Sediment Load Reduction Research Articles

Recent Changes of Water Discharge and Sediment Load in the Buyuan River Basin, China

Changes in stream flow and sediment load in the Buyuan River Basin is a focus due to its vital ecological niche in supply feeding and spawning habitats for upstream migrant fish species of Mekong River. Based on the precipitation and water discharge series data (19592008), and sediment load series data (19932008), the runoff and sediment discharge variation and its response to precipitation has been analyzed. The results shown that, during the study period of 19592008 at the Manan gauging station, annual runoff of Buyuan River has generally decreased, but not significant at the 95% confidence level. However, the decline trend of sediment load was obvious for gauged period from 1993 to 2008. The seasonal runoff distribution in the Buyuan River was uneven and demonstrated an observably unimodal distribution. Both water discharge and sediment load are largely derived from wet season (from Jun to Nov), especially in flood season from July to September. During the study period 1993-2008, both annual and monthly observed value of water discharge and sediment load had coincident changing processes. Double mass curves indicated decreasing precipitation was the main reason for runoff reduction in the Buyuan River Basin; declining rainfall and water discharge were two key factors for sediment load reduction. In 2002, however, sediment discharge undergone abrupt change due to huge peak discharge and two consecutive years strong precipitation in the Buyuan River Basin.

Optimal Location of Sediment-Trapping Best Management Practices for Nonpoint Source Load Management

Selecting approaches to managing nonpoint source pollution is challenging due to the complex generation and transport processes that influence the quantity of pollutant that eventually reaches a receiving water. Pollutant transport is influenced by land surface characteristics along the transport path, and these effects should be considered when optimizing management approaches for nonpoint source pollution. A fully distributed sediment-generation and transport watershed model is presented within an optimization framework to enable the development of spatially precise solutions to sediment-trapping best management practice (BMP) placement at the watershed scale. To focus on the BMP-siting problem, a stylized representation of a BMP is assumed to be capable of reducing sediment mass by a fixed fraction, and optimal arrangements of this assumed BMP type are developed with a genetic algorithm. The results suggest that the optimal location for sediment mass reduction is not necessarily at locations of only high mass generation or at locations of only high transport capacity. The results also suggest that there are efficient locations for management that produce a relatively large reduction in storm sediment load.

Scenario-testing of agricultural best management practices in Lake Erie watersheds

Current research has shown that reductions in nonpoint nutrient loading are needed to reduce the incidence of harmful algal blooms and hypoxia in the western and central basins of Lake Erie. We used the Soil and Water Assessment Tool (SWAT) to test various sediment and nutrient load reduction strategies, including agricultural best management practice (BMP) implementation and source reduction in various combinations for six watersheds. These watersheds, in order of decreasing phosphorus loads, include the Maumee, Sandusky, Cuyahoga, Raisin, Grand, and Huron, and together comprise 53% of the binational Lake Erie Basin area. Hypothetical pristine nutrient yields, after eliminating all anthropogenic influences, were estimated to be an order of magnitude lower than current yields, underscoring the need for stronger management actions. However, cover crops, filter strips, and no-till BMPs, when implemented at levels considered feasible, were minimally effective, reducing sediment and nutrient yields by only 0–11% relative to current values. Sediment yield reduction was greater than nutrient yield reduction, and the greatest reduction was found when all three BMPs were implemented simultaneously. When BMPs were targeted at specific locations rather than at random, greater reduction in nutrient yields was achieved with BMPs placed in high source locations, whereas reduction in sediment yields was greatest when BMPs were located near the river outlet. Modest nutrient source reduction also was minimally effective in reducing yields. Our model results indicate that an “all-of-above” strategy is needed to substantially reduce nutrient yields and that BMPs should be much more widely implemented.

