L’érosion des sols est intense dans les petits bassins versants montagneux en climat tropical. Lorsqu’ils atteignent les rivières, ces sédiments posent divers problèmes à l’aval. L’identification des sources de sédiments et la compréhension de leur dynamique au sein des bassins versants est donc cruciale pour proposer des mesures de lutte efficaces contre ces problèmes. Cette thèse vise à tester et à développer des méthodes de traçage qui consistent à comparer les propriétés physico-chimiques des sols à celles des sédiments transportés dans les rivières pour en définir les sources et pour en contraindre les temps de transfert. Le site d’étude est un bassin de 22,4 km² au Laos équipé d’un emboîtement de stations de suivi et soumis à un régime de mousson. Trois campagnes de prélèvements ont permis de collecter des échantillons de sols de surface, de ravines et de berges ainsi que des eaux de pluies, de ruissellement et de rivières, les matières en suspension (MES) associées et des laisses, au cours de 3 crues de début de mousson en mai 2012 et juin 2013. Les échantillons ont été analysés afin de déterminer leurs activités en radionucléides apportés au sol par les pluies de manière ponctuelle (¹³⁷Cs) ou continue (⁷Be, ²¹ºPb), la teneur et la composition de la matière organique (C organique, N, C/N, δ¹³C et δ¹⁵N), la granulométrie des MES et la composition de l’eau (conductivité électrique, δ¹⁸O). Plusieurs expériences méthodologiques ont permis de tester et valider les hypothèses qui sous-tendent l’utilisation du marquage en ⁷Be et en excès de ²¹ºPb des sédiments (adsorption rapide sur les particules de sol, rapport ⁷Be / ²¹ºPbxs des particules transportées par le ruissellement comparable à celui de la pluie correspondante). Durant la crue de mai 2012, la contribution des sédiments récemment érodés est élevée en début d’événement (25-35%), mais elle est ensuite diluée par la remobilisation de particules déposées sur le lit du cours d’eau ou stockées dans les zones de dépôt. La caractérisation combinée du carbone organique particulaire (COP) et de l’activité en ¹³⁷Cs des particules a mis en évidence la prédominance de l’érosion des sols de surface marqués par leur signature C₃ à l’amont, et l’augmentation de la contribution de l’érosion des berges (marquage C₄) dans la partie aval du bassin. Les valeurs les plus élevées des taux d’exportation de sédiments (43,3 Mg km-2) et de C (0,83 MgC km-2), du coefficient de ruissellement (11,7%) et du pourcentage d’eau de ruissellement dans l’écoulement total (78-100%) ont été observées au niveau de la station drainant en grande partie des versants sous teck. Les teneurs en C mesurées et les flux de COP calculés ici sont beaucoup plus élevés que ceux qui ont été estimés lors de travaux réalisés il y a 10 ans dans le même bassin versant, lorsque la surface couverte de teck y était beaucoup plus faible (2,5% en 2002-2003 contre 32% en 2012). Ainsi, à l’aval, le taux d’exportation spécifique de C lors d’une seule crue en 2012 s’est révélé être 2,6 fois plus important que celui observé au cours de l’année hydrologique 2002-2003 (34 crues érosives). Lors des deux crues de juin 2013, la variabilité spatio-temporelle des retombées de ⁷Be et ²¹ºPb a pu être quantifiée. Ces résultats indiquent qu’il est indispensable de mesurer le signal d’entrée du marquage ⁷Be /²¹ºPb de chaque événement et qu’il est préférable de réaliser un échantillonnage en plusieurs points du bassin plutôt que de le fractionner en un seul point au cours du temps. Durant ces crues, la composition des sédiments a pu être étudiée le long d’un continuum de dix stations emboitées (0,1 ha à 19,8 km²). Les contributions respectives des différentes sources de sédiments, et leur propagation à travers le bassin, ont pu être déterminées à l’aide des mêmes traceurs que lors de la crue de mai 2012. À l’avenir, la combinaison de traceurs proposée dans cette thèse pourrait être appliquée à d’autres évènements ou d’autres sites. / Soil erosion is particularly intense in small mountainous tropical catchments. When supplied to the rivers, eroded sediment leads to numerous problems downstream (transfer of pollutants, increase of water turbidity, dam siltation…). Identifying sediment sources and understanding their dynamics across catchments constitutes a prerequisite to design and implement efficient measures to reduce these problems. This thesis aimed at testing and developing fingerprinting methods, by comparing physico-chemical properties of soils and riverine suspended sediment to define their origin and constrain their transfer across catchments. The study site is a 22.4 km² catchment in northern Laos equipped with nested monitoring stations and submitted to monsoon rainfalls. Three field campaigns were conducted to collect samples of surface soils, gullies, riverbanks, and rainwater, overland flow, river water, suspended matter and deposited sediment during three floods at the beginning of the monsoon in May 2012 and June 2013. Collected samples were analysed to determine fallout radionuclide activities (¹³⁷Cs, ⁷Be and ²¹ºPb), organic matter composition (organic carbon and nitrogen concentrations, C/N ratio, δ¹³C et δ¹⁵N), particle size distribution and water composition (electric conductivity and δ¹⁸O). Several methodological experiments were performed to check assumptions underlying the use of atmospheric 7Be and ²¹ºPb as tracers of sediment (quick adsorption of rainwater radionuclides to soil particles; similarity between /210Pbxs activity ratio measured in overland flow particles and corresponding rainfall signature). During the flood studied in May 2012, the contribution of recently eroded sediment was high at the beginning of the rising stage (25-35%), but was then diluted by sediment remobilization from the riverbed and deposition areas (swamps). The combined determination of particulate organic carbon (POC) and ¹³⁷Cs activity of sediment indicated the dominance of surface soil ( C₃ vegetation labelling) erosion in upstream parts and the increase of riverbank (labelled by C₄ plants) erosion contribution in the downstream part of the catchment. The highest runoff coefficient (11.7%), sediment specific yield (43.3 Mg km-2), total organic carbon specific yield (0.83 MgC km-2) and overland flow