Rhone River Research Articles

Shallow-water polychaete assemblages in the northwestern Mediterranean Sea and its possible use in the evaluation of good environmental state

Sarda, R., Serrano, L., Labrune, C., Gil, J., March, D., Amouroux, J.M., Taboada, S., Bonifacio, P. and Gremare, A. 2014. Shallow-water polychaete assemblages in the northwestern Mediterranean Sea and its possible use in the evaluation of good environmental state. Memoirs of Museum Victoria 71: 289–301. Forty-four shore-normal transects along the Northwestern Mediterranean coast between the mouth of the Rhone River (France) and Valencia City (Spain) were sampled during the REDIT-I (September 1998, [R1]) and REDIT-II (December 1999, [R2]) campaigns. Polychaete distribution patterns on shallow littoral fine sands (10 to 50 m water depths) were analyzed at a regional scale. A total of 359 species of polychaetes were identified which represents 38% of all polychaete species recorded in the western Mediterranean. Four main soft-bottom communities were identified from the samples in the area: Littoral Fine Sands, Littoral Sandy Mud, Terrigenous Coastal Mud and Detritic Sand. Predominantly sandy environments were characterized by Ditrupa arietina and Owenia fusiformis while Lumbrineris latreilli, Hilbigneris gracilis and Sternaspis scutata were numerically dominant in muddy environments. Biological diversity assessments at different temporal and spatial scales are required by the European Marine Strategy Framework Directive (2008/56/EC) in accordance with criteria and methodological standards of Good Environmental Status (GEnS). Selected indicators for descriptors are explored based on this mesoscale assessment.

Development of a 3D Coupled Physical-Biogeochemical Model for the Marseille Coastal Area (NW Mediterranean Sea): What Complexity Is Required in the Coastal Zone?

Terrestrial inputs (natural and anthropogenic) from rivers, the atmosphere and physical processes strongly impact the functioning of coastal pelagic ecosystems. The objective of this study was to develop a tool for the examination of these impacts on the Marseille coastal area, which experiences inputs from the Rhone River and high rates of atmospheric deposition. Therefore, a new 3D coupled physical/biogeochemical model was developed. Two versions of the biogeochemical model were tested, one model considering only the carbon (C) and nitrogen (N) cycles and a second model that also considers the phosphorus (P) cycle. Realistic simulations were performed for a period of 5 years (2007–2011). The model accuracy assessment showed that both versions of the model were able of capturing the seasonal changes and spatial characteristics of the ecosystem. The model also reproduced upwelling events and the intrusion of Rhone River water into the Bay of Marseille well. Those processes appeared to greatly impact this coastal oligotrophic area because they induced strong increases in chlorophyll-a concentrations in the surface layer. The model with the C, N and P cycles better reproduced the chlorophyll-a concentrations at the surface than did the model without the P cycle, especially for the Rhone River water. Nevertheless, the chlorophyll-a concentrations at depth were better represented by the model without the P cycle. Therefore, the complexity of the biogeochemical model introduced errors into the model results, but it also improved model results during specific events. Finally, this study suggested that in coastal oligotrophic areas, improvements in the description and quantification of the hydrodynamics and the terrestrial inputs should be preferred over increasing the complexity of the biogeochemical model.

Invertebrate distribution across nested geomorphic features in braided-river landscapes

AbstractRiver landscapes are increasingly viewed as a collection of nested geomorphic features, the hydrological effects of which juxtapose to create a mosaic of aquatic habitat conditions. We examined how the combined hydrological influences of stream reaches and gravel bars affect the composition of hyporheic invertebrate assemblages along the longitudinal and vertical dimensions of river landscapes. We worked in 12 braided-river valley segments of tributaries to the Rhone River, France. Valley segments were bounded downstream by geological knickpoints so that bedform-induced exchange flows beneath gravel bars and riffles were predictably embedded in exchange flows occurring at the valley-segment scale. In upstream reaches (UR) and downstream reaches (DR) of each valley segment, we collected invertebrates at the heads and tails of gravel bars in the hyporheic zone and at the upstream and downstream ends of riffles in surface channels adjacent to the bars. Patterns of vertical hydraulic gradient and spec...