Historical reconstruction of organic carbon decay and preservation in sediments on the East China Sea shelf

AbstractSediment cores were collected from the East China Sea inner shelf in 2010 to study the decay and preservation of organic carbon (OC). The highest sediment mass accumulation rate (0.61 ± 0.20 g cm−2 yr−1), derived from 210Pb, was found near the river mouth and decreased alongshore to the south (0.17 ± 0.004 g cm−2 yr−1), and in an offshore direction (0.31 ± 0.08 g cm−2 yr−1). Average total OC content was higher at inner shelf stations (0.52%) than those offshore (0.38%). The δ13C was more depleted at nearshore (−23.49‰ to −21.97‰) than offshore (−22.49‰ to −21.60‰) stations. Principal component analysis indicated that terrestrial OC, as indicated by lignin‐phenols (Λ8) values, was preserved in sediment closer to the coast (0.22–0.44), while offshore sediment was more composed of lignin‐poor (0.12–0.24) degraded OC that was likely hydrodynamically sorted. Marine‐derived OC, as indicated by plant pigments, was significantly more abundant in the sediment mixed layer than the underlying accumulation layer. Historical flooding events were detected in Λ8 profiles in two of the six cores located at midshelf stations. Despite the magnitude of the 2010 flood in East China, we did not see any signature of this event with the chemical biomarker in these two cores. This may suggest that reduced sediment loading due to recent dam construction may have greatly decoupled river inputs with sediment loading to shelf sediment. The total OC standing stock since 1900 was approximately 1.62 ± 1.15 kg C m−2, about one tenth of all the middle and lower lakes in the Changjiang catchment basin. This work further supports the need for more research to better understand how the reduced inputs of fluvial input of sediments from Chinese rivers (due to river diversions and dams) affect carbon cycling in the East China Sea.

Can control of soil erosion mitigate water pollution by sediments?

The detrimental impact of sediment and associated pollutants on water quality is widely acknowledged, with many watercourses in the UK failing to meet the standard of ‘good ecological status’. Catchment sediment budgets show that hill slope erosion processes can be significant sources of waterborne sediment, with rates of erosion likely to increase given predicted future weather patterns. However, linking on-site erosion rates with off-site impacts is complicated because of the limited data on soil erosion rates in the UK and the dynamic nature of the source–pathway–receptor continuum over space and time. Even so, soil erosion control measures are designed to reduce sediment production (source) and mobilisation/transport (pathway) on hill slopes, with consequent mitigation of pollution incidents in watercourses (receptors).The purpose of this paper is to review the scientific evidence of the effectiveness of erosion control measures used in the UK to reduce sediment loads of hill slope origin in watercourses. Although over 73 soil erosion mitigation measures have been identified from the literature, empirical data on erosion control effectiveness are limited. Baseline comparisons for the 18 measures where data do exist reveal erosion control effectiveness is highly variable over time and between study locations. Given the limitations of the evidence base in terms of geographical coverage and duration of monitoring, performance of the different measures cannot be extrapolated to other areas.This uncertainty in effectiveness has implications for implementing erosion/sediment risk reduction policies, where quantified targets are stipulated, as is the case in the EU Freshwater Fish and draft Soil Framework Directives. Also, demonstrating technical effectiveness of erosion control measures alone will not encourage uptake by land managers: quantifying the costs and benefits of adopting erosion mitigation is equally important, but these are uncertain and difficult to express in monetary terms.

Sediment loads response to climate change: A preliminary study of eight large Chinese rivers

Climate change characterized by increasing temperature is able to affect precipitation regime and thus surface hydrology. However, the manner in which river sediment loads respond to climate change is not well understood, and related assessment regarding the effect of climate change on sediment loads is lacking. We present a quantitative estimate of changes in sediment loads (from 1.5 Gt yr−1 pre-1990 to 0.6 Gt yr−1 from 1991–2007) in response to climate change in eight large Chinese rivers. Over the past decades, precipitation change coupled with rising temperatures has played a significant role in influencing the sediment delivery dynamics, although human activities, such as reservoir construction, water diversion, sand mining and land cover change, are still the predominant forces. Lower precipitation coupled with rising temperatures has significantly reduced sediment loads delivered into the sea in semi-arid climates (4–61%). In contrast, increasingly warmer and wetter climates in subtropical zones has yielded more sediment (0.4–11%), although the increase was offset by human impact. Our results indicate that, compared with mechanical retention by reservoirs, water reduction caused by climate change or human withdrawals has contributed more sediment reduction for the rivers with abundant sediment supply but limited transport capacity (e.g., the Huanghe). Furthermore, our results indicate that every 1% change in precipitation has resulted in a 1.3% change in water discharge and a 2% change in sediment loads. In addition, every 1% change in water discharge caused by precipitation has led to a 1.6% change in sediment loads, but the same percentage of water discharge change caused largely by humans would only result in a 0.9% change in sediment loads. These figures can be used as a guideline for evaluating the responses of sediment loads to climate change in similar climate zones because future global warming will cause dramatic changes in water and sediment in river basins worldwide at rates previously unseen.

