Fluxes and storage of fine-grained sediment and associated contaminants in the Na Borges River (Mallorca,Spain)

Samples of suspended and fine channel bed sediment were used to examine the spatial and temporal variability in the amount of fine-grained sediment and associated contaminants temporarily stored in the Na Borges River (319 km²) in Mallorca and the relationship of such channel storage to the fluxes of fine sediment and associated contaminant through the system. This Mediterranean groundwater-dominated river drains a predominantly agricultural catchment, although urbanisation during the twentieth century has changed the catchment hydrology. A re-suspension technique was used to obtain estimates of channel storage at monthly intervals during the 2004–2005 hydrological year at eight locations along the main stem of the stream (i.e. 26 km). The estimates of fine sediment storage ranged between 0 and 13,000 g m^− 2, with a mean value of 2400 g m^− 2. Only Cu exceeded the critical threshold (36 μg g^− 1), established by existing guidelines for the contaminant content of fluvial sediment. The results demonstrated significant spatial and temporal variability, in response to the influence of urban point sources, agricultural practises, seasonal groundwater interactions and the first-flush effect. The amount of fine sediment entering storage during the study period was 515.2 t, representing the net increase in storage over the study period. As a result, the mean specific storage was 21 t km^− 1. Suspended-sediment load and temporary fine sediment storage are the two basic components of the channel sediment budget that interact to determine sediment transport through a channel system, and they can therefore be used to compute the total input of sediment and associated contaminants to the system. Accordingly, storage values were compared with estimates of suspended sediment load and associated contaminant load values at three measuring stations along the river. During the study period, storage in the main channel system represented 87% of the sediment input and 68% of the contaminant input, indicating that deposition was more important than transport. The low gradient of the main channel and the low return period (i.e. 0–2.5 years) of the flood events that occurred during the study period meant that remobilised bed sediment and associated contaminants were redeposited downstream rather than being flushed to the catchment outlet as suspended sediment. Furthermore, the river bed is dry during the summer months, allowing sealing and crusting processes to stabilise the sediment deposited during the wet season and thereby reduce its availability for remobilisation at the beginning of the next wet season. Together, these factors promote sediment deposition and storage, with the result that sediment progressively accumulate over several hydrological years until a major flood event (i.e. return period ≈ 5 years) evacuates the stored sediment.     

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.     

Rainfall,runoff and sediment transport relations in a mesoscale mountainous catchment: The River Isábena (Ebro basin)

This paper examines the relations between rainfall, runoff and suspended sediment transport in the Isábena basin during a quasi-average hydrological year. The Isábena is a mesoscale river basin that drains a mountainous area comprising patches of highly erodible materials (badlands). The paper includes an analysis of the different hydrological and sedimentary responses of the catchment to a similar rainfall. Thirty-four floods were studied, with a very variable response observed. Runoff coefficients ranged from 0.32% to 33%. The sedimentary response was also highly variable, with maximum suspended sediment concentrations (SSC) oscillating between < 0.1 and 90 g l^− 1 and flood sediment loads varying from 27 to 54,000 t per hydrological event. Most sediment load was concentrated in spring when competent floods occur frequently. Pearson correlation matrix and backward stepwise multiple regression indicate that the hydrological response of the catchment is strongly correlated with total precipitation, event duration, and rainfall of the previous days. Very low correlation was observed with rainfall intensity. The relation between rainfall and sediment transport followed the same trend. Sediment variables (e.g., total load and SSC) were significantly correlated with variables such as total rainfall and rainfall over the previous days, although the significance level was lower in comparison with the runoff related variables. There was again no correlation between sediment variables and rainfall intensity. On-going research in the area suggests that, apart from rainfall, factors such as sediment availability in the badlands and accumulation of sediment in the channels influences the river's sedimentary response. The non-linear hydrosedimentary response is reflected in the wide range of runoff coefficients and sediment loads that have been observed in response to similar amounts of precipitation.     

