s or at SSC of more than ca. 40 kg/m³, the grain size increases with water discharge or SSC.During the low-stage season, the relatively clear baseflow may scour the coarse bed materials, so the suspended sediment is relatively coarse. In the rainy season, rainstorm runoff washes out fine loess materials to the river, making suspended sediment fine. During relatively strong rainstorms, there often occur hyperconcentrated flows at SSC of more than 300 kg/m³. The relatively coarse grains could then remain suspended in the mixture of water and fine suspended sediment.     

Determining contemporary and historical sediment sources in a large drainage basin impacted by cumulative effects: the regulated Nechako River,British Columbia,Canada

Purpose

Sediment dynamics in most large river basins are influenced by a variety of different natural and anthropogenic pressures, and disentangling these cumulative effects remains a challenge. This study determined the contemporary and historical sources of fine-grained (<?63-μm) sediment in a large, regulated river basin and linked changes in sources to activities in the basin. The river has seen declines in chinook salmon, sockeye salmon, and the endangered Nechako white sturgeon populations, and sediment (both fine-grained and sands) transport and deposition have been identified as potential causes of these declines.

Materials and methods

Samples of suspended sediment and potential source materials were collected from numerous sites distributed throughout the upper Nechako River Basin in British Columbia, Canada. A floodplain sediment core was also collected in order to reconstruct sediment sources over the last ~?70 years. Discriminating fingerprint properties were used within the MixSIAR model to apportion sources among sub-basins and land-use types. Results were compared to records of precipitation and Nechako River discharge trends, and to changes in landscape development.

Results and discussion

Contributions from the erosion of channel banks dominated the suspended sediment load at most sites. Changes in sediment sources during the 2015 field season reflected snowmelt and patterns of water release from the Nechako Reservoir that affected the sediment-carrying capacity of tributaries and the Nechako River main stem. Spatial variations in 2015 also reflected the distribution of land use (e.g., forested or agricultural land) as well as topography (e.g., slope steepness). Over the last ~?70 years, variations in sediment sources and the characteristics of the sediment (e.g., organic matter content and particle size composition) were linked to the construction of the Kenney Dam (operational in 1954) and the impacts of deforestation by the forestry and agricultural industries. Superimposed on these have been wildfires and a major mountain pine beetle infestation leading to higher erosion rates in the affected areas.

Conclusions

The sediment source fingerprinting technique, in combination with historical information on the hydrometeorology and the land use and river management in the basin, has provided valuable information with which to understand sediment dynamics in the Nechako River Basin. Such an approach can help to disentangle how large river systems respond to a combination of natural and anthropogenic pressures.

     

Increase in suspended sediment discharge of the Amazon River assessed by monitoring network and satellite data

This study addresses the quantification of the Amazon River sediment budget which has been assessed by looking at data from a suspended sediment discharge monitoring network and remote sensing estimates derived from MODIS spaceborne sensor. Surface suspended sediment concentration has been sampled every 10 days since 1995 (390 samples available) by the international HYBAM program at the Óbidos station which happens to be the last gauged station of the Amazon River before the Atlantic Ocean. Remote sensing reflectance is derived from continuous time series of 554 MODIS images available since 2000 and calibrated with the HYBAM field measurements. Discharge shows a weak correlation with the suspended sediment concentration during the annual hydrological cycle, preventing us from computing sediment discharge directly from the water discharge. Accordingly, river sediment discharge is assessed by multiplying daily water discharge measurements by the suspended sediment concentration averaged on a monthly basis. Comparisons of annual sediment discharge assessed using both field and satellite datasets show a very good agreement with a mean difference lower than 1%. Both field and satellite-derived estimates of the sediment concentration of the Amazon River are combined to get an uninterrupted monthly average suspended sediment discharge from 1995 to 2007. Unlike the water discharge which exhibits a steady trend over the same period at Óbidos, the 12-year suspended sediment discharge increases by about 20% since 1995, significant at the 99% level. In particular, the inter-annual variability is much more significant in the sediment discharge than in the river discharge.     

Importance of bank erosion for sediment input, storage and export at the catchment scale

Purpose

The importance of bank erosion was quantified during three periods (October 2006–April 2007, May 2007–April 2008 and May 2008–April 2009) in the 486 km² catchment area of River Odense, Denmark. A catchment sediment budget was established including other sediment sources such as tile drains and surface runoff, in-channel and overbank sinks and storage and the resulting bed load and suspended sediment load exported from the catchment.

Material and methods

Bank erosion and sedimentation were measured using ca. 3,000 erosion pins established in 180 pin plots, each consisting of three vertical lines of pins. Thirty-six representative reaches, each with a length of 100 m, were selected by a stratified random procedure in GIS. Bed load and suspended sediment export from the catchment were measured using a bed load sampler and from continuous measurements of turbidity at the outlet gauging station.

Results and discussion

The gross sediment input from bank erosion during the three study periods amounted to 21,100–25,200 t in the River Odense catchment, which is considerably higher than the estimated input of sediment from tile drains and surface runoff, which amounted to 220–500 t and 0–100 t, respectively. The measured bed load (20–490 t) was five to 60 times lower than the suspended sediment export from the catchment (1,240–2,620 t) during the three study periods, with the largest difference occurring in the driest year. Sediment sinks and storage were of high importance for the catchment sediment budget as the measured in-channel storage of sediment on stream banks was as high as 16,200–20,100 t, and the overbank sediment sink was estimated at 360–3,100 t.

