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The paper emphasizes the main lessons learned from hydromorphological monitoring following the removal of a medium-sized dam (7.29 m) located on a medium energy gravel bed river over a four year period (2015–2019). The Pierre Glissotte dam was previously located on the upper Yonne river (Morvan massif), where it was an obstacle to sediment continuity and was almost completely filled with sand and silts. The dam was removed in two steps, the first in July 2015 and the second in October 2017. Several methods were used (topographical surveys, SFM photogrammetry, RFID tracking, hydrological monitoring) to characterize river adjustments, i.e. the nature of the morpho-sedimentary dynamics, their rates, their temporal and spatial variations, and their control mechanisms. The results highlight the complex and nonlinear response of the Yonne river and the relevance of a regular prolonged monitoring. The changing patterns in space and over time, underline the vast range of uncertainties surrounding this type of restoration and the difficulty involved in predicting post-removal hydromorphology around the dam (return to pre-dam functioning, no changes, new equilibrium conditions). For instance, up to now, the study shows that intense morpho-sedimentary dynamics in the reservoir and effective restoration of bedload continuity do not necessarily lead to changes in the downstream conditions (bed mobility and morphological configuration) previously shaped under the influence of the dam, thus mitigating the success of the river restoration operation.

Few studies address past droughts by combining different geographical and temporal scales, and a contextualization of social and environmental conditions. This paper proposes a study of the 1857–1858 dry spell in eastern France (Alsace), its natural (climatological and hydrological) as well as socio-political consequences. This episode is one of the major droughts that affected the north of France in the nineteenth century, as much by its duration as by its intensity. Here, we do not only consider the drought as a natural hazard producing physical damage (lack of water, weakening of river discharge) or social tensions (competition for the water, conflicts), but we place it in the particular context of the basins it affects, in order to better evaluate its natural and social intensity. First, we highlight the importance of the 1857–1858 drought from the regional to the Alsatian scale, by unravelling the spatial and temporal distribution of precipitations, based on available instrumental measurements. Depending on the stations, the rainfall deficit established between 10 and 40% of the average rainfall amount in 1857 or 1858. Suitable, narrative sources from municipal, departmental and national archives allow us to specify the position of water stakeholders and to analyse the discourses as well as their social and political reactions. Comparing natural and social data, it appears that rivalries for water resource did not necessarily match the meteorological rhythm of the dry spell. The severity of the drought varies depending on economic needs. Three periods seem very demanding for water consumers: August–September 1857, February 1858, and August–October 1858. The structural responses appear to be linked to the economic context, both at a local and national scale. Finally, our approach shows that drought partly is a socially constructed phenomenon, and one to consider carefully, when it comes to both contemporary and past events. Hence, to better understand drought episodes, we need to disentangle the socio-natural interactions at the local level.

International audience; In southern Tunisia, agricultural land has been subject to significant anthropogenic pressures since the early 20th century, which have contributed to significant changes in land use. The region's strong demographic growth has led to socio-spatial changes (sedentarisation, the descent of mountain populations down to the foothills and the Djeffara plain, the development of agriculture in the jessour (water and soil conservation works that block the gullies dug in the loess) at the expense of the rangelands. The current landscapes of southern Tunisia, from the Matmata region to the surroundings of Tataouine, now display signs of a rural exodus that is reflected in the abandonment of the jessour. This exodus has significant landscape and environmental impacts that can be measured through the use of diachronic vertical imagery (aerial photographs, satellite images). It can be observed that from the 1950s onwards, jessour developed strongly in the ravines, in line with demographics. The rural exodus that appeared in the 1970s led to the concentration of populations in the new towns, which did not prevent water and soil conservation works from continuing to be used and maintained. While some areas, particularly in the southern part of the study area, are still seeing the appearance of new jessour, most of them are abandoned and subjected to regressive erosion due to the suffocation that rapidly destroys them.; Dans le Sud Tunisien, les terres agricoles sont soumises depuis le début du XXe siècle à d’importantes pressions anthropiques qui ont contribué à modifier largement l’occupation des sols. La forte croissance démographique de la région a entraîné des mutations socio-spatiales (sédentarisation, descente des populations montagnardes vers les piémonts et la plaine de la Djeffara, développement de l’agriculture dans les jessour (ouvrages de conservation des eaux et des sols qui barrent les ravines creusées dans les lœss) au détriment des parcours). Les paysages actuels du Sud Tunisien, de la région de Matmata aux environs de Tataouine, montrent aujourd’hui les signes d’un exode rural qui se traduit notamment par l’abandon des jessour. Cet exode a des impacts paysagers et environnementaux conséquents, qu’il est possible d’appréhender et de quantifier grâce à l’utilisation de l’imagerie verticale diachronique (photographies aériennes, images satellitaires). On observe ainsi qu’à partir des années 1950, les jessour se développent fortement dans les ravines, en lien avec la démographie. L’exode rural qui apparaît dans les années 1970 se traduit par la concentration des populations vers les villes nouvelles, qui n’empêche pas que les ouvrages de conservation des eaux et des sols continuent à être utilisés et entretenus. Si de nos jours certains secteurs, en particulier au sud de la région étudiée, voient encore l’apparition de nouveaux jessour, la plupart d’entre eux sont abandonnés et soumis à une érosion régressive due à la suffosion qui les détruit rapidement.

