Emmanuèle Gautier

Surface water -groundwater interactions in an alluvial plain: Chemical and isotopic systematics

Our work on the Loire River forms part of a French National Research Program dedicated to wetlands and aims to better understand the global functioning of the system from the hydrological, geochemical, ecological and sociological aspects. The present study, using a coupled hydrological and geochemical (stable and Sr isotopes) approach, focuses on the ‘Soulangy’ site with its secondary anastomosing channels just below the confluence of the Loire and Allier rivers, and also on the ‘Dorna``nt’ site with two unconnected oxbow lakes 50 km upstream of the confluence. The stable isotopes of water (δ18O, δ2H) show that the alluvial (or riverbank) aquifer feeds the Loire River during the summer, but is not recharged by the river during flood periods in the winter; the alluvial groundwater thus has a purely local origin from precipitation. The major elements reveal an anthropogenic input of Cl and more importantly of NO3, especially near farms. The 87Sr/86Sr isotopes identify different groundwater layers in the alluvium, i.e. an upper and a lower alluvial aquifer, and a perched aquifer at Dornant, that have relatively complex relationships with the surface water. The two main rivers (Loire and Allier) present distinct geochemical characteristics reflecting the different lithologies that they drain upstream. In addition, the secondary channels, lying parallel to the Loire main stream at the Soulangy site, give different geochemical signatures, which shows that they are not fed by the same overflows of the Loire; they are more-or-less well connected to the upper level of the alluvial plain, and a longitudinal study of one of these channels has revealed a Loire River influence progressively replaced by a water contribution from the upper alluvial aquifer. Similarly, the two oxbow lakes at the Dornant site are not supplied by the same water during the summer months. A conceptual scheme of the Loire hydrosystem based on δ18O and 87Sr/86Sr suggests that the isotopic variations of the Loire River can be related to a Massif Central surface-water supply for the Loire and Allier main streams and to a groundwater supply from the alluvial plains.

Water erosion in the southern French alps: climatic and human mechanisms

Abstract A large number of studies have focused on water erosion in the southern Alps because of its intensity and the high erodability affecting numerous outcrops in this region. The aim of this paper is to determine the relations between natural and human factors on the one hand, geomorphologic behaviour of hillslopes and fluvio-torrential dynamics at two time scales (geological and historical) on the other hand. This paper specially focuses on the recent weakening of torrential activity and its linkage with reforestation and rural desertion on the watersheds. Another objective is to compare these results with a rich mainly French scientific production on this theme. Experimental data on: – current erosion measurements and the analysis of its processes on the one hand, – and geomorphologic background of river beds and slopes, are compared and analysed using archives and the present knowledge on historical climate. It has been shown that the overexploitation of watersheds leads to an increase in soil loss, in the number and intensity of floods in alpine valleys, as well as to an enhancement of sediment transport. However, climatic changes during the Holocene and during the past millennium have resulted in changes in morphological behaviour of the hillslopes upstream and of river beds downstream. Moreover, a strong sedimentary deficit can be observed over the last three or four decades in all the Alpine massifs. The consequences of this sediment deficit are firstly an entrenchment of river beds, which threatens bridges and embankments. This recent trend is probably due to the excessive extraction of material in gravel pits and the sedimentation in the numerous dams built between 1920 and 1980 in the entire alpine range. However, the entrenchment appeared recently on non-exploited rivers; therefore, the deficit is also due to the success of the natural and artificial reforestation and torrent correction since the end of 19th century.

Runoff evolution due to land-use change in a small Sahelian catchment

Abstract Land-use changes have been significant these last decades in West Africa, particularly in the Sahel region; in this area, climatic and demographic factors have led to a rise in cropped areas in recent decades causing strong changes in the water cycle and in river regimes. This study compares the rainfall–runoff relationships for two periods (1991–1994 and 2004–2011) in two small and similar neighbouring Sahelian catchments (approx 0.1 km2 each). This allows identification of the different hydrological consequences of land-use/land-cover change, particularly the fallow shortening and the consequent degradation of topsoil. The main land surface change is a 75% increase in crusted soil area. Runoff increased by more than 20% on average between the two periods while flood duration decreased by 50% on average. However, runoff values remained largely constant in the lower part of the northern basin due to a strong increase in in-channel infiltration. Editor D. Koutsoyiannis; Associate editor T. Wagener

The Geology of Mars: Siberian rivers and Martian outflow channels: an analogy

11.1 IntroductionTheoriginsofMartianoutflowchannelshavebeenthesubjectofconsiderabledebate. Our examination of Martian conditions has led us to the idea thatmany commonalities exist between the Martian and terrestrial periglacialclimates. Particularly, the formation processes for Martian outflow channelsand Siberian valleys seem to be similar on both planets. Therefore in thischapter we propose an analogy between the Lena River and the Martianoutflow channel Ares Vallis. The fluctuating flows of the Lena River mightalso afford an analog to the Martian channel-forming flows. Both hydro-systems were or are associated with a periglacial environment characterizedby a deep and continuous permafrost.11.2 Periglacial environments in Yakutia and on MarsCentral Yakutia (Figure 11.1) is a very specific morphoclimatic zone, boundon the east by the Verkhoyansk Mountains (elevation 2900 m) and on thewest by the Siberian shield of (200 m to 1000 m in elevation). The extremecontinental Yakutian climate in Siberia is characterized by long and coldperiods with minimum surface temperatures of 72 C. In summer, themaximum surface temperature can reach 38 C. Low precipitation (less than200 mm per year), and high evaporation and sublimation rates are char-acteristics of a very dry climate (Katasonow and Soloviev, 1969).Thelowmeanannualsurfacetemperaturetogetherwiththethinsnowcoverfavor the existence of a deep permafrost layer. With a permanent freezing