Impact of winter storms on sediment erosion in the Rhone River prodelta and fate of sediment in the Gulf of Lions (North Western Mediterranean Sea)

In this study a three-dimensional sediment transport model was developed. The model accounts for both current and wave forcing on the sediment and was implemented over the Gulf of Lions. A two-way nesting technique was used to focus on the Rhone River prodelta which is considered as a sink for riverine sediment. In addition, to understand the resuspension of trapped sediment over the Rhone prodelta, an in situ experiment, called SCOPE, was conducted during the winter 2007–2008. The experiment consisted of measuring hydro-sedimentary parameters using a mooring station comprising a current profiler (ADCP) and an altimeter (acoustic transducer) located in the eastern part of the Rhone prodelta. The three-dimensional transport model was validated using these data, and used to investigate the effect of sediment dynamics at the prodelta and Gulf of Lions scale. Both modelling and data analysis highlighted the impact of the two strong storms from the south-east which characterised the experimental period. Erosion of bed material (about 2cm) and an increase in suspended material (up to about 50–100mg/l) in the water were the result of each storm as recorded at the mooring station. The erosion capacity due to waves, combined with a strong current, due to both wind and wave forcing, resulted in strong south-westward export over the whole prodelta. Each storm was responsible for an off-prodelta export estimated at around 2.1Mt. This study demonstrates that the Rhone River sediments trapped over the Rhone prodelta are subject to strong resuspension during episodic events.

Wave reworking of abandoned deltas

River deltas and individual delta lobes frequently face reduction of sediment supply, either from the geologic process of river avulsion or, more recently, due to human activities such as river damming. Using a process-based shoreline evolution model, we investigate wave reworking of delta shorelines after fluvial input elimination. Model results suggest that littoral sediment transport can result in four characteristic modes of delta abandonment, ranging from diffusional smoothing of the delta (or delta lobe) to the development of recurved spits. A straightforward analysis of delta shape and wave characteristics provides a framework for predicting the mode of delta abandonment. The observed morphologies of historically abandoned delta lobes, including those of the Nile, Ebro, and Rhone rivers, fit within this framework. Our results provide quantitative insight into the potential evolution of active delta environments in light of future extreme reduction of fluvial sediment input.

Modeling the spatial and temporal population dynamics of the copepod Centropages typicus in the northwestern Mediterranean Sea during the year 2001 using a 3D ecosystem model

The spatio-temporal dynamics of a simulated Centropages typicus (Kröyer) population during the year 2001 at the regional scale of the northwestern Mediterranean Sea are addressed using a 3D coupled physical–biogeochemical model. The setup of the coupled biological model comprises a pelagic plankton ecosystem model and a stage-structured population model forced by the 3D velocity and temperature fields provided by an eddy-resolving regional circulation model. The population model for C. typicus (C. t. below) represents demographic processes through five groups of developmental stages, which depend on underlying individual growth and development processes and are forced by both biotic (prey and predator fields) and abiotic (temperature, advection) factors from the coupled physical–biogeochemical model. The objective is to characterize C. t. ontogenic habitats driven by physical and trophic processes.The annual dynamics are presented for two of the main oceanographic stations in the Gulf of Lions, which are representative of shelf and open sea conditions, while the spatial distributions over the whole area are presented for three dates during the year, in early and late spring and in winter. The simulated spatial patterns of C. t. developmental stages are closely related to mesoscale hydrodynamic features and circulation patterns. The seasonal and spatial distributions on the Gulf of Lions shelf depend on the seasonal interplay between the Rhône river plume, the mesoscale eddies on the shelf and the Northern Current acting as either as a dynamic barrier between the shelf and the open sea or allowing cross-shelf exchanges. In the central gyre of the northwestern Mediterranean Sea, the patchiness of plankton is tightly linked to mesoscale frontal systems, surface eddies and filaments and deep gradients. Due to its flexibility in terms of its diet, C. t. succeeds in maintaining its population in both coastal and offshore areas year round. The simulations suggest that the winter–spring food conditions are more favorable on the shelf for C. t., whereas in late summer and fall, the offshore depth-integrated food biomasses represent a larger resource for C. t., particularly when mesoscale structures and vertical discontinuities increase food patchiness. The development and reproduction of C. t. depend on the prey field within the mesoscale structures that induce a contrasting spatial distribution of successive developmental stages on a given observation date. In late fall and winter, the results of the model suggest the existence of three refuge areas where the population maintains winter generations near the coast and within the Rhone River plume, or offshore within canyons within the shelf break, or in the frontal system related to the Northern Current. The simulated spatial and temporal distributions as well as the life cycle and physiological features of C. t. are discussed in light of recent reviews on the dynamics of C. t. in the northwestern Mediterranean Sea.

A LATTICE BOLTZMANN SIMULATION OF THE RHONE RIVER

We present a very detailed numerical simulation of the Rhone river in the Geneva area, using a Lattice Boltzmann (LB) modeling approach. The simulations of water ways are important to better predict and control their behavior when subject to exceptional event or new management strategies. Here, we investigate the current computing limits of using a three-dimensional (3D), free surface model to simulate a high resolution flow over a long section of the river, on a massively parallel computer. We argue that in a near future, computers will be powerful enough to tackle such a simulation. We also compare our results with a two-dimensional (2D) shallow water model to determine in which range a 3D free surface approach provides better insights. Finally, we discuss the advantage of a multi-scale approach for this type of problems.