Conclusions

Bank erosion was the dominant sediment source (90–94 %) in the River Odense catchment during the three study years. In-channel and overbank sediment sinks and storage dominated the sediment budget as 79–94 % of the sediment input from all sources was not exported from the catchment during the three study years. Such a large attenuation of sediment in river channels and on floodplains is extremely important for fluvial habitats and ecology. Moreover, it has strong implications for attempts to document changes in sediment export following implementation of mitigation measures.     

Sediment source analysis in the Linganore Creek watershed, Maryland, USA, using the sediment fingerprinting approach: 2008 to 2010

Purpose

Fine-grained sediment is an important pollutant in streams and estuaries, including the Chesapeake Bay in the USA. The objective of this study was to determine the sources of fine-grained sediment using the sediment fingerprinting approach in the Linganore Creek watershed, a tributary to the Chesapeake Bay.

Materials and methods

The sediment fingerprinting approach was used in the agricultural and forested, 147-km² Linganore Creek watershed, Maryland from 1 August 2008 to 31 December 2010 to determine the relative percentage contribution from different potential sources of fine-grained sediment. Fine-grained suspended sediment samples (<63 μm) were collected during storm events in Linganore Creek using an automatic sampler and manual isokinetic samplers. Source samples were collected from 40 stream bank sites, 24 agricultural (cropland and pasture) sites, and 19 forested sites. Suspended sediment and source samples were analyzed for elements and stable isotopes.

Results and discussion

Results of sediment fingerprinting for 194 samples collected in 36 separate storm events indicate that stream banks contributed 53% of the annual fine-grained suspended sediment load, agriculture contributed 44%, and forests contributed 3%. Peak flows and sediment loads of the storms correlate to stream bank erosion. The highest peak flows occurred in the winter and, along with freeze–thaw activity, contributed to winter months showing the highest rate of stream bank erosion. Peak flow was negatively correlated to sediment sources from agricultural lands which had the greatest contribution in non-winter months. Caution should be observed when trying to interpret the relation between sediment sources and individual storms using the sediment fingerprinting approach. Because the sediment fingerprinting results from individual storms may not include the temporal aspects of the sourced sediment, sediment that is in storage from previous events, remobilized and sampled during the current event, will reflect previous storm characteristics. Stream bank sediment is delivered directly to the channel during an event, whereas the delivery of upland sediment to the stream is lower due to storage on hillslopes and/or in channels, sediment from stream banks are more likely to be related to the characteristics of the sampled storm event.

Conclusions

Stream banks and agricultural lands are both important sources of fine-grained sediment in the Linganore Creek watershed. Peak flows and sediment loads for the 36 storms show a significant relation to sediment sources from stream bank erosion. Attempting to link upland sediment sources to flow and seasonal characteristics is difficult since much of the upland sediment eroded in an event goes into storage. By averaging sediment sources over several storms, it may be possible to determine not only the sediment sources that are directly contributed during the current event but also sediment from previous events that was in storage and remobilized.     

Relationship between suspended sediment load, channel geometry and land area increment in the Yellow River Delta

The annual river discharges and suspended sediment loads into the Yellow River Delta show a declining tendency with some distinct fluctuations over the last 50 years. The decrease of river discharge and suspended sediment load and the change in the river channel must influence the evolution of the Yellow River Delta. During this period several new river mouths formed via channel switch at the river delta, and the old watercourse was gradually abandoned. Recently, in years with very low annual suspended load, erosion of land area has been recorded for the delta. The aims of this work are (1) to determine what is the critical suspended sediment load needed in order to maintain the land balance of the Yellow River Delta for two periods before and after 1976 (the last time the channel shifted), and (2) to examine the variation in the channel geometry and gradient in response to changes in suspended sediment load and delta area at the river mouth. In order to estimate these critical values, we used statistical method to analyze the relationships between land area increment, and suspended sediment load and channel geometry. In order to examine the variation in the channel geometry, the channel cross-sections of the Q1 and Q6 were compared. The results show that to maintain the land area balance between 1953 and 1973, when the river mouth was the Diaokouhe, the critical annual suspended sediment load entering the delta was 4.21  10⁸ tonnes/a. After the main channel switched to Qingshuigou in 1976, the critical value to maintain the Qingshuigou mouth between 1976 and 1997 was 1.51  10⁸ tonnes/a. To maintain the land area balance for the entire Yellow River Delta between 1976 and 1997 the critical suspended sediment load was 3.18  10⁸ tonnes/a. The annual mean channel thalweg elevation and channel gradient at the river delta increase with increasing land area increment at the Qingshuigou mouth. The critical channel gradient at the channel reach between cross sections Q1 and Q6 is 0.000095. The channel has narrowed during the time period from 1976 to 1997. Also, lateral channel migration has decreased remarkably, resulting in enhanced hydraulic efficiency of the deltaic channel and artificial levees. This channel geometry evolution was influenced by river adjustment and human activities. These results are of importance for the management of the lower Yellow River channel and the delta. Future water diversion or river damming should consider the balance between suspended sediment delivery and delta growth.