Geochemical tools for the stratigraphic correlation of floodplain deposits of the Morava River in Strážnické Pomoraví, Czech Republic from the last millennium

The floodplain of the Morava River in Strá?nické Pomoraví, south-east Moravia, Czech Republic contains a very valuable record of regional environmental change, which goes back to several thousand years. Its interpretation has been limited by poor stratigraphic correlation and dating of the sediments. We present a geochemical solution to this challenge. We studied 8 outcrops of floodplain deposits from 4 localities along a 6 km long part of the current meander belt of the Morava River in Strá?nické Pomoraví using geochemical proxy analyses, magnetic susceptibility measurements, ¹⁴C dating of wood remnants, and sediment micromorphology. The proxy methods are based on elemental analysis (EDXRF) and analysis of the cation exchange capacity of clay minerals; granulometric analyses provided the basis for lithological and facies assignment of the sediments. Our geochemical and mineralogical interpretations have further been tested by microstratigraphically studying the optical properties of the fine fraction. Horizons older than about 3 centuries were ¹⁴C dated using wood remnants and the age of deposits from the last century was determined on the base of several proxies reflecting their industrial contamination by heavy metals and magnetic particles. The mean depositional rate over the period from about 1000 to about 1900 AD ranged from 0.2 to 0.6 mm y^− 1, depending on the sedimentary facies. The coeval lithological change in the majority of the studied sections indicated a change of the meander belt structure at between ~ 1200 AD and ~ 1600 AD probably as a consequence of changes of channel structure. The alluvial deposition in the 20th century was strongly affected by the river regulation.     

Boundary conditions of morphodynamic processes in the Mura River in Slovenia

Many alpine gravel-bed rivers have been altered in the past due to human interventions. A typical transboundary alpine river in Central Europe flowing over Austria, Slovenia, Croatia and Hungary is the Mura River (length: 465 km, catchment area: 14,304 km²). The main problem of the river before leaving Austria is bed degradation (average 0.5 m from 1970 to 2000) as a combined consequence of river regulation works and the reduced sediment supply from the upstream reaches due to the construction of hydro power plants. River restoration measures for the river reach on the border between Austria and Slovenia (SLO–A Mura reach) were proposed in 2000 to support both ecological and flood protection purposes that apply the concept “of self-restoration”. In order to apply similar process-oriented restoration strategies in the Mura River downstream in Slovenia, we consider the potentials for self-forming river processes. We have analysed the sediment granulometry and morphology of the Mura River in Slovenia, and discussed morphodynamic processes to detect potential for the Mura River recovery into a more diverse morphological structure. The cross section area increased by 8% between 1979 and 2005 on average, mainly due to riverbed degradation. On average, the thalweg of the Mura riverbed in Slovenia had degraded by 0.28 m in the 1979–2007 period, with the highest degradation of 2.28 m in one cross section and some cross sections being stable. The high river degradation trend from the SLO–A Mura reach is slowly shifting to the downstream direction into the Mura River in Slovenia. Nevertheless, the clear downstream coarsening of river sediments turns into the normal trend of sediment fining in the Mura River reach in Slovenia. The “self-restoration” potential in the Mura River reach in Slovenia is larger than on the SLO–A Mura reach due to still active morphological fluvial processes. These processes can be enhanced by the inflow of fresh coarse sediments from the SLO–A Mura reach, where active measures for re-establishing sediment transport are under way. In the long term, this will not work as proposed, if no sediment inflow is re-established from the Mura headwaters in Austria.     

Coarse bed material patch evolution in low-order,ephemeral channels

In river channel beds composed of a wide range of grain sizes, the bed material is often arranged in discrete patches discernable by relative texture. These bed material patches are the primary source of entrainable coarse sediment within the channel system and their composition and size have been found to influence the composition and rate of sediment transport. Twelve coarse (gravel–cobble) sediment patches distributed throughout the channel network within a 4.53 ha watershed in southeastern Arizona were monitored for 2 years. Changes in patch area and grain size were measured and painted patch grains were monitored to confirm that patch grains were mobilized during flow. Individual coarse bed material patches exhibited variable persistence during flows with return frequencies ranging from approximately 1 year to 4.6 years. While no patch fully dispersed during the study period, two new patches formed. Most coarse patches remained relatively stable in area and grain-size distribution despite the entrainment of patch grains as lost grains were sufficiently replaced with sediment from upstream. Because of the grain replacement process and the effect of other sediment supply dynamics, the changes in patch area and grain-size distribution display a complex relationship with the magnitude of predicted grain mobilization within each patch. Results indicate that relative stability varies from patch to patch, influenced by the balance of patch grains transported out of the patch and the deposition of new grains into the patch. Predictive models of coarse sediment transport and yield that assume the channel bed is a fixed source of sediment supply may not adequately capture the sediment dynamics within patchy channel beds and should be used with caution when applied to these environments due to the possibility of patch instability as documented in this study.     