L’article présente le suivi hydromorphologique de la première phase d’arasement du barrage hydroélectrique de Pierre Glissotte (7,29 m de hauteur). Construit sur l’Yonne supérieure (Morvan) dans les années 1920, l’ouvrage représentait un obstacle aux continuités biologique et sédimentaire et sa retenue (~40 000 m³) était presque entièrement comblée par des sédiments. L’objectif est de comprendre les modalités et rythmes d’ajustement hydromorphologique de l’Yonne suite au démantèlement des 3 m supérieurs du barrage. Les diverses méthodes utilisées (relevés topographiques, photogrammétrie SfM, marquages RFID, suivis hydrologiques) ont permis de caractériser l’évolution rapide de l’ancienne retenue (incision de 1,20 à 4 m, formation puis élargissement d’un chenal, augmentation du D50, etc.). Malgré de faibles débits, la remobilisation des sables et limons comblant l’ancien réservoir a débuté dès la vidange. La charge grossière (graviers, cailloux, galets), piégée en amont et queue de retenue, a aussi été rapidement réintroduite dans le système, avec des vitesses d’avancée des traceurs RFID mobilisés comprises entre 138 et 347 m/an les trois premiers mois, là aussi sans épisode hydrologique remarquable. Au bout de 9 mois, près de 23 % du volume de sédiments contenu dans le réservoir avait été érodé (9 600 m³), à la faveur d’une faible cohésion des dépôts, d’un charriage actif sur l’Yonne et de plusieurs épisodes de crues. En aval du barrage, si les écoulements très concentrés des premières semaines ont déposé jusqu’à 0,37 m d’épaisseur de sédiments fins dans certaines zones, aucun exhaussement du lit n’a pu être constaté 15 mois plus tard. The article focuses on the hydromorphological monitoring of the first stage of the removal of the Pierre Glissotte hydroelectric dam (7.29 m high) located on the upper Yonne River (Morvan massif). The dam, built in the 1920s, acted as an obstacle for biological and sediment continuity and the reservoir (~40,000 m3) was almost completely filled with sediment. Monitoring was undertaken to understand how the Yonne River has adjusted geomorphologically from the beginning of the dam demolition (3 meters removed). Various methods were used (topographical surveys, SFM photogrammetry, RFID tracking, hydrological monitoring) and they enabled the rapid evolution of the former reservoir to be characterized (incision from 1.20 m to 4 m, channel widening, D50 coarsening, etc.). Sand and silt filling the reservoir started to be evacuated as soon as the dewatering began, during low flows. Coarse material (granules, pebbles and cobbles), placed in the upper part of the reservoir, was also moved quickly, with an average velocity between 138 and 347 m.y‑¹for the mobilized RFID tracers during the first three months, without any important hydrological event. After 9 months, about 23% of the reservoir sediment fill was eroded (9,600 m3). Erosion was favoured by the poor cohesiveness of the deposits, active bedload transport on the Yonne River, and several high-flows and flood events. Downstream of the dam, fine sediment-laden flows occurring over the first weeks left 0.37 m thick deposits in certain areas. However, 15 months later, no aggradation of the downstream riverbed had yet been noticed.

International audience; The geomorphological response of valley bottoms in eastern France to climatic fluctuations of the Little Ice Age (LIA) was investigated using sedimentological analysis together with optically stimulated luminescence (OSL) and radiocarbon dating. Diachronic mapping of land use since the beginning of the nineteenth century was also carried out. Since A.D. 1500, the valley bottoms experienced three cycles of aggradation and subsequent incision , each characterized by paired periods of high and low detritic activity. While the impact of human activity on the aggradation of the alluvial plain is observed, the vertical dynamics of the valley bottom deposits seemingly were also linked to the hydroclimatic fluctuations during the LIA. The sensitivity to these fluctuations was increased by human activity at the scale of the basin. Variations of the winter North Atlantic Oscillation (NAO) and solar activity from the last five centuries correlate with wet and cold phases during which valley bottoms accumulated , and dry and warm phases during which the streams incised into the valley floors. This fluvial sensitivity to the meteorological conditions induced temporal variations in sedimentary supply originating from either direct input from remnants of periglacial alluvial sheets or local rocky outcrops and/or from indirect input from the erosion of alluvial and colluvial deposits. These two components, combined with the sheet runoff over the ploughlands, express the complex coupling between hillslopes and valley bottoms in the headwater catchments. This caused a cascade-shaped transit of the sediments characterized by alternating phases of storage and removal. Keywords: Little Ice Age Headwater valley bottom deposits Aggradation-incision cycle OSL and radiocarbon dating Landscape history Massif Central