Géoarchéologie du Rhône dans le secteur du pont Saint-Bénézet (Avignon, Provence, France) au cours de la seconde moitié du deuxième millénaire apr. J.-C. : étude croisée de géographie historique et des paléoenvir

Le pont d’Avignon, également appelé pont Saint-Bénézet, fait ici l’objet d’une approche géoarchéologique associant des données issues des études combinées de géographie historique et d’analyse des paléoenvironnements fluviaux rhodaniens. Le but de cet article est 1) de proposer une restitution du tracé du Rhône à l’époque moderne dans le secteur du fort Saint-André (situé à 1 km en amont du pont) et 2) de déterminer l’emplacement des anciennes arches du pont aujourd’hui enfouies sous les alluvions du Rhône ou immergées dans les bras de Villeneuve-lès-Avignon et d’Avignon. Les résultats de l’analyse des cartes anciennes, des relevés bathymétriques et de la caméra acoustique permettent de révéler pour la première fois la présence de blocs de maçonnerie à intervalles réguliers dans les bras de Villeneuve-lès-Avignon et d’Avignon. Une interprétation concernant la forme du pont Saint-Bénézet peut ainsi être proposée et offre une bonne correspondance avec l’iconographie et les cartes établies au cours des XVIe et XVIIe s. En complément, cinq carottages, d’une profondeur maximale de 7,6 m, ont été réalisés au pied du fort Saint-André, ont permis de restituer la mobilité spatiale du Rhône en amont du pont Saint-Bénézet. La réalisation de datations par le radiocarbone permet de livrer une séquence chronostratigraphique de référence pour le secteur. L’ensemble de ces données géoréférencées a par la suite été intégré dans un Système d’Information Géographique (SIG) afin de restituer l’évolution moderne (XVe s.-actuel) du Rhône et du pont. La corrélation puis la spatialisation des données de carottages avec celles de la tomographie de résistivité électrique (TRE) attestent la présence d’un bras majeur du Rhône (bras de Villeneuve) au pied du fort Saint-André dès le début du XVIe s., comme indiqué par l’iconographie et les cartes, et révèle aussi une migration rapide vers l’est dès le XVIIe s. avec un abandon du chenal daté du début du XVIIIe s.

Spatial heterogeneity of benthic methane dynamics in the subaquatic canyons of the Rhone River Delta (Lake Geneva)

Heterogeneous benthic methane (CH4) dynamics from river deltas with important organic matter accumulation have been recently reported in various aquatic and marine environments. The spatial heterogeneity of dissolved CH4 concentrations and associated production and diffusion rates were investigated in the Rhone River Delta of Lake Geneva (Switzerland/France) using sediment cores taken as part of the eLEMO Project. Benthic CH4 dynamics within the complex subaquatic canyon structure of the Rhone Delta were compared (1) between three canyons of different sedimentation regimes, (2) along a longitudinal transect of the ‘active’ canyon most influenced by the Rhone River, and (3) laterally across a canyon. Results indicated higher CH4 diffusion and production rates in the ‘active’ compared to the other canyons, explained by more allochthonous carbon deposition. Within the active canyon, the highest diffusion and production rates were found at intermediate sites further along the canyon. Stronger resuspension of sediments directly in front of the river inflow was likely the cause for the variable emission rates found there. Evidence also suggests more CH4 production occurs on the levees (shoulders) of canyons due to preferred sedimentation in those locations. Our results from the heterogeneous Rhone delta in Lake Geneva further support the concept that high sedimentary CH4 concentrations should be expected in depositional environments with high inputs of allochthonous organic carbon.

The shifting territorialities of the Rhone River’s transboundary governance: a historical analysis of the evolution of the functions, uses and spatiality of river basin governance

For the past decade, water policies have been strongly influenced by the concept of integrated water resource management (IWRM), and the river basin has been regarded as the most relevant scale for water governance. This article is based on the case of the Rhone River. Through historical analysis (from 1870 to the present), we study how the river’s functions evolve, how water users compete to secure their needs, and the effects on the governance structure and on its spatiality. While this governance structure has remained stable for decades, we show how the evolution of water policies (and the emergence of IWRM) and of environmental concerns strongly modified the strategies of actors. We also demonstrate how the governance structure as well as its space and scale of regulation tends to change with the attempt of central States to get back to the centre of the configuration of actors.