Modeling of hydrophobic cohesive sediment transport in the Ells River Alberta,Canada

Purpose

In this paper, a novel modeling approach is applied to assess the unique transport characteristics of hydrophobic (bitumen containing) cohesive sediment for the Ells River, AB, Canada. The modeling offers a new way of treating the transport and fate of fine sediment in rivers and points to the importance of including a sediment entrapment process in the modeling of the Ells River sediment dynamics.

Materials and methods

The modeling approach involves combining two existing models (RIVFLOC and MOBED). Using fine sediment transport parameters derived from laboratory flume experiments (e.g., settling velocity of sediment as a function of floc size and the critical shear stresses for deposition) and the calculated flow field from the MOBED model (using field survey data such as, cross-sectional geometry, river slope, grain size of bed material, and discharge), the RIVFLOC model is used to predict the transport characteristics (including entrapment) of the hydrophobic Ells River sediment.

Results and discussion

The application of the connected RIVFLOC and MOBED models, demonstrated the unique hydrophobic sediment dynamics of the Ells River. The model showed no deposition (in the classical sense) of the hydrophobic sediment as the bed shear stresses, even at base flow, are well above the critical bed shear for deposition (flocculation is shown to occur, but its impact on settling is negligible given the high shear stresses). However, the model showed the possibility of fine sediment ingression into the river bed (interstitial voids) due to the entrapment process which is known to occur at bed shear stresses well above the critical shear stress for deposition.

Conclusions

The salient features of RIVFLOC and MOBED models and their applications for understanding the transport and fate of unique hydrophobic fine sediments are presented. The models are shown to be useful for the understanding and projection of flow characteristics and sediment dynamics (including entrapment), and will be of benefit for the adaptive management of riverine monitoring programs given various flow scenarios including extreme events and climate change.

     

Boundary conditions of morphodynamic processes in the Mura River in Slovenia

Many alpine gravel-bed rivers have been altered in the past due to human interventions. A typical transboundary alpine river in Central Europe flowing over Austria, Slovenia, Croatia and Hungary is the Mura River (length: 465 km, catchment area: 14,304 km²). The main problem of the river before leaving Austria is bed degradation (average 0.5 m from 1970 to 2000) as a combined consequence of river regulation works and the reduced sediment supply from the upstream reaches due to the construction of hydro power plants. River restoration measures for the river reach on the border between Austria and Slovenia (SLO–A Mura reach) were proposed in 2000 to support both ecological and flood protection purposes that apply the concept “of self-restoration”. In order to apply similar process-oriented restoration strategies in the Mura River downstream in Slovenia, we consider the potentials for self-forming river processes. We have analysed the sediment granulometry and morphology of the Mura River in Slovenia, and discussed morphodynamic processes to detect potential for the Mura River recovery into a more diverse morphological structure. The cross section area increased by 8% between 1979 and 2005 on average, mainly due to riverbed degradation. On average, the thalweg of the Mura riverbed in Slovenia had degraded by 0.28 m in the 1979–2007 period, with the highest degradation of 2.28 m in one cross section and some cross sections being stable. The high river degradation trend from the SLO–A Mura reach is slowly shifting to the downstream direction into the Mura River in Slovenia. Nevertheless, the clear downstream coarsening of river sediments turns into the normal trend of sediment fining in the Mura River reach in Slovenia. The “self-restoration” potential in the Mura River reach in Slovenia is larger than on the SLO–A Mura reach due to still active morphological fluvial processes. These processes can be enhanced by the inflow of fresh coarse sediments from the SLO–A Mura reach, where active measures for re-establishing sediment transport are under way. In the long term, this will not work as proposed, if no sediment inflow is re-established from the Mura headwaters in Austria.     

陕南山区侵蚀环境与水土流失特征

<正> 陕西省的南部山区,北靠秦岭,南临巴山,汉江、丹江、嘉陵江从境内流过,区内辖汉中、安康、商洛三个地区及宝鸡市的太白、凤县,共30个县(市)。秦岭山脉是黄河、长江流域和我国地理分带的天然分界,所以陕南山区的水土流失也表现出独有的特征。     

Assessing the Role of a Limestone Quarry as Sediment Source in a Developing Tropical Catchment