Changes in the sediment load of the Lancang-Mekong River over the period 1965–2003

Changes in the sediment load of a river can have important impacts on river channel evolution, nutrient fluxes, aquatic ecology and delta erosion and sedimentation, and the possibility of changes in the sediment load of the Lancang-Mekong River has attracted increasing concern in recent years. Existing studies present conflicting findings regarding the nature and magnitude of recent changes in the sediment load of the Lancang-Meking River and the authors have attempted to assemble the most reliable data on annual sediment loads for the period 1965–2003, to assess recent trends in the sediment load of the river. The changes in annual sediment load at 7 stations on the river are analyzed. Important sediment contributing areas are found in the reaches between Gajiu and Yunjinghong, Chiang Saen and Luang Prabang and downstream of Nong Khai. The sediment load increased at Gajiu, Yunjinghong and Chiang Saen over the period 1985–1992 because of serious soil erosion caused by the expansion of cultivation, the replacement of natural forest by plantations and land disturbance associated with hydropower dam construction. A marked reduction in sediment load occurred at Gajiu after the impoundment of the Manwan Hydropower dam on the Lancang River, but this reduction was not evident downstream at Yunjinghong and the stations further downstream. Significant increases in sediment load appeared at Mukdahan and Khong Chiam. These contrasting patterns of change reflect the influence of sediment contributions from the intervening catchment areas and channel systems as well as storage and remobilization of sediment from the channel system and the impact of hydraulic works such as irrigation systems. The long term mean annual sediment load of the Mekong River at its mouth is estimated to be ca. 145×106 t a−1, which is lower than previously reported values and it seems likely that this will be reduced in the foreseeable future.

The impact of Best Management Practices on simulated streamflow and sediment load in a Central Brazilian catchment

The intense use of water for both public supply and agricultural production causes societal conflicts and environmental problems in the Brazilian Federal District. A serious consequence of this is nonpoint source pollution which leads to increasing water treatment costs. Hence, this study investigates in how far agricultural Best Management Practices (BMPs) might contribute to sustainable water resources management and soil protection in the region. The Soil and Water Assessment Tool (SWAT) was used to study the impact of those practices on streamflow and sediment load in the intensively cropped catchment of the Pipiripau River. The model was calibrated and validated against measured streamflow and turbidity-derived sediment loads. By means of scenario simulations, it was found that structural BMPs such as parallel terraces and small sediment basins (‘Barraginhas’) can lead to sediment load reductions of up to 40%. The implementation of these measures did not adversely affect the water yield. In contrast, multi-diverse crop rotations including irrigated dry season crops were found to be disadvantageous in terms of water availability by significantly reducing streamflow during low flow periods. The study considers rainfall uncertainty by using a precipitation data ensemble, but nevertheless highlights the importance of well established monitoring systems due to related shortcomings in model calibration. Despite the existing uncertainties, the model results are useful for water resource managers to develop water and soil protection strategies for the Pipiripau River Basin and for watersheds with similar characteristics.

Environmental and Economic Trade-Offs in a Watershed When Using Corn Stover for Bioenergy

There is an abundant supply of corn stover in the United States that remains after grain is harvested which could be used to produce cellulosic biofuels mandated by the current Renewable Fuel Standard (RFS). This research integrates the Soil Water Assessment Tool (SWAT) watershed model and the DayCent biogeochemical model to investigate water quality and soil greenhouse gas flux that results when corn stover is collected at two different rates from corn-soybean and continuous corn crop rotations with and without tillage. Multiobjective watershed-scale optimizations are performed for individual pollutant-cost minimization criteria based on the economic cost of each cropping practice and (individually) the effect on nitrate, total phosphorus, sediment, or global warming potential. We compare these results with a purely economic optimization that maximizes stover production at the lowest cost without taking environmental impacts into account. We illustrate trade-offs between cost and different environmental performance criteria, assuming that nutrients contained in any stover collected must be replaced. The key finding is that stover collection using the practices modeled results in increased contributions to atmospheric greenhouse gases while reducing nitrate and total phosphorus loading to the watershed relative to the status quo without stover collection. Stover collection increases sediment loading to waterways relative to when no stover is removed for each crop rotation-tillage practice combination considered; no-till in combination with stover collection reduced sediment loading below baseline conditions without stover collection. Our results suggest that additional information is needed about (i) the level of nutrient replacement required to maintain grain yields and (ii) cost-effective management practices capable of reducing soil erosion when crop residues are removed in order to avoid contributions to climate change and water quality impairments as a result of using corn stover to satisfy the RFS.