Over a historical timescale, landscapes have been strongly affected by fluctuations in climate and by the impact of human societies. This study examines the historical evolution of mid-altitude fluvial systems in the Western Alps and Cevennes (SE Massif Central) in the context of marked climate and anthropogenic change at the end of the Little Ice Age (late 19th century). This work contributes to the reconstruction of river paleodynamics by investigating the nature and chronology of geomorphological changes in upper river basins. In order to produce a detailed reconstruction of hydromorphological changes, we combined two approaches: the study of historical archives and the use of bioindicators (the dating of terraces using dendrochronology and of silt benches using lichenometry in order to reconstruct the evolution of the river channels). The 19th century is a particularly propitious period for the study of surface dynamics because archives have preserved a wealth of qualitative and quantitative data concerning rivers (economic statistics, meteorological and hydrological observations, illustrative documents, descriptions by contemporary observers). It is also a period for which reliable dating methods are available for detrital deposits in rivers. The period provides new information on how the transition between the Little Ice Age and current climate conditions affected the evolution of drainage basins and stream channels by highlighting a succession of phases in the erosive process (e.g., the preparatory role of the drought of 1830–1838 in the erosion crisis of 1855–1870) and refining the chronology of events (very early onset of riverbed incision). The results highlight the effect of climate (small hydroclimatic fluctuations), amplified by strong anthropization, on the rhythm of landscape change and on the relative stabilization of the landscape at the end of the 19th century. In addition, the synchronization of phenomena on the two sides of the Rhone Valley shows that these changes took place on a regional scale.

International audience; Timber floating industry of the Morvan Massif (central France) has been studied through several historical and economical studies. Its hydrological and geomorphological aspects and consequences are however still relatively unknown. For more than two centuries, water releases were regularly created from small artificial reservoirs in order to ease the transportation of logs through the small tributaries towards the Yonne River. The aim of this study was to reconstruct discharges of these artificial floods from historical data. They show that discharges ranged between 2 m3/s and 3 m3/s immediately downstream the reservoirs. Such discharge may be considered as major flood when considering catchments of only few square kilometres. In the headwater stream the discharges of those artificial floods may therefore be considered as extreme events in comparison to the natural regime. Further downstream, on the main course of the Yonne River, 2-3 m3/s correspond to ordinary floods in natural regimes. This discrepancy between the very upper part of the course and the downstream part underwent a major difference in terms of geomorphological impacts. An estimation of the specific streampower of those artificial floods and geomorphologic field survey suggest that the small head water streams have been highly eroded propagating a sediment wave further downstream.; Le flottage du bois dans le Morvan a fait l'objet de nombreuses études historiques ou économiques. Toutefois, la modification de l'écou-lement pour le flottage à bûches perdues n'a jamais été véritablement étudiée d'un point de vue hydrologique. Pendant plus de deux siècles, des lâchers d'eau répétés ont été effectués à partir d'étangs artificiels afin de faciliter le transport des bûches dans les petits ruis-seaux morvandiaux. Grâce aux données historiques existantes, le débit de ces lâchers a pu être évalué : il était compris entre 2 m 3 /s et 3 m 3 /s à l'aval immédiat des étangs, ce qui est considérable dans de petits ruisseaux dont le bassin-versant ne couvre que quelques kilo-mètres carrés. La comparaison de ces chiffres avec l'écoulement naturel des rivières étudiées fait apparaître une différenciation très nette entre les cours d'eau de tête de bassin, où les crues artificielles étaient exceptionnellement fortes, et l'Yonne amont, où les crues pour le flottage demeuraient dans la classe des événements ordinaires. Ces manipulations d'écoulements ont entraîné des perturbations disparates sur le milieu fluvial, les ruisseaux de tête de bassin ayant été les plus perturbés. Par ailleurs, l'estimation de la puissance spécifique critique dissipée à l'aval des barrages et les observations géomorphologiques montrent que la capacité de cisaillement des écoulements artificiels était très importante. On peut ainsi faire l'hypothèse que les lâchers d'eau destinés au flottage ont favorisé l'éro-sion des vallons et alimenté une vague sédimentaire.