Impact assessments on river systems of the combined effect of bed and suspended sediment loads from quarries are difficult to find. In this study, bed and suspended loads were measured to determine the impact of a 20‐ha limestone quarry on the river system of its 5,000‐ha steep, diverse land use/land cover but mostly forested catchment. A network of hydrologic and sediment monitoring instruments was deployed over the catchment during two separate study periods when sediment loadings were measured from captured storms. Results showed that the quarry stood to make a disproportionately large contribution to the catchment's estimated 2·1 Mg ha⁻¹ yr⁻¹ suspended sediment load. Large storm events contributed most of the loadings with five events supplying 92% of total loadings at the outlet. A paired method approach to compare suspended sediment loads between two subcatchments showed that during eight storm events, the quarry yielded between 2 and 49·2 Mg ha⁻¹ per event, whereas the forest never yielded more than 0·1 Mg ha⁻¹. Furthermore, the contribution of sediments from the quarry to bed load was more than 75% at a section located 1·2 km downstream. Future management activities to reduce sediment and bed loads, not only from this catchment but also from all others with similar land use/land covers, should focus on improving quarry operations. Copyright © 2014 John Wiley & Sons, Ltd.     

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.     

Spatial and temporal variability of sediment and dissolved loads from two alpine watersheds of the Lesser Himalayas

Estimation of sediment load from Himalayan basins is of considerable importance for the planning, designing, installation and operation of hydro-power projects, including management of reservoirs. In the present study, an assessment of physical and chemical load, sediment yield and erosion rate has been undertaken at eight different locations in the Sainj and Tirthan watersheds. The analysis revealed that the maximum load was transferred during the monsoon season. Moreover, the estimated average chemical erosion rate of the Sainj (83 t km^− 2 yr^− 1) and Tirthan (80 t km^− 2 yr^− 1) watersheds were higher than that of the Indian average (69 t km^− 2 yr^− 1) representing all the rivers. Both watersheds were eroding physically and chemically at a faster rate than that of the world global average erosion rate (185 t km^− 2 yr^− 1). The flattish nature of the channels in some segments of these watersheds showed a lower transport of sediments, where as the constricted segments having steep bed slopes increased the velocity of flow and the sediment transport rate. These findings have important implications for water resource management in the context of sediments mobilization, erosion, channel management, ecological functions and operation of the hydro-power projects in the Lesser Himalayan region.     

黄河上游内蒙古河段塑槽输沙需水量分析

为了解冲积性河流塑槽输沙需水量,合理调配水资源,以内蒙古河段为例,依据挟沙水流能量平衡方程,研究了冲积性河流挟沙水流的能量耗散原理及水流塑槽和输沙能量的分配模式,给出了反应冲积性河流冲淤状态的塑槽输沙需水量计算方法,由平滩流量、河道来沙量和冲淤量三者组成的关系确定塑槽输沙需水量。利用内蒙古河段1960-2013年实测资料,计算得到内蒙古河段塑槽输沙水量,汛期来沙量0.7～1.1亿t,塑造2 000～3 000 m~3/s的中水河槽、淤积水平控制30%以下,需要的汛期塑槽输沙水量为94.6～141.2亿m~3;分析了不同条件下汛期塑槽输沙水量变化规律:来沙量一定,同样的中水河槽规模,控制淤积水平越低,需要的塑槽输沙水量越大;来沙量一定,同样的淤积水平,塑造中水河槽规模越大,需要的塑槽输沙水量越大;控制同样淤积水平,塑造同样中水河槽规模,汛期来沙量越大,需要的塑槽输沙水量越大。内蒙古河段塑槽输沙需水量计算结果和结果变化规律可为黄河上游内蒙古河段水资源配置提供参考。     

First data on the coastal dynamics and the sedimentary characteristics of the area influenced by the River Irminio basin (SE Sicily)

The various lithological outcrops in the drainage basin of the River Irminio are described for their contribution to the riverborne material and therefore to the sedimentary balance of the coastal strip. The main contribution to the present sedimentary balance of the subaqueous delta cannot be attributed to the River Irminio, owing to the extreme difference between fluvial and marine sediments. The river's hydraulic characteristics have not always been constant but have tended to decrease in three stages: the first, with a strong competence, shown by the Quaternary alluvial sediments with pebbles; the second, up to 1985, of medium competence, shown by sandy-gravelly deposits in the coastal strip and by finer sediments in the distal areas; the third, at the present day, with low competence, shown by a prevalently suspended load sediment with practically no fluvial influence and with a sedimentation dominated by the littoral currents; this is because between the 1960s and 1970s, owing to interventions along the river channel, the sediments from the basin were reduced to the point of non-existence and a new sedimentary cycle was set up on the delta with allochthonous sediments, exclusively made up of quartz clasts, whereas in the distal zones the mineralogical and compositional characteristics of the ancient river mouth were maintained.     