Identification of fluvial sediment sources in the Kharaa River catchment, Northern Mongolia

AbstractFine‐grained sediments constitute a major stressing factor for the aquatic ecosystem in the 15 000 km2 Kharaa River catchment in Northern Mongolia. The objectives of this study were to identify the suspended sediment (SS) sources and quantify the sediment budget of the basin. Sediment sources were identified with the help of sediment tracing methods utilizing Be‐7, Cs‐137 and Pb‐210 radionuclides. High‐resolution discharge data were used in combination with daily suspended solid measurements to calculate the SS budget. These calculations were compared with the monthly archive data on SS and discharge to investigate temporal load variations. In addition, the sediment budget model SedNet was used to estimate the SS budget and test its applicability in a cold semi‐arid region. Results of the sediment tracing showed that riverbank erosion generates 74.5% of the suspended sediment load, whereas surface erosion contributes 21.7% and gully erosion only 3.8%. In the most intensely used agricultural tributary catchment Zagdelin Gol, upland erosion contributed only 12.7% to the total SS losses. The calculated mean annual sediment load for the years 1990–2002 was 20.3 kt⋅a−1. The SedNet model computed SS export from the catchment in the same order of magnitude as measured data (16.2 kt⋅a−1). The results help to identify effective management measures to reduce sediment loads and mitigate its impact on the aquatic environment. Copyright © 2012 John Wiley & Sons, Ltd.

Model for Prioritizing Best Management Practice Implementation: Sediment Load Reduction

Understanding the best way to allocate limited resources is a constant challenge for water quality improvement efforts. The synoptic approach is a tool for geographic prioritization of these efforts. It uses a benefit-cost framework to calculate indices for functional criteria in subunits (watersheds, counties) of a region and then rank the subunits. The synoptic approach was specifically designed to incorporate best professional judgment in cases where information and resources are limited. To date, the synoptic approach has been applied primarily to local or regional wetland restoration prioritization projects. The goal of this work was to develop a synoptic model for prioritizing watersheds within which suites of agricultural best management practices (BMPs) can be implemented to reduce sediment load at the watershed outlets. The model ranks candidate watersheds within an ecoregion or river basin so that BMP implementation within the highest ranked watersheds will result in the most sediment load reduction per conservation dollar invested. The model can be applied anywhere and at many scales provided that the selected suite of BMPs is appropriate for the evaluation area’s biophysical and climatic conditions. The model was specifically developed as a tool for prioritizing BMP implementation efforts in ecoregions containing watersheds associated with the USDA-NRCS conservation effects assessment project (CEAP). This paper presents the testing of the model in the little river experimental watershed (LREW) which is located near Tifton, Georgia, USA and is the CEAP watershed representing the southeastern coastal plain. The application of the model to the LREW demonstrated that the model represents the physical drivers of erosion and sediment loading well. The application also showed that the model is quite responsive to social and economic drivers and is, therefore, best applied at a scale large enough to ensure differences in social and economic drivers across the candidate watersheds. The prioritization model will be used for planning purposes. Its results are visualized as maps which enable resource managers to identify watersheds within which BMP implementation would result in the most water quality improvement per conservation dollar invested.

