Search

Abstract This work is the first geomorphological analysis of La Narse de la Sauvetat , a hydromorphic basin located in the southern Limagne plain (French Massif Central) which had never been studied despite its great palaeoenvironmental interest. We explore the potential of its sedimentary archives to provide valuable data on Holocene geomorphic dynamics and their sensitivity to local and regional hydro-climatic changes. In order to characterize the nature and the morphostratigraphic and pedosedimentary evolution of the basin, we used an integrative approach involving geomorphological mapping, hand auger stratigraphic cross-sections, topographic analysis based on LiDAR data and stereophotogrammetric reconstruction, geophysical prospection and radiocarbon dating. Results revealed a palaeotopography fossilized under 6 m of sedimentary infilling, with a clear shift from fluvial to hydromorphic conditions circa 2800 cal BCE, separated by a level with colluvial and lacustrine features. A detailed analysis of the eastern border of the basin also suggests the existence of a former open valley in place of the current closed depression, now locked in by a topographic threshold forming a dam. The sedimentary aggradation of the basin started with an alluvial phase during the Upper Pleistocene and probably continued into the Middle and Late Holocene, consistently with regional alluvial dynamics. In the early Subboreal, a massive landslide occurred on the western slopes of the Puy-de-Corent volcanic plateau, thereby damming the valley and forming a closed depression. This episode coincided with the mid-Holocene climatic shift and an associated phase of slope instability and increased landslides in Western Europe due to changing climatic conditions. This depression evolved into a hydromorphic basin with several high energy hydro-sedimentary episodes after 2800 cal BCE; however hydro-sedimentary conditions gradually became more stable towards the Late Holocene. Around 500 cal CE, the drainage deteriorated towards palustrine conditions and caused the formation of a marsh and its consequent evolution into a shallow lake throughout the Middle Ages. This lake was drained in the late XVIIIth Century. The long-term sedimentary record of La Narse de la Sauvetat , sensitive to climatic instability periods of the Middle Holocene, makes this landslide-dammed basin a valuable site with an important potential to further development of palaeoenvironmental and geoarchaeological studies. Graphical abstract Unlabelled Image Highlights • A palaeovalley was found under Pleistocene and Holocene alluvio-colluvial sediments. • A landslide dammed the valley in the early Subboreal, forming a short-lived lake. • The depression became hydromorphic between 2800 cal BCE and late Antiquity. • The basin was a marsh then a shallow lake during the Middle Ages. • La Narse de la Sauvetat has high potential for further palaeoenvironmental research. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents the first results of the geoarchaeological study of an unusual hilltop wetland, located within the protohistoric proto‐urban site of Corent in the French Massif Central. This small depression offers an exceptionally local sedimentary record that provided valuable data on long‐term human–environment interactions, as well as proto‐urbanization of the first millennium B.C.E. Field survey revealed a major archaeological discovery: a large ensemble of 114 Iron Age storage pits excavated in clayey deposits. The geoarchaeological analysis of stratigraphic logs and cross‐sections completed by geophysical survey and radiocarbon dating allowed us to refine the chronology of these structures. Here, we suggest these structures are probably contemporary with the Hallstatt occupation of the site (600–425 B.C.E.), consistent with the emergence at Corent of a short‐lived proto‐urban environment during this period. These investigations also allowed us to characterize the main evolution phases of the basin and the diversity of human impacts from first disturbance in the Early Neolithic to its final destruction in the Roman period. These findings highlight the nonlinear nature of the socio‐environmental interactions and the definitive shift to an “anthroposystem” as a consequence of major disturbance in the first Iron Age, centuries before the development of urban settlements in the oppida period. [ABSTRACT FROM AUTHOR]

This paper investigates post-dam geomorphic and vegetation changes in the Sauce Grande River, a meandering dryland river impounded by a large water-conservation dam. As the dam impounds a river section with scarce influence of tributaries, sources for fresh water and sediment downstream are limited. Changes were inspected based on (i) analysis of historical photographs/imagery spanning pre- (1961) and post-dam (1981, 2004) channel conditions for two river segments located above and below the dam, and (ii) field survey of present channel conditions for a set of eight reference reaches along the river segments. Whilst the unregulated river exhibited active lateral migration with consequent adjustments of the channel shape and size, the river section below the dam was characterized by (i) marked planform stability (93 to 97%), and by (ii) vegetation encroachment leading to alternating yet localized contraction of the channel width (up to 30%). The present river displays a moribund, stable channel where (i) redistribution of sediment along the river course no longer occurs and (ii) channel forms constitute a remnant of a fluvial environment created before closing the dam, under conditions of higher energy. In addition to providing new information on the complex geomorphic response of dryland rivers to impoundment, this paper represents the very first geomorphic assessment of the regulated Sauce Grande and therefore provides an important platform to underpin further research assessing the geomorphic state of this highly regulated dryland river. [ABSTRACT FROM AUTHOR]

