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International audience; Mitigating erosion, mass movements, and geohazards in high mountains is increasingly conceived within frameworks of ecological restoration, that is, recovering the form and function of ecosystems that have been damaged by degradation (Hubble et al. 2017). From a geomorphological point of view, ecological restoration involves both the prevention and control of slope and riverbank instabilities as well as the confinement of runoff and sediment regimes to the capacity river channels. In this regard, the practice of soil and water bioengineering is rapidly emerging as a short-term hazard control that can enable long-term ecological recovery (Rey et al. 2019). Vegetation, as a chief ecological engineer, is key to soil and water bioengineering applications. However, the application of soil and water bioengineering in high mountains is limited by severe ecological conditions, making plant establishment and ecological recovery times are slow in high mountains (Dupin et al. 2019).While applications of sediment source fingerprinting using for example geochemical or radionuclide soil signatures yield a rough distinction between sediment sources, they cannot reflect the multiple vegetation covers that are relevant source types and should be discriminated in high mountain environments to prioritize restoration works. Vegetation may be the most distinctive feature of high mountains, where the underlying lithology is heterogeneous and soils are mainly shallow and poorly developed. Because there are strong interrelations between land cover and geomorphological processes in high mountain environments (Geertsema and Pojar 2007; Giaccone et al. 2019; Lizaga et al. 2019), the use of land cover-based sediment tracers would be particularly meaningful. eDNA has the highest source discrimination potential in that regard, providing information up to the species level and reflecting changes in vegetation on over short timescales. Furthermore, eDNA signals in sediments will be strongest from areas experiencing higher erosion rates and which are highly connected with the hydrographic network. The use of eDNA sediment source fingerprinting would thus allow the investigation of complex and often poorly understood relationships between vegetation cover, restoration activities, and geomorphological response at the catchment scale.To improve the success rates of restoration activities, collaboration between scientists and stakeholders can accelerate technology transfer rates (Stokes et al. 2014; Giupponi et al. 2019; Rey et al. 2019). However, time and budget constraints often hamper in-situ monitoring of soil and water bioengineering applications, and very few monitoring programs exist (Giupponi et al. 2019). Knowledge of success rates is, however, essential for restoration (Frankl et al. 2021). To this end, sediment source fingerprinting has been shown to provide a valid framework for supporting soil restoration activities (Mukundan et al. 2012). Environmental DNA has already been used to successfully monitor restoration programs, but with a focus on fungal species (Yan et al., 2018). We opinionate that eDNA fingerprinting – as an emerging technique – could be particularly useful to support soil restoration and hazard control in high mountain environments.

International audience; Mountains in Europe are highly valued as they provide diverse living and recreational opportunities and unique landscapes, are key economic assets, and because they are treasures of unique flora and fauna. Their vulnerable environment is, however, threatened by the frequent occurrence of shallow landslides and water erosion which produce large amounts of sediment during floods. The urgency to mitigate natural hazards calls for an improved understanding of how physical and biological dimensions of soil restoration interact. We address this issue by investigating how environmental DNA (eDNA) or DNA of organisms isolated from environmental samples can be used to trace hotspots of soil erosion in the Bastan catchment in the Pyrenees (France). Based on the persistence eDNA from vascular plant litter in soils and sediments, and the possibilities offered by DNA metabarcoding to characterise whole plant communities to the species level, we argue that eDNA can be used as a high-resolution fingerprinting method for identifying and tracing sediment sources. As such, bridging the gaps between physical and biological connectivity features at the catchment scale will allow us to develop tangible soil restoration scenarios which incorporate hazard protection, landscape and biodiversity restoration.

https://www.soil-discuss.net/soil-2020-22/soil-2020-22-RC2-supplement.pdf; General comments This study quantifies soil redistribution due to both water and tillage erosion processes in a 4.2-ha catchment (corresponding to one single field) located in an intensively cultivated region of Northeastern Germany. To reach this goal, Pu-239+240 inventory measurements have been conducted (and interpolated) and an inverse modelling approach (based on the SPEROS-Pu conceptual model) has been undertaken. Overall, the study was well designed (through a nice nested sampling approach with five densified sub-grids). In total, soil cores were collected at 219 locations (including 10 detailed depth profiles sampled at different topographic locations C1

International audience; The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident that occurred in March 2011 released significant quantities of radionuclides in the environment. So far, most of the research focused on radio-cesium and rarely on actinides. Until now, most of the studies on uranium and plutonium released by FDNPP were conducted on bulk environmental samples (soil, sediment, biota, etc.) and rarely on individual particles The investigation of individual particles allows working on the FDNPP signature alone compared to studies of bulk material which may also incorporate the signature of global fallout. Accordingly, the objective of the current research is to identify and characterize actinide-bearing particles in soil samples collected in the vicinity of FDNPP to get a better understanding of their formation mechanisms and of their fate in the environment. In order to identify and locate actinide-bearing particles in soil samples, we developed and implemented a method based on alpha-autoradiography (Jaegler et al., 2019), which allows identifying and locating specifically alpha-emitters, including plutonium isotopes 239Pu, 240Pu and 241Pu using a CR39 SSNTD device (Solid State Nuclear Tracks Detector). Before alpha-autoradiography samples were dried, grinded with an agate mortar and sieved to several cutoffs: 1mm, 700µm, 400µm, 200µm, 100µm and 63µm. Cutoffs were chosen according to the sizes of actinide-bearing particles detected by Satou et al (2018). The absence of significant loss of uranium particles by the sieving process was demonstrated by the analysis of a test soil sample with a 137 Cs activity below 650 Bq/kg spiked with depleted uranium reference particles. After sieving, the test sample was deposited onto a carbon planchet for secondary ion mass spectrometry analysis in order to determine the particle recovery yield. Then, the detection of actinide-bearing particles by means of alpha autoradiography was performed on various soil samples collected in the vicinity of FDNPP. Longer exposure times logically improve the detection efficiency of alpha-emitters-bearing particles. However, the exposure time should not exceed two months to limit the impact of emissions from naturally-occurring alpha-emitters.