Surprisingly small increase of the sedimentation rate in the floodplain of Morava River in the Strážnice area,Czech Republic,in the last 1300 years

Sediment profiles from the floodplain of Morava River in the Czech Republic have been collected from exposed river banks (4–6 m long sections) and cores (2–4 m deep) and investigated using a set of geochemical proxies validated by granulometry and conventional geochemical analysis, outlined in our previous paper. The work was conducted to evaluate the increase in sedimentation rate during Medieval and modern time periods. Correlation of sediments along the current channel belt allows identification of two most important synchronous changes in the channel structure over the past 1300 years: in the 13th century and at the end of the 16th century. These changes could be related to central European climatic extremes rather than to land cover/land use practises. Analysis of the pollen record in peaty deposits at the floodplain edge allows excluded dramatic deforestation in Medieval times. Maps of the area from the last five centuries revealed direct and indirect signs of past avulsions and clearly show how the original multichannel system was transformed into a single meandering channel in the early 20th century. The extrapolated aggradation rate (net vertical accretion of floodplain fines except for levee sediments) increased from 0.2–0.3 cm/year in 700 AD to 0.3–0.4 cm/year in 2000 AD depending on the grain size of the sediment. This is the smallest yet reported enhancement of siliclastic deposition, although Morava River watershed has been intensively used for agriculture and its land cover has changed in a manner similar to west and central European rivers.     

Sediment transport from continuous monitoring in a perennial Mediterranean stream

Sediment transport in the Ribera Salada stream was estimated from continuous suspended and bedload monitoring between 2005 and 2008. The Ribera Salada basin is a mountainous perennial river in the Southern Pyrenees and is representative of the extensive forestry land use in this upland region. Water and sediment fluxes have been analysed with the aim of determining the thresholds, duration, ranges and relative contributions of the two sediment transport modes (suspended and bedload), and their variability in relation to the basin's annual hydrology. The stream's hydrology in the first two sampling years was average in the context of a 10-yr series of the basin, while 2007–2008 can be classified as a wet year. The specific total sediment load during the study period amounted to 12 t km⁻²y⁻¹, a low value compared with similar Mediterranean counterparts. The maximum load (31.5 t km⁻²y⁻¹) was observed during the wettest year of the monitoring programme. During average years most of the total load (> 90%) was transported in suspension; consequently, suspended sediment transport is more frequent through time. In contrast, bedload occurs sporadically during floods that exceed certain hydraulic thresholds, corresponding to a flow equalled or exceeded 4% of the time, that yields an average shear stress of ∼ 35 N m⁻². Under such conditions bedload transport becomes relevant and, if the threshold is frequently exceeded as in relatively wet years such as 2007–2008, may constitute the majority of the total sediment load (74%). This paper provides new evidence of the distinct role of sediment transport modes in stable fluvial environments where only sporadic inputs of surplus energy (flow discharge) determine the dominant mode, magnitude and duration of their respective contribution.     

Increasing magnetic susceptibility of the suspended particles in Yangtze River and possible contribution of fly ash

The magnetic properties of soil have been increasingly applied as a rapid and economic way to monitor environment pollution. Sediments from a growing islet in the lower reach of Yangtze River as well as the suspended particles in the surrounding river water were used to identify anthropogenic influence on the magnetic susceptibility (MS) of the Yangtze River sediment. Results show that newly deposited sediments in 2004 have significantly higher MS (~ 150 × 10⁻⁸ m³ kg⁻¹) on average than that of the ancient deposit (~ 50 × 10⁻⁸ m³ kg^− 1). Scanning electron microscope (SEM) of the extracted magnetic particles from newly deposited sediments and fly ash samples indicates large contribution of fly ash for the samples with elevated MS. Dependence of MS on grain size is evident, which enable calculation of the MS of suspended particles from river sediment. A value of 32 × 10⁻⁸ m³ kg^− 1is inferred for the suspended particles in ancient Yangtze River. The records from 2004 to 2010 indicate progressive increase in the MS of the suspended particles in Yangtze River from 67 to 96 × 10⁻⁸ m³ kg^− 1, which is much higher than that of the ancient. Mass balance calculation based on the increasing MS suggests that at least 7% of the fly ash produced within the catchment of Yangtze River was released into the environment.     