BSTRACT MISSIN [ABSTRACT FROM AUTHOR]

• Geoarchaeological study of a pedosedimentary archive in a protohistoric wetland. • Human activities drove local environmental evolution over five millennia. • Neolithic anthropogenic erosion contributed to degrade Early Holocene andic soils. • Bronze Age agro-pastoral land use induced development of reducing conditions. • Iron Age activities led to strong environmental impact and backfill of the basin. This paper presents a multi-proxy geoarchaeological analysis of the complex pedo-sedimentary archives of a small pond situated within the protohistoric site of Corent, in central France. A robust chrono-stratigraphic framework was developed integrating all chronological data from previous studies. Micromorphological, geochemical and sedimentological analyses were performed on a radiocarbon-dated core. Results are discussed alongside palynological data from previous work on the same core, in order to reconstruct human-environmental interactions on the Corent volcanic plateau since the Neolithic. After intense soil erosion recorded during the Early and Middle Neolithic, the Late Neolithic is characterized by vegetation recovery and soil stability in the basin contemporaneous with site abandonment, perhaps due to pronounced soil degradation in the plateau. During the Early Bronze Age, agropastoral land use contributed to major environmental changes in the pond's basin, including soil profile sealing and gleyfication. Environmental impact patterns detected in the pedo-sedimentary record indicate settlement intensification during the Middle Bronze Age. After the substantial impacts of an intense anthropogenic disturbance in the Early Iron Age, the Lac du Puy becomes a vertic depression recurrently flooded while a late Iron Age Gallic oppidum develops on the plateau, and is finally backfilled at the end of the Iron Age. The detailed multi-proxy picture from this intra-urban wetland helps nuance our understanding of the environmental impacts of prehistoric societies in Western Europe. Indeed, our results highlight the importance of keeping a long term perspective when dealing with past human-environment interactions. [ABSTRACT FROM AUTHOR]

We analysed the late-Holocene pedo-sedimentary archives of La Narse de la Sauvetat, a hydromorphic depression in the southern Limagne plain (central France), where chronologically accurate studies are scarce. The multi-proxy geoarchaeological and palaeoenvironmental analysis of two cores from different areas of the basin was carried out through sedimentological, geochemical, micromorphological and malacological investigations. Integration of these datasets supported by a robust radiocarbon-based chronology allowed discussion of socio-environmental interactions and anthropogenic impacts from Late Neolithic to Early Middle Ages. Until the Middle Bronze Age, there was no clear evidence of anthropogenic impact on soils and hydro-sedimentary dynamics of the catchment, but two peaks of high alluvial activity probably related to the 4.2 and 3.5 kyr. BP climate events were first recorded in Limagne. Significant anthropogenic impacts started in the Late Bronze Age with increased erosion of the surrounding volcanic slopes. However, a major threshold was reached c. 2600 cal BP with a sharp increase in the catchment erosion interpreted as resulting from strong anthropogenic environmental changes related to agricultural activities and drainage. This implies an anthropogenic forcing on soils and hydro-sedimentary systems much earlier than was usually considered in Limagne. These impacts then gradually increased during Late Iron Age and Roman periods, but environmental effects were certainly contained by progress in agricultural management. Late Antiquity environmental changes are consistent with regional trend to drainage deterioration in lowlands, but marked asynchrony in this landscape change suggests that societal factors implying differential land management were certainly predominant here. [ABSTRACT FROM AUTHOR]

Expected runout distances and related return periods are the most important parameters needed for zoning in terrain prone to snow avalanching. Hazard mapping procedures usually allocate areas of land to zones with a different degree of danger based on return periods estimated for given snow volumes in the starting zone or with statistical/dynamical models. On forested avalanche paths, dendrogeomorphology has a great potential to add critical input data to these calculations in terms of recurrence intervals or return periods. However, quite paradoxically, recurrence interval maps of snow avalanches have only rarely been retrieved from tree-ring analysis and mostly represent the inverse of the mean frequency of avalanches that could be retrieved locally rather than the return period. The purpose of this study therefore was to propose a consistent approach for tree-ring based recurrence interval mapping of snow avalanche events. On the basis of 71 snow avalanches retrieved from 2570 GD growth disturbances identified in 307 larch trees from three avalanche paths located in the vicinity of Täsch (Canton of Valais, Swiss Alps), we first followed the classical approach used in dendrogeomorphology and derived recurrence interval maps through interpolation from recurrence intervals observed at the level of individual trees. We then applied an expert delineation of the spatial extent of past events based on the location of disturbed trees. Our results show that the second step improved representation of expected patterns of recurrence intervals that typically increase as one moves down the centerline of the avalanche path. Despite remaining limitations and uncertainties precluding from direct use of our maps for hazard mapping purpose, these results suggest that dendrogeomorphic time series of snow avalanches can yield valuable information for the assessment of recurrence intervals of avalanches on forested paths for which only very limited or no historical data exists, and that this data can be obtained independently from meteorological data or numerical modeling. [ABSTRACT FROM AUTHOR]