International audience

The Be-10, Cs-137 and Pb-210(xs) radionuclide fallout has been used for the last several decades to quantify various soil and geomorphological processes on different time scales. However, a basic assumption of the studies relying on these radionuclides is that they have a strong affinity for soil particles and that their mobility in soil solution and losses through leaching can be neglected. Another area of the scientific literature deals with the radionuclide mobility in soils as solute. In that context, the objective of this work is to determine the pedological conditions under which this hypothesis of poor solute mobility of radionuclides is valid. To this end, meteoric Be-10, Cs-137 and Pb-210(xs) concentrations were measured in six soil profiles representative of 5 soil types contrasted in terms of physico-chemical properties: an Andosol and a Luvisol under pasture, a Ferralsol and a Leptosol under forest and a Podzol both under forest and cultivation. The main soil properties (soil pH, organic carbon (OC) content, particle size distribution and specific extractions) were measured. The < 2 mu m fraction was extracted to measure radionuclide activities and undertake mineralogical analysis. Results show that meteoric Be-10 is significantly leached from soils whose pH(w) is lower than 5, regardless of the < 2 mu m particle proportion and Fe oxides content. Significant Cs-137 losses through leaching can generally be neglected except in sandy soils whose pH(w) is lower than 4.5 (Podzol). No significant Pb-210(xs) losses were evidenced. For the three radionuclides considered, the major part of their budget is associated with the < 2 mu m fraction. However, concerning the Andosol, the proportion of radionuclide budget associated with the < 2 mu m fraction represents < 40%. With regards to the forested Podzol, two thirds of the Pb-210(xs) budget is associated with the litter. Well-crystallized Fe oxides, illite and interlayered clay minerals as well as allophane, imogolite and other Al-phases in the Andosol and kaolinite in highly weathered acidic soils (Ferralsol) were found to efficiently retain Be-10. Finally, litter degradation and the content of large particulate organic matter were shown to control Pb-210(xs) concentrations. As expected, our results highlight strong contrasts in the retention of the considered isotopes according to soil physico-chemical properties. Accordingly, their mobility and losses through solute transport should be considered when using them for quantifying solid transport and future mass transport models must be improved, in particular through the addition of a solute transfer term. Otherwise, soil redistribution might be strongly overestimated for Podzols, Ferralsols and also most probably for other acidic tropical soil types (Nitisols, Acrisols, Plinthisols).

International audience; Rural landscapes were extensively modified by human activities in Western Europe since the beginning of the 20 th century. Cultivated areas expanded in wetlands traditionally used as grassland. These expansions were associated with extensive anthropogenic modifications of the landscape, for instance through the implementation of artificial tile drainage networks, increasing water drainage from soils. These networks increase the water and sediment connectivity from plots to rivers, creating new transfer pathways beneath the soil surface. As traditional soil conservation measures, such as grass strips, were designed to limit transfers by surface runoff, tile drain make sediment particles bypass these measures. Moreover, these transfers mobilize small particles (in the order 10-5-10-6 m) with a high adsorption potential, making them possibly detrimental for the environment. It is therefore crucial to understand the water and sediment transfer dynamics occurring through tile drainage, and their importance at both the plot and catchment scales. Although tile drainage was studied at the plot scale, for instance to analyze nutrients transfers, the high frequency behavior of sediment transfer from tile drainage, and its potential impacts at the catchment scale, is still poorly understood. Our research quantified sediment fluxes with a high frequency (10 minutes time step) during three years at five nested stations including a tile drain outlet across a small (25 km 2) agricultural catchment (Loire River basin, France) representative of lowland cultivated environments. This catchment was largely modified by anthropogenic features: streams were redesigned and more than 67% of the fields are equipped with tile drainage networks. Flows from the drain were measured during 11.4% of the monitoring time. The nested tile drainage-catchment monitoring network allowed quantifying both the sediment fluxes from tile drain, an often overlooked process, and its proportion in the associated catchment sediment fluxes. Most of sediment fluxes (79 ± 9%) were measured during flood events (n = 44), mostly occurring in winter (75%) and spring (20%). Seasonality therefore controlled most of the variations of sediment fluxes in this context, meaning that the hydrosedimentary response of the catchment should be studied during long observations periods.

International audience; An important step in the sediment source fingerprinting approach is the selection of the appropriate tracing parameters to maximise source discrimination. In order to reduce uncertainties and increase discrimination between sources it may be necessary to use multiple tracing parameters. Accordingly, this study investigates the discrimination and apportionment of sediment sources in a rural agricultural catchment obtained by combining colour, magnetic, and geochemical fingerprinting approaches. The Conceição River catchment (804 km 2) has predominantly deep and highly weathered Ferralsols with land-use consisting of croplands (73%), pastures (18%), forests (8%) and other uses (1%). A total of 189 samples were taken from the main sediment sources, including: Crop-lands (CR, n=78), pastures (P, n=24), unpaved roads (UR, n=38), gullies (G, n=15) and stream bank (SB, n=34). Sediment samples were taken from the surface bed (n=10) of the river and with time integrated samplers (n=4). Twenty-two geochemical tracers, 6 magnetic properties and 24 colour parameters were analyzed. Tracers were selected following a three step procedure, including: (i) a conservative range test (95% confident interval), (ii) a Kruskal-Wallis H test, and (iii) discriminant function analysis (DFA). The DFA was performed using four different sets of variables: (i) geochemical variables only (G); (ii) geochemical+magnetic+colour (GMC); (iii) geochemi-cal+colour (GC); (iv) geochemical+magnetic (GM). The selected tracers were introduced into a modified version of the classical Solver-based mixing model that in order to determine the relative contribution of different sources to in-stream sediment through simultaneously minimizing mixing model difference. The G and GC DFAs both resulted 69% of samples correct classified (SCC) as no colour parameters were selected by the DFA. The GMC and GM approaches improved the discrimination, both resulting in 76% of SCC. For the G and GC approaches, the average source contribution for the 12 sediment samples were P 47%, SB 29%, CR 19%, UR 5% and G 0%. For the GMC and GM approaches, the contribution of each source was P 41%, SB 37%, CR 14%, UR 8% and G 0%. These results are counterintuitive to field observations where cropland is anticipated to contribute more sediment than pastures. Future research should use artificial mixtures to validate these results. Both magnetic and colour parameters hold potential to improve discrimination between sources, particularly magnetic parameters in catchments with high weathered soils.