Cumulative sediment trapping by reservoirs in large river basins: A case study of the Yellow River basin

Reservoir sedimentation has been a serious problem for reservoir operation and watershed management worldwide, which highlights the importance of accurate estimate of the deposited sediment amount. With reservoir information derived from remote sensing dataset and observed hydrological records of water and sediment, this paper is concerned with the cumulative reservoir sediment trapping in the Yellow River basin. The river basin was divided into 12 sub-basins, upon which the reservoirs' sediment trapping efficiency (TE) and their interception effects on inflowing sediment were estimated with sediment records extracted from 179 field sampling stations. The results show that, with the sediment TE greater than 80% for all the sub-basins, theoretically the basin-wide reservoirs are able to trap most sediment. For the whole river basin, the sediment TE averages 95.2%, which indicates that the reservoirs can collectively make a significant anthropogenic signature on basin-wide sediment delivery. A basin-wide sediment yield map was generated to estimate the reservoir sedimentation amount. During 1950–1970, annually about 2.483Gt of sediment was transported into channels from sloping lands. Taking into account the actual water storage changes and the reservoir construction history, the actual reservoir sedimentation rate was estimated at 0.59Gt/yr after corrections, which represents 47.6% of the river basin's total sediment load reduction. Globally, reservoir sedimentation in the Yellow River basin represents about 12–15% of the global mean rate. Up to 2010, approximately 19.32Gt of sediment has been trapped by Yellow River reservoirs, and totally about 40.32Gt of sediment has been artificially fixed if silt check dams are also considered. With huge amounts of sediment deposited, these reservoirs have been losing their storage capacity to sedimentation at a rate of 0.6% per year. The magnitude is expected to enhance in future following new reservoir completions. Thus, more efforts are strongly needed to explore the associated responses.

Temporal variations of water discharge and sediment load of Huanghe River, China

Based on the data from gauging stations, the changes in water discharge and sediment load of the Huanghe (Yellow) River were analyzed by using the empirical mode decomposition (EMD) method. The results show that the periodic oscillation of water discharge and sediment load of the Huanghe River occurs at the interannual, decadal, and multi-decadal scales, caused by the periodic oscillations of precipitation, and El Nino/Southern Oscillation (ENSO) af- fects water discharge by influencing precipitation distribution and contributes to periodic varations in precipitation and water discharge at interannual timescale. The water discharge and sediment load of the Huanghe River have decreased since the 1960s under the influence of precipitation and huamn activities, and human activities attribute more than precipitation to the reduction in the water discharge and sediment load, furthermore, water abstraction and water-soil conservation practices are the main causes of the decrease in water discharge and sediment load, respectively. The reduction in sediment load has directly impacted on the lower reaches of the Huanghe River and the river delta, causing considerable erosion of the river channel in the lower reaches since the 1970s along with River Delta changing siltation into erosion around 2000.

Determining the relative contributions of climate change and multiple human activities to variations of sediment regime in the Minjiang River, China

AbstractIn‐stream sediment transport plays an important role in delivery of sediment‐associated terrestrial elements. Investigating the history of fluvial sediment regime responding to changes in natural and anthropogenic driving forces provides a theoretical basis for establishment of optimal strategies on catchment management. The present study aims to systematically detect the patterns of change in sediment load at two key hydrological stations (Pengshan and Gaochang) in the Minjiang River and quantitatively evaluate the relative contributions of regional precipitation change and multiple local human activities to the observed sediment variations. Abrupt change in annual sediment load was detected in 1990 at Pengshan and in 1968, 1980 and 1992 at Gaochang. Compared with the baseline period of 1957–1990, precipitation decline and human activities had respectively contributed to 5 × 106 t and 2 × 106 t of reduction in mean annual sediment load at Pengshan during 1991–2007. For the entire Minjiang basin, taking 1956–1968 as the baseline period, precipitation decline and human activities had relatively contributed to 10 × 106 t and 18 × 106 t of reduction in mean annual sediment load at Gaochang during 1969–1980. During 1981–1992, precipitation decline had relatively contributed to 5 × 106 t of reduction in mean annual sediment load, but human activities had led to 3 × 106 t of increase in mean annual sediment load. During 1993–2009, 13 × 106 t and 17 × 106 t of reduction in mean annual sediment load may be attributed to precipitation decline and human activities, respectively. Copyright © 2012 John Wiley & Sons, Ltd.