Within riparian corridors, biotic-abiotic feedback mechanisms occur between woody vegetation strongly influenced by hydrogeomorphic constraints (e.g., sediment transport and deposition, shear stress, hydrological variability), fluvial landforms, and morphodynamics, which in turn are modulated by the established vegetation. During field investigations in spring 2015, we studied 16 alluvial bars (e.g., point and lateral bars) within the dynamic riparian corridor of the Allier River (France) to assess the aptitude of three pioneer riparian Salicaceae species ( Populus nigra L., Salix purpurea L., and Salix alba L.) to establish and act as ecosystem engineers by trapping sediment and constructing fluvial landforms. Our aim is to empirically identify the preferential establishment area (EA; i.e., the local areas where species become established) and the preferential biogeomorphic feedback window (BFW; i.e., where and to what extent the species and geomorphology interact) of these three species on alluvial bars within a 20-km-long river reach. Our results show that the EA and BFW of all three species vary significantly along the longitudinal profile, i.e., upstream-downstream exposure on the alluvial bars, as well as transversally, i.e., the main hydrological connectivity gradient from the river channel toward the floodplain. In the present-day context of the Allier River, P. nigra is the most abundant species, appearing to act as the main engineer species affecting landform dynamics at the bar scale; S. purpurea is established and acts as an ecosystem engineer at locations on alluvial bars that are most exposed to hydrosedimentary flow dynamics, while S. alba is established on the bar tail close to secondary channels and affects the geomorphology in mixed patches along with P. nigra . Our study highlights the role of functional trait diversity of riparian engineer species in controlling the extent of fluvial landform construction along geomorphic gradients within riparian corridors exposed to frequent hydrogeomorphic disturbances. [ABSTRACT FROM AUTHOR]

Over the last twenty years, significant technical advances turned photogrammetry into a relevant tool for the integrated analysis of biogeomorphic cross-scale interactions within vegetated fluvial corridors, which will largely contribute to the development and improvement of self-sustainable river restoration efforts. Here, we propose a cost-effective, easily reproducible approach based on stereophotogrammetry and Structure from Motion (SfM) technique to study feedbacks between fluvial geomorphology and riparian vegetation at different nested spatiotemporal scales. We combined different photogrammetric methods and thus were able to investigate biogeomorphic feedbacks at all three spatial scales (i.e., corridor, alluvial bar and micro-site) and at three different temporal scales, i.e., present, recent past and long term evolution on a diversified riparian landscape mosaic. We evaluate the performance and the limits of photogrammetric methods by targeting a set of fundamental parameters necessary to study biogeomorphic feedbacks at each of the three nested spatial scales and, when possible, propose appropriate solutions. The RMSE varies between 0.01 and 2 m depending on spatial scale and photogrammetric methods. Despite some remaining difficulties to properly apply them with current technologies under all circumstances in fluvial biogeomorphic studies, e.g. the detection of vegetation density or landform topography under a dense vegetation canopy, we suggest that photogrammetry is a promising instrument for the quantification of biogeomorphic feedbacks at nested spatial scales within river systems and for developing appropriate river management tools and strategies. [ABSTRACT FROM AUTHOR]

ALS-derived raster visualization techniques have become common in recent years, opening up new possibilities for subtle landform detection in earth sciences and archaeology, but they have also introduced confusion for users. As a consequence, the choice between these visualization techniques is still mostly supported by empirical knowledge. Some attempts have been made to compare these techniques, but there is still a lack of analytical data. This work proposes a new method, based on gradient modelling and spatial statistics, to analytically assess the efficacy of these visualization techniques. A selected panel of outstanding visualization techniques was assessed first by a classic non-analytical approach, and secondly by the proposed new analytical approach. The comparison of results showed that the latter provided more detailed and objective data, not always consistent with previous empirical knowledge. These data allowed us to characterize with precision the terrain for which each visualization technique performs best. A combination of visualization techniques based on DEM manipulation (Slope and Local Relief Model) appeared to be the best choice for normal terrain morphometry, occasionally supported by illumination techniques such as Sky-View Factor or Negative Openness as a function of terrain characteristics. [ABSTRACT FROM AUTHOR]