International audience; The hydrological dynamics of a small rural river catchment in southern Brazil have been used to understand the impact of agricultural activity on soil and water resource degradation. The monitoring and modeling strategies employed to better understand the integrated phenomena focus on surface flow dynamics and the associated processes, such as erosion, sediment yield (SY), and chemical element transfer. In this context, monitoring and modeling techniques are combined in order to include the main hydrological processes involved in soil and water resource degradation. The project began in 2002 and over 500 rainfall-runoff-sediment events and suspended sediment concentration make up the hydrological database under different land use and soil management conditions. The 1.2 km2 catchment of the study is characterized by the presence of shallow soils, high slope, intense agricultural activity, and high SY (∼150 t.km-2.y-1). Techniques that enable the identification of the origin and redistribution of sediments (e.g fingerprinting approach and Cs137) and hydrograph analysis are being used to further understand the dynamics of SY in the catchment. The comprehension of the factors involved in the SY dynamics has allowed the improvement and application of different mathematical models (e.g., LISEM and WATERSHED) to simulate surface flow and SY. These models are used to represent the influence of soil and water conservation practices while considering the interaction between the different landscape components (crops, roads, drainage network, etc.). From this set of data and tools, the capacity of the agricultural production system, drainage network, wetlands and riparian forests in accelerating or retarding water flow, sediment, and chemical elements in the landscape are evaluated. All of these studies have been used to improve soil and water conservation practices at catchment scale, meeting the expectations of farmers in erosion control and water storage as well as of the society interested in maintaining the hydrological functions of soils in catchment scale.

International audience; A major challenge in geomorphology is reconciling the disparity between runoff coefficients and erosion ratesfrom the field to the catchment scale. In the European loess belt, a decrease of runoff coefficients and erosion ratesoccurs with increasing spatial scales. Indeed, it is important to understand the scale dependency of run-off anderosion to manage the off-site impacts of accelerated soil erosion. Accordingly, a continuous simulation of the scaledependency of runoff and erosion, from the field scale to the catchment scale (100 – 1000 km2) was conducted withtheWater and Sediment (WaterSed) model for two catchments (Andelle-756 km2, and Austreberthe-214 km2) over12 years (>1000 events). Scale effects were evident with a 100-fold decrease in runoff coefficients and sedimentdelivery ratios between the field scale and the catchment scale. In spite of a low variability of the annual rainfall(19%), the inter-annual variability of the runoff volume (37%) and erosion rates (92%) at catchment outlets werehigh. The inter-annual variability of runoff and erosion was closely linked to the number of extreme events peryear and their distribution through the year, in particular during periods with highly crusted soil surface states.For these high magnitude events, a complex distributed modelling approach was not necessary as the ability ofthe soil surface and of the landscape to retain overland flows are largely exceeded. The seasonality of soil surfacecharacteristics also affected the scale dependency of runoff and erosion, from the field scale to the catchment scale.However, this is only observed up to a certain spatial extent, i.e. where hillslope erosion processes are governedby hortonian overland flow. When saturation flows are generated, topography and soil depth become the dominantfactors. Understanding runoff and sediment response at different scales should focus on the location and amountof runoff and sediment production within the catchment and the capacity of the downstream flow path to retainor transfer overland flow and sediment. Furthermore, this study demonstrates the need to implement model thatrepresent both hortonian and saturation flows when simulating erosion events at the catchment scale.

International audience; The excessive supply of contaminants from urban areas to rivers during the last centuries has led to deleterious impacts on aquatic ecosystems. The sources, the behavior and the dynamics of these contaminants must be better understood in order to reduce this excessive anthropogenic pollution. Accordingly, the current research investigated the particle-bound trace element (TE) contamination of the 900-km² Orge River (Seine basin, France) and the potential sources of these particles (agricultural or forest soils, channel banks, road deposited sediments), through the analysis of multiple fallout radionuclides, elemental geochemistry and lead isotopic composition on suspended particulate matter (SPM) collected during a hydrological year at four stations following an increasing urbanization gradient (300 to 5000 inhab.km-2). Fallout radionuclide measurements showed an increasing contribution of recently eroded particles from urban areas to the SPM in downstream direction. However, this contribution varied depending on hydrological conditions. A greater contribution of particles originating from urban areas was observed during low stage periods. On the contrary, the contribution of agricultural soils and channel banks that are less enriched in contaminants and fallout radionuclides was higher during seasonal floods, which explained the dilution of radionuclide contents in sediment transiting the river during those events. Trace element contamination of SPM in Cu, Zn, Pb and Sb increased from moderate to significant levels with urban pressure in downstream direction (with corresponding enrichment factors raising from 2 to 6). In addition, Pb isotopic ratios indicated that the main source of Pb corresponded to the "urban" signature found in road deposited sediments. The low variations in lead isotope ratios found in the SPM for contrasting hydrological conditions demonstrated the occurrence of a single source of Pb contamination. These results demonstrate the need to better manage urban runoff during both flood and low precipitation events to prevent the supply of diffuse particle-bound contamination to rivers draining urban areas.

International audience; On account of the virtual absence of 236U in nature and its abundance closely related to its provenance, the 236U/238U ratio has been commonly employed as a tracer to provide information in source identification for safeguard purposes, nuclear forensic studies and environmental monitoring. Due to their high performance capabilities, ICP-MS techniques are currently the most widely employed to perform radionuclide determination. However, in the case of 236U and due to its very low relative abundance, its determination is limited by the formation of 235U1H+ in the plasma and by the influence of the tail of the major isotope 238U (abundance sensitivity).In this work, we present an approach based on the ICP-MS/MS technology to determine 236U/238U ratios in environmental samples. The combination of two quadrupole-based mass filters, before (Q1) and after (Q2) the cell, in the MS/MS configuration provides abundance sensitivity values lower than 10-10, avoiding the influence of the 238U tail. In order to reduce the influence of the hydridebased interferences, different desolvation systems have been evaluated in relation to the sensitivity and the hydride rate obtained. In-cell ion-molecule reactions with O2 and CO2 have also been assessed to detect the uranium isotopes in mass-shift mode (Q1 U+ et61664; Q2 UO+). Best conditions provided a hydride rate value (235U16O1H+/235U16O+) in the order of 10-7. Finally, the methodology was validated with IRMM certified standards, and successfully applied to the determination of 236U/238U ratios in U samples from soils originating from the radioactive contamination plume caused by the Fukushima Daiichi Nuclear Power Plant accident. Ratios down to 10-9 have been obtained with precisions lower than 20.