FIELD SCALE MODELING TO ESTIMATE PHOSPHORUS AND SEDIMENT LOAD REDUCTIONS USING A NEWLY DEVELOPED GRAPHICAL USER INTERFACE FOR SOIL AND WATER ASSESSMENT TOOL

Streams throughout the North Canadian River watershed in northwest Oklahoma, USA have elevated levels of nutrients and sediment. Soil and Water Assessment Tool (SWAT) was used to identify areas that likely contributed disproportionate amounts of Phosphorus (P) and sediment to Lake Overholser, the receiving reservoir at the watershed outlet. These sites were then targeted by the Oklahoma Conservation Commission (OCC) to implement conservation practices, such as conservation tillage and pasture planting as part of a US Environmental Protection Agency Section 319(h) project. Conservation practices were implemented on 238 fields. The objective of this project was to evaluate conservation practice effectiveness on these fields using the Texas Best Management Evaluation Tool (TBET), a simplified Graphic User Interface (GUI) for SWAT developed for field-scale application. TBET was applied on each field to predict the effects of conservation practice implementation on P and sediment loads. These predictions were used to evaluate the implementation cost (per kg of pollutant) associated with these reductions. Overall the implemented practices were predicted to reduce P loads to Lake Overholser by nine percent. The ‘riparian exclusion’ and ‘riparian exclusion with buffer’ practices provided the greatest reduction in P load while ‘conservation tillage’ and ‘converting wheat to bermuda grass’ produced the largest reduction in sediment load. The most cost efficient practices were ‘converting wheat to bermuda grass’ or ‘native range’ and ‘riparian exclusion’. This project demonstrates the importance of conservation practice selection and evaluation prior to implementation in order to optimize cost share funds. In addition, this information may lead to the implementation of more cost effective practices and an improvement in the overall effectiveness of water quality programs.

Soil Erosion Control and Sediment Load Reduction in the Loess Plateau: Policy Perspectives

The sediment load of the Loess Plateau has shown a sharp decreasing trend in the past decades. This paper gives a comprehensive review of the driving factors of the sediment decrease with respect to human activities, including soil and water conservation, dam construction, and vegetation restoration. Also presented is an overview of the main achievements of soil and water conservation and the main soil erosion control programmes implemented in the Loess Plateau. The paper concludes with suggestions for further policy modifications that could move management towards ecological sustainability and will be greatly beneficial to the regional water resources management and restoration of an eco-environmental system in the Loess Plateau.

Exposition professionnelle au trichloroéthylène et pathologies cancéreuses du col utérin, une étude cas témoins

This study addresses the problem of modelling sediment erosion at the catchment scale, in order to predict the possible impact of reservoirs and land use changes on sediment load in South East Asia. The investigated basin is the Lo river basin, (38,165 km2 in Viet Tri), a left tributary of the Red River, where the Thac Ba and the recently built Tuyen Quang reservoirs have already been changing downstream sediment load since 1971 and 2005 when they were, respectively, in operations. The RUSLE equation is adopted in a distributed GIS framework to assess catchment erosion, and is coupled with a sediment accumulation and routing scheme to model suspended sediment load in the Lo basin at a monthly scale. Monthly precipitation were collected and used as input to the model. Suspended sediment load data, measured at eight gauging stations in Vietnam, from 1959 to 2007, were compared with the model’s simulated sediment yield. Resulting average Nash–Sutcliffe efficiency is 0.45, ranging from 0.33 to 0.62 and bias is 5% at Vu Quang, close to the basin outlet, and results 1.4% by averaging biases at the eight stations, thus confirming that the model is adequate. Effects of reservoirs were analysed by modelling erosion and sediment yield passing from natural to impounded conditions and resulted in a suspended sediment load reduction of about 95% and 75% downstream, respectively, the Thac Ba reservoir in the Chay river and the Tuyen Quang reservoir in the Gam river. Land use change scenarios, parameterized on the basis of observed land use changes in the impounded basin and assuming that 20% of forest area is converted into rice and agricultural crops and 15% into bushes, shrubs and meadows, are expected to induce a 28% increase of suspended sediment load which can compensate, at least in part, sedimentation in reservoirs. Also agricultural and hillslope maintenance practices can modify sediment erosion in the basin.