The sediment budget of a large river regulated by dams (The lower River Ebro, NE Spain)

Purpose

The aim of this work was to develop a comprehensive fluvial suspended sediment budget for a large regulated river, the lower River Ebro (NE Spain).

Materials and methods

The sediment loads of the Ebro mainstem and its main tributaries were estimated from continuous records of water discharge and turbidity (appropriately transformed to suspended sediment concentrations). Records were obtained at ten monitoring sections during the relatively dry 2008–2011 period.

Results and discussion

The sediment load estimated for the River Ebro upstream of the Mequinenza Reservoir is remarkable (i.e. mean suspended load of 0.6?×?10⁶?t?year^?1), despite the fact that the site is already affected by a sediment deficit due to upstream reservoirs. Further downstream, and owing to their humid characteristics, the contribution of the Pyrenean tributaries (Segre and Cinca Rivers) is much larger compared with their Iberian Massif counterparts (Matarranya and Algars Rivers), with sediment loads of 0.49?×?10⁶ and 2,260 t, respectively. The suspended sediment load trapped in the Mequinenza-Ribarroja-Flix Dam Complex for the study period was estimated at 2.3?×?10⁶?t. Below the dams, the sediment load was reduced by 95 % but increased gradually in a downstream direction due to the erosion processes that clear water (i.e. very low sediment concentrations) flood flows exert on the river bed and banks and the episodic contribution from ephemeral tributaries.

Conclusions

Reservoirs have reduced the overall sediment load and the natural variability of flow and sediment transport in the River Ebro. In addition, the sediment budget revealed that floods were not the only drivers of the sediment dynamics in the lower Ebro. For instance, the particular location of the monitoring sections showed that episodic contributions from small tributaries alter the general sediment load of the river during certain torrential events.     

Land degradation,soil conservation and the sediment load of the Yellow River,China: Review and assessment

Ninety per cent of the sediment load of the Yellow River, the world's muddiest river, comes from the loess plateau region in the middle course of the river. Control of this sediment supply is essential to avoid the danger of flooding in the lower reaches on the South China Plain. Since 1971, sediment loads entering the lower reaches have decreased, by 20 per cent as a result of lower precipitation, and by 27 per cent through soil conservation works and reservoir construction. Reductions in erosion can be obtained by restraining the formation of overland flow through promotion of an effective vegetation cover and elimination of the soil surface cover to encourage infiltration. Reservoir operation is a key to control of sediment movement through tributaries and along the main river. Water demands often conflict with sediment control, and, as a consequence, reservoir operation systems have changed several times in the past decade. The possibility of a further reduction in the sediment load of the Yellow River exists, but much depends on how rapidly control of grazing and deforestation leads to the establishment of plant cover on the loess plateau.     

Effects of anthropogenic activities on the Lower Sakarya River

We investigate the effects of anthropogenic activities on the Lower Sakarya River. The impacts of dam, levee, and bridge constructions, sand-gravel mining activities and water withdrawals during the industrialization period of the Sakarya River Basin have been explored by analyzing flow, sediment and channel cross section data from different periods in time by comparing pre- and post-1975 periods. The year 1975 is roughly determined to be commencement of heavy human activities. Assessment of data shows that average annual flow is reduced by almost 20% after 1975. Due to increased regulations after 1975 flow became less variable, i.e. low-flows are increased and high-flows are reduced. Flow showed less variation with seasons during the post-1975 period compared to the pre-1975 period. Close inspection of precipitation and temperature patterns over the course of this period indicates that these changes in the flow regime cannot be attributed to natural causes and must be instigated by anthropogenic activities. Analyses of sediment data point toward a consistent reduction in sediment concentration and loadings with years in the Lower Sakarya River. Sediment rating curves developed for pre- and post-1975 exhibit a similar pattern. The impact of the anthropogenic activities on the river cross section is also examined by employing data from 1965, 2003 and 2006 at various points along the river profile. We found as much as 1 m aggradation at the thalweg elevation along the river profile starting from the river mouth up to the 12th km. Degradation in thalweg elevation is observed upstream of the 12th km, as much as 7 m at some locations. This research clearly undermines how human activities can alter the river hydrology and morphology. The adverse impacts of these modifications on the stream ecology in the Lower Sakarya River unfortunately remain unresolved.