International audience; Soil erosion and subsequent sediment transfer in rivers may be intense in tropical regions exposed to heavy rainfall. In New Caledonia, a French island located in the southwest of the Pacific Ocean, these problems are exacerbated by nickel open-mining activities. After more than 150 years of exploitation, the excessive sediment input from mining areas has strongly modified the morphology of rivers (hyper-sedimentation) and impacted the island's ecosystems designated a UNESCO World Heritage Site. Despite the severity of these problems, the contribution of mining activities to sediment transiting rivers of this region has not yet been quantified, and this estimation is required to guide the implementation of efficient management measures to mitigate the fine sediment supply to New Caledonian rivers and lagoons. To this end, a pilot sediment tracing study has been conducted in one of the first sites exploited for nickel mining, the 400-km$^2$ Thio River catchment. Sediment deposited during the February 25, 2015 flood was collected according to a 'tributary tracing ap-proach' on the main stem of the Thio River and its main tributaries classified in two types (rivers draining mining sites vs. those devoid of mining activity and where sediment is supplied by landslides, cattle trampling and erosion following forest fires). As nickel mining has mainly developed on peridotite massifs enriched in Mn and Fe oxides, mining sources generate orange or red-coloured sediments, whereas the areas devoid of mining activities rather supply grey material to the rivers. Accordingly, the applicability of alternative sediment tracing method such as spectrocolorimetry in the visible (VIS) range of the spectrum was investigated. The results of this method were compared to those obtained with conventional methods based on fallout radionuclides. Thorium (Th) contents estimated based on the measurement of Th-228 by gamma spectrometry provided a better discrimination between both sources than Cs-137 and unsupported Pb-210. This is likely explained by the low levels of fallout radionuclides (mainly Cs-137) in this region of the world, and by the natural Th enrichment in the volcano-sedimentary formation rocks, devoid of mining activities. On the contrary, the peridotite massif rocks concentrating the nickel mining activities were depleted in Th. Accordingly, the application of a distribution modelling approach based on Th contents showed that tributaries draining mining sites were the main source of sediment supplied to the Thio River (mean 59 %), although the contributions of no-mining sources was far from negligible. Similar results were found with the spectrocolorimetry parameters. After validation, these two methods will be applied to a sediment core collected in the Thio river deltaic plain to characterize potential changes in sediment source contributions over time. Potential applications in other catchments draining mines in New Caledonia will also be explored.

International audience; Fieldwork was conducted every 6 months between 2011 and 2017 to monitor the dispersion of sediment contaminated with radionuclides in rivers draining the main Fukushima radioactive pollution plume (~500 km²). Fine sediment was systematically collected at the same locations (n=47) and analysed for gamma-emitting radionuclides (including 137Cs and 110mAg). A selection of samples was also analysed for carbon/nitrogen concentrations and isotopes. Organic matter analyses showed that paddy fields provided the main source of contaminated sediment to the rivers shortly after the accident. The spatial variations of the 110mAg:137Cs activity ratio in soils were used to demonstrate the very rapid export of sediment to the Pacific Ocean. However, this tool could only be used during the first campaigns, because of the quick decay of 110mAg. Overall, radiocesium concentrations measured in sediment deposits decreased by ~90% between 2011 and 2017. This may be explained by remediation works, by a massive export of material during typhoons and by the occurrence of landslides or channel bank erosion that supply material sheltered from the Fukushima fallout to the river network. Consequently, 6 years after the accident, most of the residual radioactive contamination is found in forests and in dam reservoirs. The analysis of sediment cores collected in a reservoir confirmed the significant storage of contaminated material in these lakes and that paddy fields provided their main source. Ongoing research focuses on the development of a soil erosion model and on the quantification of the impact of remediation works.

International audience; Fieldwork was conducted every 6 months between 2011 and 2017 to monitor the dispersion of sediment contaminated with radionuclides in rivers draining the main Fukushima radioactive pollution plume (~500 km²). Fine sediment was systematically collected at the same locations (n=47) and analysed for gamma-emitting radionuclides (including 137Cs and 110mAg). A selection of samples was also analysed for carbon/nitrogen concentrations and isotopes. Organic matter analyses showed that paddy fields provided the main source of contaminated sediment to the rivers shortly after the accident. The spatial variations of the 110mAg:137Cs activity ratio in soils were used to demonstrate the very rapid export of sediment to the Pacific Ocean. However, this tool could only be used during the first campaigns, because of the quick decay of 110mAg. Overall, radiocesium concentrations measured in sediment deposits decreased by ~90% between 2011 and 2017. This may be explained by remediation works, by a massive export of material during typhoons and by the occurrence of landslides or channel bank erosion that supply material sheltered from the Fukushima fallout to the river network. Consequently, 6 years after the accident, most of the residual radioactive contamination is found in forests and in dam reservoirs. The analysis of sediment cores collected in a reservoir confirmed the significant storage of contaminated material in these lakes and that paddy fields provided their main source. Ongoing research focuses on the development of a soil erosion model and on the quantification of the impact of remediation works.

International audience; In France, since the beginning of 20 th century, rural landscapes have been completely modified by human activities. These practices have resulted in profound sedimentary and morphological alterations (channel bed incision, deposition of fine sediment, bank erosion, etc.), detrimental to the achievement of good water status. Several research efforts have already investigated either global budgets at the river basin or continental scale or local detailed budget at the plot to the field scale. However, very few studies have tried to analyse the connectivity between fluxes and storages and to draw the links between temporal and spatial scales. In this broad context, the purpose of this study is to examine source-to-sink dynamic of the sediment cycle for the Loire River Basin. This project is broken down into two steps: the first step's aim is to understand poorly studied processes such as sediment production by agricultural drainage or bank erosion by catchment monitoring. The second step is to elaborate a distributed model of sediment connectivity from hillslopes to basin outlet. For the first step, two catchment sites are studied, using historical data or monitoring: the linear of two small streams (" La Ligoire ") and the Louroux lake catchment. Most of those two catchments are intensively cultivated and have been extensively submitted to subsurface drainage using drain tiles. The objectives of this part are threefold: quantify incision and deposition processes since the channelization of the streams, quantify in-channel deposition rates of fine sediments, and explain the spatial distribution of these deposits. For the second step, the modeling approach is based on the use of indicators to describe hillslope processes, potential downstream retention, attempting to link river basin characteristics to a prediction of sediment exports in rivers. It provides insight in the identification of the most influent sediment redistribution processes on the total sediment fluxes and on the differences between various basin typologies. The Ligoire study shows an important stream incision and the influence of water obstacles on sediment distribution. The Louroux monitoring permits to differentiate between the different sources of sediment and highlight the importance of surface erosion during flood events and of bank erosion during low flows. The increased export of the sediment is primarily due to the very high human-made connectivity of these landscapes that was originally created to evacuate the excess water during the humid seasons. The analysis of particulate sediment fluxes datasets at different spatial and temporal scales permits to identify some of the dominant processes, and also to distinguish natural from anthropogenic influences. Concerning upland physical soil surface erosion rates, we find that the average travel distance of eroded particles may be limited, implying a strong decrease in physical erosion rates when moving from the local scale to the river basin scale. The results also highlight the predominant role of surface connectivity to characterize the fraction of sediment exported out of river drainage areas by physical soil surface erosion.