Actuellement, un consensus a été atteint pour protéger et restaurer les bras morts. Par conséquent, il est important de comprendre le fonctionnement de ces zones humides. Á notre connaissance, la distribution spatio-temporelle des diatomées, des macrophytes et des macroinvertébrés benthiques, en relation avec la géomorphologie n'a pas été étudiée de manière approfondie. Dans les zones amont et intermédiaire, la faible température de l'eau et les fortes concentrations ioniques ont révélé une connexion avec l'eau de nappe. De plus, le seuil géomorphologique, qui caractérise la zone intermédiaire, a créé les mêmes phénomènes de circulation de l'eau dans les sédiments que dans les rivières, avec un écotone « nappe phréatique/milieu hyporhéique/eau de surface » et produit des différences physico-chimiques entre l'amont et l'aval du bras mort et ainsi modifie la distribution des organismes. En aval, les apports d'eau du chenal principal de la rivière Allier ont permis le développement de taxons observés dans la rivière. Toutes ces connexions hydrologiques et la sédimentation ont modifié les caractéristiques physiques, chimiques et géomorphologiques, créant ainsi un gradient de qualité de l'eau, qui influence la répartition des communautés biologiques. Currently, a consensus has been reached to protect and restore cut-off meanders. Therefore, it is important to understand how these wetlands function. To our knowledge, the spatial and temporal distribution of both benthic diatoms and macroinvertebrates, associated with macrophytes communities, linked to the geomorphology, has not been studied extensively. In the upstream and intermediate zones, the low water temperature and the high conductivity revealed a connection with the groundwater. Moreover, the geomorphological riffle characterizing the intermediate zone created the same downwelling and upwelling processes as in running water, with a groundwater/hyporheic/surface water ecotone and produced physico-chemical differences between the upstream and the downstream zones and thereby modify the diatom, macroinvertebrate and macrophyte distribution. Downstream, water input from the main channel of the Allier River allowed the development of taxa observed in the River. All these hydrological connections and the sedimentation modified the physical, chemical and geomorphological characteristics, thereby, creating a gradient of water quality, influencing the distribution of the biotic communities. [ABSTRACT FROM AUTHOR]

A study has been conducted on an alpine river deeply changed by human interventions. Using hydrological, geomorphological and phytoecological methods as well as historical reconstruction, the metamorphosis of the river and the causes of changes are discussed. The example of the river metamorphosis shows that the floodplain is composed of patches in close juxtaposition. The concepts of 'space-time' and 'series'is introduced to understand the particular dynamics of patches in anenvironment dramatically disturbed by man. In addition, the interestof such an approach for floodplain management is highlighted. [ABSTRACT FROM AUTHOR]

Within riparian corridors, vegetation establishment is controlled by hydrogeomorphology which in turn is modulated by plant species acting as ecosystem engineers. The potential of different plant species to affect geomorphology according to their exposure to different level of mechanical stress was poorly considered. The comprehension of the variation of morphological and biomechanical response trait attributes of plants is crucial to better understand how pioneer riparian plants succeed in establishing on alluvial bars and affect fluvial geomorphology. We investigated the response of vegetation to hydrogeomorphic constraints and its engineering effect at two spatial scales within the dynamic Allier River, France. At the broader reach scale, we studied on sixteen alluvial bars the aptitude of three dominant pioneer riparian Salicaceae species (Populus nigra L., Salix purpurea L. and Salix alba L.) to establish and to act as ecosystem engineers by trapping fine sediment. At the finer bar scale, we quantified the relation between response trait attributes of young (1-2yrs) P. nigra plants and their exposure to three different levels of mechanical stress. Our results show (i) that the capacity of riparian plants to establish and act as ecosystem engineers depends of species ecology, the age of individuals and on their locations and (ii) that P. nigra plants develop different response traits depending on their exposure to mechanical stress. From a functional trait perspective, our results suggest that interspecific functional trait diversity and intraspecific plasticity of riparian engineer species plays an important role in vegetation resilience and resistance. Trait diversity and plasticity also control the overall capacity of plants to enhance fluvial landform construction at the scale of the alluvial bar. Keywords: riparian Salicaceae species; fluvial landforms; ecosystem engineer; response and effect plant traits; multi-scale study [ABSTRACT FROM AUTHOR]