Targeting resource investments to achieve sediment reduction and improved Great Barrier Reef health

Concerns about excessive sediment loads entering the Great Barrier Reef (GBR) lagoon in Australia have led to a focus on improving ground cover in grazing lands. Ground cover has been identified as an important factor in reducing sediment loads, but improving ground cover has been difficult for reef stakeholders in major catchments of the GBR. To provide better information an optimising linear programming model based on paddock scale information in conjunction with land type mapping was developed for the Fitzroy, the largest of the GBR catchments. This identifies at a catchment scale which land types allow the most sediment reduction to be achieved at least cost. The results suggest that from the five land types modelled, the lower productivity land types present the cheapest option for sediment reductions. The study allows more informed decision making for natural resource management organisations to target investments. The analysis highlights the importance of efficient allocation of natural resource management funds in achieving sediment reductions through targeted land type investments.

Gender of large river deltas and parasitizing rivers

Deltas are the most dynamic part of large rivers and the characteristics of deltas reflect the basic nature of morphodynamics, ecology and anthropogenic influence. The authors investigated many deltas of large rivers, including the Yellow, Yangtze, Pearl, Rhine, Nile, Mississippi, Luanhe and Ebro rivers. Data were collected and sediment, water, benthic invertebrates and fish were sampled. Statistical analysis showed that deltas can be classified into male deltas and female deltas. The deltas of the Yellow, Mississippi, Luanhe, Ural and Ebro rivers are male deltas. Male deltas extend into the ocean, each with only one or two channels, forming a fan-shape. Its development process is accompanied with periodic nodal or random avulsions. Male deltas develop if the sediment load/water ratio is high and the tidal range is low. The deltas of the Yangtze, Rhein-Meuse-Scheldt, Pearl and Irrawaddy rivers are female. Female deltas consist of complex channel networks and numerous bars and islands. Female deltas develop if the load/water ratio is not high and tidal current is relatively strong. Female deltas provide stable and multiple habitats for various bio-communities. Therefore, the biodiversity of female deltas and the taxa richness of benthic invertebrates and fish are much higher than that of male deltas. Human activities reduced sediment load and change the delta gender from male to female. Male rivers have high levees in their lower reaches and estuary. Several rivers originate from the levees of a male river and flow parallel with the river into the sea. Appearance, persistence, length and stability of these rivers depend on the male river to which they are attached. Therefore, these rivers are named parasitizing rivers. In general, parasitizing rivers have no tributaries and almost no drainage area. The runoff of these rivers comes from the rainfall on the surrounding area, and therefore, the flow discharge exhibits very sharp peaks during rainfall. The Yellow and Luanhe rivers have many parasitizing rivers. Some of the parasitizing rivers are quite long, with a length of 400-500 km, such as the Tuhai River and Majia River. Management of parasitizing rivers must be integrated into management strategies of their father rivers.

A RUSLE approach to model suspended sediment load in the Lo river (Vietnam): Effects of reservoirs and land use changes

Summary This study addresses the problem of modelling sediment erosion at the catchment scale, in order to predict the possible impact of reservoirs and land use changes on sediment load in South East Asia. The investigated basin is the Lo river basin, (38,165 km 2 in Viet Tri), a left tributary of the Red River, where the Thac Ba and the recently built Tuyen Quang reservoirs have already been changing downstream sediment load since 1971 and 2005 when they were, respectively, in operations. The RUSLE equation is adopted in a distributed GIS framework to assess catchment erosion, and is coupled with a sediment accumulation and routing scheme to model suspended sediment load in the Lo basin at a monthly scale. Monthly precipitation were collected and used as input to the model. Suspended sediment load data, measured at eight gauging stations in Vietnam, from 1959 to 2007, were compared with the model’s simulated sediment yield. Resulting average Nash–Sutcliffe efficiency is 0.45, ranging from 0.33 to 0.62 and bias is 5% at Vu Quang, close to the basin outlet, and results 1.4% by averaging biases at the eight stations, thus confirming that the model is adequate. Effects of reservoirs were analysed by modelling erosion and sediment yield passing from natural to impounded conditions and resulted in a suspended sediment load reduction of about 95% and 75% downstream, respectively, the Thac Ba reservoir in the Chay river and the Tuyen Quang reservoir in the Gam river . Land use change scenarios, parameterized on the basis of observed land use changes in the impounded basin and assuming that 20% of forest area is converted into rice and agricultural crops and 15% into bushes, shrubs and meadows, are expected to induce a 28% increase of suspended sediment load which can compensate, at least in part, sedimentation in reservoirs. Also agricultural and hillslope maintenance practices can modify sediment erosion in the basin.