Poster; International audience; Managing paleoscience data is highly challenging to the multiplicity of actors in play, types of sampling, analysis, post-analysis treatments, statistics etc. However, a well-structured curating of data would permit innovative developments based on data and/or sample re-use, such as meta-analysis or the development of new proxies on previously studied cores. In this paper, we will present two recent initiatives that allowed us tackling this objective at a French national level: the “National Cyber Core Repository” (NCCR) and the “LTER-France retro-observatory” (ROZA).NCCR was developed under the umbrella of the French National Center fo Coring and Drilling (C2FN) thanks to the national excellence equipment project CLIMCOR. It aims at gathering on a unique website the locations and metadata of any scientific coring/drilling performed by French teams or using French facilities, whatever the type of archive it is (lake/marine sediment; ice etc.). It uses international standard, notably IGSN (for samples), ORCID (for persons) and DOI (for campaigns). NCC follows the INSPIRE ISO 19115 standard in order to catalogue the data. For continental sediment, NCCR may be fed directly on the field through a specifically developed mobile application.Based on NCCR, further initiatives may be led. In particular, under the umbrella of LTER-France (Long Term Ecological Research), we developed ROZA in order to facilitate the re-use of data and samples. Here the idea is to capitalise the knowledge on a given lake from which several sediment cores can be taken through time. In that aim we selected at least one lake from each of the 13 areas composing the network LTER-France. To enter the database, a set of mandatory data must be provided under a pre-determined format. In that case, the insertion of ROZA within the network LTER will favor to use of paleodata by non-paleodata scientists, in particular ecologists.

International audience; Following the shift towards more intensive agriculture in cultivated lowlands in Europe, field sizes have increased and stream valley meanderings have been removed and realigned along new straight field borders. These modifications have led to profound alterations of the hydromorphology of the streams. To test the impact of these modifications, the long-term and current volumes of sediment originating from stream banks were calculated as they provided potential sources of sediment in a large pond located at the outlet of a small agricultural lowland basin under strong anthropogenic pressure. Bank erosion was measured using several methodologies, i) over a short period using a set of erosion pins along a small stream (1400 m long) to quantify the material exported during a single winter season (2012/2013); ii) over the last 69 years using an original approach involving the comparison of a compilation of three-dimensional historical stream redesign plans from 1944 vs. new measurements conducted in 2013 (DGPS and LiDAR data); iii) over several decades by using tracers (137Cs) that can differentiate between surface and subsoil erosion. At the catchment scale, total sediment exports were estimated from 1945 to 2013 combining seismic imagery and core dating in the lake. Sediment exports decreased with time, from 300 t. km–2.yr–between 1954 and 1980 to 95 t. km–2.yr–1 between 1980 and 2013. Today, erosion rates recorded at the outlet of the catchment vary between 90–102 t.km–2.yr–1. Therefore, the order of magnitude of the mean export rate is approximately 180 t. km–2.yr–1 for the last 70 years.The contribution of channel banks to this sediment export was the highest (~30%) between 1954 and 1980 when the ditches were constructed. For the entire period since the landscape modification, the contribution of bank erosion is lower but still reaches 20%. Bank erosion can therefore be considered as a significant contributor to the sediment budget of the lowland catchments that have been redesigned after the 2nd world war in Western Europe.

International audience; Artificial fallout radionuclides (FRNs) such as Caesium-137 and Plutonium-239+240 released as products of the thermonuclear weapons testing that took place from the mid-1950s to the early 1980s and from nuclear power plant accidents (e.g. Chernobyl) are useful tools to quantify soil redistribution. In combination with geostatistics, FRNs may have the potential to bridge the gap between small scale process oriented studies and modelling that simplifies processes and effects over large spatial scales. An essential requirement for the application of FRNs as soil erosion tracers is the establishment of the baseline fallout at undisturbed sites before its comparison to those inventories found at sites undergoing erosion/accumulation. For this purpose, undisturbed topsoil (0-20cm) samples collected in 2009 within the framework of the Land Use/Cover Area frame Survey (LUCAS) have been measured by gamma-spectrometry and ICP-MS to determine 137Cs (n=145) and 239+240Pu (n=108) activities. To restrict the analysis to undisturbed reference sites a geospatial database query selecting only sites having a slope angle

International audience; Intensification of agriculture after WWII increased soil erosion and sediment supply to rivers in Northwestern Europe. The Louroux Pond catchment (25 km 2), located in the Loire River basin (central France), is representative of these intensively cultivated environments and subject to severe soil erosion and river siltation. This catchment was equipped with an extensive network of tile drains that were installed after 1945 to produce crops in this former wetland. In order to decrease the deleterious impacts induced by accelerated soil erosion in this region, there is a need to better understand sediment dynamics during flood events. To this end, natural fallout radionuclides were analysed in both sediment sources and suspended matter transiting the river network to quantify the respective proportions of recently eroded vs. resuspended material. Accordingly, 7Be and 210Pbxs concentrations were measured in overland flow and suspended sediment collected in the pond tributaries during a succession of floods that occurred during winter in 2013-2014 and in 2016. The results show that the mean fraction of recently eroded material transiting these rivers increased from 40 ± 20% to 80 ± 20 in 2013-2014, and from 65 ± 20% to 80 ± 20% in 2016. These results demonstrate an initial flush of sediment previously accumulated in the river channel during the first winter floods. Then, the fraction of sediment recently eroded from the hillslopes significantly increased during subsequent events. This research illustrates the added value of combining continuous river monitoring and fallout radionuclide analyses to improve our knowledge of sediment dynamics and to protect soil and water resources in these environments.

International audience; Soil erosion is recognized as one of the main processes of land degradation in agricultural areas. High suspended sediment loads, often generated from eroding agricultural landscapes, are known to degrade downstream environments. Accordingly, there is a need to understand soil erosion dynamics during flood events. Suspended sediment was therefore sampled in the river network and at tile drain outlets during five flood events in a lowland drained catchment in France. Source and sediment fallout radionuclide concentrations (Be-7, Pb-210(xs)) were measured to quantify both the fraction of recently eroded particles transported during flood events and their residence time. Results indicate that the mean fraction of recently eroded sediment, estimated for the entire Louroux catchment, increased from 45 +/- 20% to 80 +/- 20% between December 2013 and February 2014, and from 65 +/- 20% to 80 +/- 20% in January 2016. These results demonstrate an initial flush of sediment previously accumulated in the river channel before the increasing supply of sediment recently eroded from the hillslopes during subsequent events. This research highlights the utility of coupling continuous river monitoring and fallout radionuclide measurements to increase our understanding of sediment dynamics and improve the management of soil and water resources in agricultural catchments.