contribution (78–100%) were found at the station draining mainly the areas covered with teak plantations. Total organic carbon concentrations and POC specific yields calculated during this thesis are much higher than those obtained during a previous study carried out 10 years earlier in the same catchment, when teak plantations covered a much lower proportion of its area (2.5% in 2002-2003 vs 32% in 2012). At the downstream station, carbon specific yields for the studied flood in 2012 were twice higher than the annual ones observed in 2002-2003 (34 erosive events). During the two floods analysed in June 2013, the spatial and temporal variability of ⁷Be and ²¹ºPb wet deposition was investigated and quantified. The results indicate that the input signal of ⁷Be/²¹ºPb labelling associated with each storm should be determined and that spatially distributed collection of rainfall should be privileged to the sequential time-fractionated sampling at a single location. During these floods, the composition of exported suspended sediment could be studied along a continuum of ten nested stations draining areas between 0.1 ha and 19.8 km². Respective contributions of the different surface and subsurface sources of sediment and their propagation across the catchment, were successfully determined with the same tracers as during the May 2012 event. In future, the panel of parameters used in this thesis could be applied to characterise suspended matter behaviour during floods occurring later in the rainy season, or along a longer continuum of nested monitoring stations.

Anthropogenic driven soil erosion has increased drastically within the last few centuries. Accelerated removal of topsoil degrades soil characteristics, decreasing the agricultural capacity to sustain food production for future human generations. In order to reduce sediment erosion to a minimum, soil movement dynamics have to be understood over relevant timescales. Emphasis is required on human land use, which has changed over time, altering erosion and deposition processes in the landscape. This thesis aims to understand the development of erosion with time and studies the effects of the anthropogenic impact on the Earth’s surface. A case study was conducted in the Christina River Basin in south-eastern Pennsylvania (USA) to evaluate the anthropogenic impact on sediment redistribution before and during European colonisation of the US East Coast. The Christina River Basin is focus of the Christina River Basin Critical Zone Observatory, where researchers of many disciplines (e.g. hydrology, geochemistry, geology, biology, geomorphology, soil science) study the interactions of the water, mineral and carbon cycle in relation to different land uses. This site is particularly suitable for this study due to the sudden change in land use with the arrival of European Settlers and the well-recorded history of land use in the area since. Besides sediment processes related to the European colonisation, special emphasis was placed on the last century and the particular impact of land management practices on the landscape. For the approach of this study, upland and depositional sediment profiles, as well as suspended sediment collected from rivers at different times were analysed for radionuclide activity. In particular, a combination of the short-lived radionuclides 7Be, 210Pb, and 137Cs with the long-lived meteoric nuclide 10Be was used to allow detection of sediment erosion as well as tracing of sediment source to streams over different time frames. Natural sediment processes were studied by investigating meteoric 10Be in different upland and valley hollow soil profiles in the Christina River Basin. Natu-ral sediment erosion rates of 17 to 18 mm per 1000 years and soil residence time of 26,000 to 57,000 years in upland sites classify the region to be characterised by slowly eroding soils. Valley hollow study sites indicate an alteration in sediment supply due to climate change within the last 80,000 years. Analysis of meteoric 10Be on suspended sediment in rivers with time (from pre-colonial to present times) and a comparison to the historical background of land use management indicated drastic changes in the sediment sources to the streams during the colonisation of the US East Coast. Such profound changes in the soil movement dynamics can be tied to a complete deforestation of the watershed during that time. However, a slow recovery of sediment sources to pre-colonial conditions within the last 100 years was observed, which may be related to the afforestation in parts of the watershed and change in farming practises. Examination of floodplains in recent history (last 100 years) using 210Pb and 137Cs identified a correlation between changes in land use and floodplain development. In particular, deforestation led to channel migration within the watershed, whereas afforestation reduced the flood magnitude. A discrimination of different sediment sources (agricultural and forest surface, channel migration) was detected on present suspended sediment by using a combination of tracers: 7Be, 137Cs, 210Pb and 10Be. About 50 % of sediment in the stream was attributed to channel migration classifying the stream as relatively unstable, whereas the second major sediment contribution was agricultural soil (32 %). As both sources are closely related to human activity in the study area, appropriate land management practices and stream channel stabilisation are of great importance for a hydro- and morphodynamic balance in future times. Results presented in this study provide evidence of the powerful impact anthropogenic land use has had on sediment movement in the Christina River Basin. More importantly, this study has shown the usefulness of combining long- and short-lived radionuclides to identify soil redistribution at different time periods and scales. Radionuclide data does not only reflect the drastic impact during European colonisation but also detects gradual changes that occur due to human action to limit soil erosion within the last century. This study indicates that some of the best management practices may minimise anthropogenic induced soil erosion, whilst simultaneously signifying the need to further improve land use management and reduce sediment erosion.