International audience; Sediment connectivity characterizes the physical transfer of sediment through different geomorphic compartments in catchments due to sediment detachment, transport and deposition. Quantifying and modelling sediment connectivity is therefore a key prerequisite to improving our understanding of the dispersion of particle-borne contaminants, especially in catchments exposed to highly erosive climates. The objective of this study is to provide novel insights into typhoon impacts on sediment connectivity from hillslopes to rivers. The dispersion of particle-bound caesium-137 (Cs-137) was investigated in two coastal catchments draining the main contamination plume from the Fukushima Daiichi Nuclear Power Plant accident. Five sampling campaigns were carried out from November 2011 to November 2015, after each typhoon season. The spatial and temporal evolution of Cs-137 contamination was investigated through the calculation of Cs-137 enrichment ratios in sediment relative to nearby soils. Rainfall erosivity (EI30) associated with the main typhoons that occurred prior to each sampling campaign were computed, mapped, and finally used to improve a topographic-based index of connectivity. From 2011 to 2015, mean contamination levels in Mano and Niida catchments decreased from 11.9kBqkg(-1) to 3.3kBqkg(-1) and from 34.1kBqkg(-1) to 8.0kBqkg(-1), respectively. Regional mean EI30 ranged from 262MJmmha(-1)h(-1) for typhoon Jelawat (in 2012) to 1695MJmmha(-1)h(-1) for typhoon Roke (in 2011). Typhoons Roke (2011) and Etau (2015) showed the highest connectivity from contaminated sources to the rivers, and induced a significant export of sediment to the ocean. In 2013 a slight increase in Cs-137 levels in river sediments occurred, likely resulting from initial decontamination works and the occurrence of two consecutive typhoons. Importantly, this research provides new insights into the connectivity of the main sources of sediments contaminated with radiocaesium in Fukushima Prefecture and their temporal evolution, which will help with ongoing decontamination efforts.

Study region: Houay Xon catchment in northern Laos.Study focus: Because agricultural headwater catchments of SE Asia are prone to erosion anddeliver a significant proportion of the total suspended sediment supply to major rivers andfloodplains, the potential sources of sediments and their dynamics were studied for twosuccessive storm flow events in June 2013. Characterization of suspended sediment loadswas carried out along a continuum of 7 monitoring stations, combining analyses of falloutradionuclides, particle borne organic matter and stream water properties.New hydrological insights: Radionuclide activities showed that remobilization of soil par-ticles deposited during the previous rainy season or supplied by riverbank erosion isthe dominant process, although pulses of surface-soil derived sediments also propagatedownstream. This interpretation is supported by suspended organic matter data that alsofingerprints the mixing of surface soil vs. subsurface particles. The study moreover high-lights the advantages and the drawbacks of combining fallout radionuclides, particle borneorganic matter composition and stream water characteristics to discriminate and quantifysediment sources and dynamics in rural areas undergoing urban sprawl.

International audience; The widespread adoption of the sediment fingerprinting approach to guide catchment management has been limited by the cost and the difficulty to prepare and process samples for geochemical and radionuclide analyses. Spectral properties have recently been shown to provide a rapid and cost‐efficient alternative for this purpose. The current research objective was (i) to quantify the sediment source contributions in a 1∙19‐km2 rural catchment of Southern Brazil by using mid‐infrared (MIR) spectroscopy and (ii) to compare these results with those obtained with geochemical approach and near‐infrared and ultraviolet–visible spectroscopy methods. The sediment sources to discriminate were cropland surface (n = 20), unpaved roads (n = 10) and stream channel banks (n = 10). Twenty‐nine suspended sediment samples were collected at the catchment outlet during nine significant flood events. The sources could be distinguished by MIR spectroscopy. Cropland and channel bank sources mainly differed in their clay mineral contents, but their similar organic matter content complicated the MIR‐model predictions. Unpaved road contributions were discriminated from the other sources by their lower organic carbon content. When the results of the current research based on MIR spectroscopy are compared with those obtained using other sediment fingerprinting approaches, based on geochemistry and near‐infrared and ultraviolet–visible spectroscopy, an overestimation of channel banks contribution and an underestimation of cropland and unpaved road contributions is found. These results suggest that MIR spectroscopy can provide a useful tool that is non‐destructive, rapid and cheap for tracing sediment sources in rural catchments and for guiding the implementation of soil and water conservation measures

International audience

International audience; Rural landscapes have been extensively modified by human activities in Western Europe since the beginning of the 20th century in order to intensify agricultural production. Cultivated areas often expanded at the expense of grassland and wetlands located in lowland areas (de Groot et al., 2002). Therefore, large modifications were made to the agricultural landscapes: stream redesign, land consolidation, removal of hedges, and installation of tile drainage networks to drain the hydromorphic soils. These changes modified sediment processes and resulted in large morphological alterations (e.g. channel bed incision, deposition of fine sediment, channel bank erosion). Accordingly, these alterations threaten water quality and prevent to meet the requirements of the European directives. Improving water quality requires a clear understanding of the hydrosedimentary dynamics in these lowland cultivated catchments. However, few studies were conducted in drained environments. To fill this research gap, a pilot study was started in cultivated catchment of the Loire River basin, France, where tile drain densities are very high (> 1.5 km/km²). Six hydro-sedimentary monitoring stations were installed in the Louroux catchment (24 km²). One of them was specifically dedicated to measuring water/sediment fluxes from tile drains. Water level and turbidity were continuously monitored and sediments were sampled during floods and low stage periods. Samples were measured for particle size distribution, and sediment tracing studies are currently being developed to quantify the contribution of potential sources (e.g. surface vs subsurface, lithologies) to river sediment. Hydro-sedimentary fluxes were quantified and modelled for some selected events. The catchment hydrosedimentary fluxes and their properties were shown to be impacted by tile drain sediment transport, especially regarding particle size distribution, with the dominant export of very fine particles (< 2 µm) from tile drains. This study demonstrates that tile drain transport should be included when developing and running hydro-sedimentary models in catchments, and particularly in lowland/wetland environments.

Consumption of water polluted by faecal contaminants is responsible for 2 million deaths annually, most of which occur in developing countries without adequate sanitation. In tropical aquatic systems, streambeds can be reservoirs of persistent pathogenic bacteria and high rainfall can lead to contaminated soils entering streams and to the resuspension of sediment-bound microbes in the streambed. Here, we present a novel method using fallout radionuclides (Be-7 and Pb-210(xs)) to estimate the proportions of Escherichia coli, an indicator of faecal contamination, associated with recently eroded soil particles and with the resuspension of streambed sediments. We show that using these radionuclides and hydrograph separations we are able to characterize the proportion of particles originating from highly contaminated soils and that from the resuspension of particle-attached bacteria within the streambed. We also found that although overland flow represented just over one tenth of the total flood volume, it was responsible for more than two thirds of the downstream transfer of E. coli. We propose that data obtained using this method can be used to understand the dynamics of faecal indicator bacteria (FIB) in streams thereby providing information for adapted management plans that reduce the health risks to local populations.

Reservoir siltation because of water erosion is an important environmental issue in Mediterranean countries where storage of clear surface water is crucial for their economic and agricultural development. The high density of gully systems observed in Mediterranean regions raises the question of their contribution to reservoir siltation. In this context, this study quantified the absolute and relative contributions of rill/interrill and gully/channel erosion in sediment accumulation at the outlet of small Tunisian catchments (0·1-10 km2) during the last 15 years (1995-2010). To this end, a fingerprinting method based on measurements of caesium-137 and total organic carbon combined with long-term field monitoring of catchment sediment yield was applied to five catchments in order to cover the diversity of environmental conditions found along the Tunisian Ridge and in the Cape Bon region. Results showed the very large variability of erosion processes among the selected catchments, with rill/interrill erosion contributions to sediment accumulated in outlet reservoirs ranging from 20 to 80%. Overall, rill/interrill erosion was the dominant process controlling reservoir siltation in three catchments whereas gully/channel erosion dominated in the other two catchments. We identified the presence of marly gypsum substrates and the proportion of catchment surface covered by soil management/conservation measures as the main drivers of erosion process variability at the catchment scale. These results provided a sound basis to propose guidelines for erosion mitigation in these Mediterranean environments and suggested to apply models simulating both rill/interrill and gully/channel erosion in catchments of the region.

International audience; Introduction: In France, since the beginning of 20th century, rural landscapes have been completely modified by human activities. These practices have resulted in profound sedimentary and morphological alterations (channel bed incision, deposition of fine sediment, bank erosion, etc.), detrimental to the achievement of good water status [1].Several research efforts have already investigated either global budgets at the river basin or continental scale or local detailed budget at the plot to the field scale. However, very few studies have tried to analyse the connectivity between fluxes and storages and to draw the links between temporal and spatial scales. In this broad context, the purpose of this study is to examine source-to-sink dynamic of the sediment cycle for the Loire River Basin. This project is broken down into two steps: the first step’s aim is to understand poorly studied processes such as sediment production by agricultural drainage or bank erosion by catchment monitoring. The second step is to elaborate a distributed model of sediment connectivity from hillslopes to basin outlet. Methods:.for the first step, two catchment sites are studied, using historical data or monitoring: the linear (21 km) of two small streams (“La Ligoire”) and the Louroux lake catchment. Most of those two catchments are intensively cultivated and have been extensively submitted to subsurface drainage using drain tiles. The objectives of this part are threefold: 1) quantify incision and deposition processes since the channelization of the streams (1970), (2) quantify in-channel deposition rates of fine sediments, and, (3) explain the spatial distribution of these deposits. For the second step, the modeling approach is based on the use of indicators to describe hillslope processes, potential downstream retention, attempting to link river basin characteristics to a prediction of sediment exports in rivers. It provides insight in the identification of the most influent sediment redistribution processes on the total sediment fluxes and on the differences between various basin typologies [2] [3].Results and discussion: The Ligoire study shows an important stream incision (around 30 cm in 40 years) and the influence of water obstacles on sediment distribution. The Louroux monitoring permits to differentiate between the different sources of sediment and highlight the importance of surface erosion during flood events and of bank erosion during low flows within small intensively cultivated plain catchments. The increased export of the sediment is primarily due to the very high human-made connectivity of these landscapes that was originally created to evacuate the excess water during the humid seasons.For the second step, the analysis of particulate sediment and dissolved fluxes datasets at different spatial and temporal scales permits to identify some of the dominant processes, and also to distinguish natural from anthropogenic influences. Concerning upland physical soil surface erosion rates, we find that the average travel distance of eroded particles may be limited, implying a strong decrease in physical erosion rates when moving from the local scale (m²) to the river basin scale (> 103 km²). Chemical erosion rates are less sensitive to scale and can either decrease or increase with increasing area in function of lithology, land management and topography. The results also highlight the predominant role of surface connectivity to characterize the fraction of sediment exported out of river drainage areas by physical soil surface erosion. For the export of dissolved sediment originating from weathering processes, the catchment physiography and connectivity does no longer play the dominant role.

International audience; During the Fukushima Daiichi nuclear power plant (FDNPP) accident, a dozen of high-altitude aerosol sampling stations, located between 850 and 3,454 m above sea level (a.s.l.), provided airborne activity levels across Europe (Fig. 1). This represents at most 5% of the total number of aerosol sampling locations that delivered airborne activity levels (at least one result) in Europe, in connection with this nuclear accident. High altitude stations are typically equipped with a high volume sampler that collects aerosols on filters. The Fukushima-labeled air mass arrival and the peak of airborne cesium-137 (137Cs) activity levels were registered in Europe at different dates depending on the location, with differences up to a factor of six on a regional scale. Besides this statement related to lowland areas, we have compared the maximum airborne levels registered at high-altitude European locations (850 m < altitudes < 3450 m) with what was observed at the closest lowland location. The vertical distribution of 137Cs peak level was not uniform even after a long travel time/distance from Japan. This being true at least in the atmospheric boundary layer and in the lower free troposphere. Moreover the relation '137Csmax vs. altitude' shows a decreasing trend (Fig. 2).

International audience; Change of land use or agricultural practices are known to have high impacts on sediment transfer in catchments and rivers. Numerous studies have particularly illustrated these effects in sloping land in tropical areas undergoing deforestation. Much less attention has been paid to lowland humid areas, where permanent land uses have been plowed more recently. However recent studies reported significant erosion rates in these environments despite the gentle topography and the temperate climate. In order to quantify these changing fluxes of sediment, several instrumentation and historical database analyses were carried out in various catchments of the Loire Valley, France. More particularly, a multiparameter analysis was conducted on sedimentary deposits of a pond created in the 11th century in a catchment representative of cultivated and drained lowland environments where an intensification of agricultural practices has occurred during the last 60 years. The results showed that the initial land consolidation period (1954-1960) was characterized by a dominance of allochtonous material input to the pond. This input represents an erosion of 1900 to 2300 t.km-$^2$ .yr-1 originating from the catchment. Then, between 1970-1990, terrigenous material flow decreased progressively and tended to stabilize, whereas eutrophication and associated primary production increased in the pond. In addition to these temporal changes, material input across the pond during the last 10 years corresponds to a loss of material in the catchment ranging between 90 and 102 t.km-$^2$.yr-1. While a strong decrease is observed, it still represents a 60-fold increase of the sediment fluxes to the pond compared to the preintensification period. Subsequent research monitoring studies permitted to differentiate between the different sources of sediment and highlight the importance of surface erosion during flood events and of bank erosion during low flows. The increased export of the sediment is primarily due to the very high human-made connectivity of these landscapes that was originally created to evacuate the excess water during the humid seasons.

International audience; Soil erosion plays an important role in sediment and carbon storage within, and exports from, catchments. In cultivated landscapes, field borders can improve the temporary storage of eroded soil particles and associated carbon, by impeding lateral soil fluxes. These local soil accumulations can lead to the development of linear landforms (such as headlands and lynchets) which will keep evolving after field border removal. A recent study performed in a representative cultivated hillslope of the SW Parisian Basin showed that 39% of the area corresponds to landforms resulting from soil accumulation induced by former and present field borders. This study demonstrated that field borders influence greatly the landscape morphology, but also the spatial distribution of soil thickness, and locally the A-horizon thickness, which are essential parameters for the prediction of SOC stocks. This study aims at characterizing and quantifying the effect of field borders and their removal on medium term topsoil erosion and deposition rates in a cultivated hillslope of the SW Parisian Basin, consolidated in 1967. Here, we used the Cs-137 technique to assess recent patterns of soil redistribution. We measured the Cs-137 inventories of 68 soil cores sampled along transects covering the area and, more specifically, linear landforms identified along present and past field borders (i.e. lynchet and undulation landforms, respectively). Then, we used a spatially-distributed Cs-137 conversion model that simulates and discriminates soil redistribution induced by water and tillage erosion processes over the last fifty years. Finally, observations and model outputs were confronted. Our results show that tillage erosion dominate the soil redistribution in the study area for the 1954-2009 period and generated about 95% (i.e. 4.50 Mg.ha −1 .yr −1) of the total gross erosion. Soil redistribution was largely affected by the presence of current and former field borders, where hotspots areas of deposition and erosion (>20 Mg.ha −1 .yr −1), respectively, were observed. Land consolidation contributed to the local acceleration of topsoil erosion through the conversion of storing areas into sediment generating areas. Though the general patterns of Cs-137 inventories in the area were correctly reproduced by the model, this latter performed weakly with a r2 of 0.20. Important discrepancies were associated with sampling points located along current field borders where data suggests that tillage erosion processes cannot be described as elsewhere, i.e. as a diffusive process. These specific processes implied here should be characterised and implemented into erosion models for simulating rates and patterns of topsoil redistribution in fragmented cultivated landscapes. In addition, the use of a DEM of the present-day morphology leads to the underestimation of soil erosion and storing within linear landforms which morphology seems to have greatly evolved since 1967. This study highlights the importance of present and former field borders on the patterns and intensities of topsoil erosion and deposition processes at landscape scale. This is of particular interest concerning the improvement of our knowledge on soil organic carbon patterns and on estimation of SOC stocks.

Fallout radionuclides Be-7 and Pb-210 have been identified as potentially relevant temporal tracers for studying soil particles dynamics (surface vs. subsurface sources contribution; remobilization of inchannel sediment) during erosive events in river catchments. An increasing number of studies compared Be-7: Pb-210 activity ratio in rainwater and sediment to estimate percentages of freshly eroded particles. However, the lack of data regarding the spatial and temporal variability of radionuclide wet deposition during individual storms has been identified as one of the main gaps in these estimates. In order to determine these key parameters, rainwater samples were collected at three stations during four storms that occurred at the beginning of the monsoon (June 2013) in the Houay Xon mountainous catchment in northern Laos. Rainwater Be-7 and Pb-210 activities measured using very low background hyperpure Germanium detectors ranged from 0.05 to 1.72 Bq L-1 and 0.02 to 0.26 Bq L-1, respectively. Water 8180 were determined on the same samples. Total rainfall amount of the four sampled storms ranged from 4.8 to 26.4 mm (51 mm in total) at the time-fractionated collection point. Corresponding cumulative 78e and 210Pb wet depositions during the sampling period were 17.6 and 2.9 Bq m(-2), respectively. The 7Be: 210Pb activity ratio varied (1) in space from 6 to 9 for daily deposition and (2) in time from 3 to 12 for samples successively collected. Intra-event evolution of rainwater 7Be and 210Pb activities as well as 8180 highlighted the progressive depletion of local infra-cloud atmosphere radionuclide stock with time (washout), which remains consistent with a Raleigh-type distillation process for water vapour. Intra-storm ratio increasing with time showed the increasing contribution of rainout scavenging. Implications of such variability for soil particle labelling and erosion studies are briefly discussed and recommendations are formulated for the collection of rainwater signature in studies based on the Be-7: Pb-210 ratio method, especially in tropical areas under high erosive pressure.

International audience; Small reservoirs of agriculture-dominated areas experience severely increased sediment input caused by soil erosion on cultivated slopes, also accompanied at some locations by gully erosion. This causes rapid decrease of the reservoir water storage and shortened periods of functioning. In this paper we discuss several examples of the 137Cs-based short-term siltation chronology of small reservoirs located in different landscape zones within the agricultural belt of European Russia. From two to four time marks could have been established in 137Cs depth distribution curves constructed from detailed depth-incremental sampling of reservoir infill sediment, allowing reconstruction of sediment microstratigraphy and deposition rates. In combination with other independent information sources this provides insight on the relative importance of recent land use changes and climatic variability in controlling sediment delivery within small agriculture-dominated fluvial systems. In combination with sediment redistribution studies, it has become possible to construct closed sediment budgets for catchments of several reservoirs and make a quantitative assessment of sediment delivery variability. Such information is important for appropriate design and management of small agricultural reservoirs in Russia.