Ecotoxicological risk assessment linked to the discharge by hospitals of bio-accumulative pharmaceuticals into aquatic media: The case of mitotane

The release of hospital wastewater into the urban sewer networks contributes to the general contamination of aquatic media by pharmaceutical residues. These residues include bio-accumulative pharmaceuticals that lead to increased risk for ecosystems because they can concentrate in organisms and food chains, and therefore reach toxic levels. In order to assess the ecotoxicological risks linked to this particular category of residues, we have developed a specific method, by combining a theoretical calculation of pollutant concentrations in organisms to estimate Body Residue (BR), and ecotoxicity biomarkers in fish cell lines, enabling the calculation of a Critical Body Residue (CBR). This method finally results in the calculation of a specific risk quotient (Qb=BR/CBR), characterizing the risk linked to this type of pollutant. This method was applied to mitotane, a bio-accumulative pharmaceutical typically found in hospital wastewater, in the framework of an exposure scenario corresponding to the discharge of all the hospital wastewaters into the Rhone River which flows through the city of Lyon, France. This approach leads to risk quotients (Qb and Qbg) much higher than those found with the classical approach, i.e. Q=PEC/PNEC (Predictive Environmental Concentration/Predictive Non Effect Concentration)=0.0006. This difference in the appreciation of risk is important when using cytotoxicity as the criterion for measuring the toxicity of mitotane (Qb=0.056) and it is even greater when the criterion used is genotoxicity (Qbg=6.8). This study must be now consolidated by taking the biomagnification of the pharmaceuticals into consideration.

The evolution of the Rhone River in the Basses Terres basin during the Holocene (Alpine foothills, France)

This paper concerns the evolution during the Holocene of the floodplain of the Rhone River in the Basses Terres basin, 70km northeast of Lyon. Geomorphological mapping of a number of paleochannels has been carried out, using topographic maps, aerial photos, and landform surveys, and mechanical cores have been taken that enable the reconstruction of several stratigraphies in cross section. The chronology of fluvial deposits relies mainly on the radiocarbon dating of organic materials collected from the abandoned fluvial fills. Paleochannel infillings were also used to record fluvial style changes and floodplain aggradation rates. The main results show that the region has a mountainous piedmont plain controlled by local factors (inherited from glacial times) that have led to the atypical predominance of a meandering river pattern controlling the evolution of the floodplain in the Basses Terres area during the Holocene. A major avulsion of the Rhone, when it switched to its present-day northern course around 2700–2600cal. BP, strongly influenced the evolution of the Holocene floodplain, causing a sharp decrease in the aggradation of the abandoned section. In addition, four major occurrences of strong and increased sediment discharge took place around 10,000–7500, 3000–2700, 2400/1500–1000, and 230–50cal. BP. Two of these (ca. 3000–2700 and 230–50cal. BP) are correlated to a braided pattern and represent temporary fluvial metamorphoses of the Rhone. The infilling rates of several paleochannels also reveal periods of strong sediment deposition around 5500–5250, 3500–3200, 1400–1200, and 600–400cal. BP, consistent with previous regional paleohydrological phases as well as new ones. These phases are synchronous with the hydrosedimentary variations identified on a regional scale in the Rhone catchment area and the northern Alps.

Distribution of organochlorine compounds in superficial sediments from the Gulf of Lion, northwestern Mediterranean Sea

Superficial sediments from Cap de Creus to the Rhone Delta, in the Gulf of Lion, Northwestern Mediterranean Sea, including the mid-shelf mud belt and the continental slope were collected between 2005 and 2008 to assess the levels, main sources and distribution patterns of organochlorine pollutants. Discharges from the Rhone River are the main source for all these compounds around the area. The spatial distribution of organochlorine pollutants was also related to their physicochemical properties and to sediment grain size and composition. The concentrations of polychlorobiphenyls (PCBs), dichlorodiphenyltrichloroethane (DDT) and its metabolites (DDD and DDE), and the chlorobenzenes (CBzs) – pentachlorobenzene (PeCB) and hexachlorobenzene (HCB) – decreased westwards along the mid-shelf mud belt. In contrast, hexachlorocyclohexane isomers (HCHs), namely lindane (γ-HCH), followed another concentration pattern suggesting a different transport mode. The major concentrations of organochlorine compounds were observed off the Rhone River mouth, in the prodelta, where PCB, DDT and CBz concentrations reached 38, 29 and 8.3ngg−1, respectively. These average concentrations in the mid continental shelf were two to ten times lower than those found in a study performed about 20years ago, albeit in almost all the sites the values of PCBs and DDTs still exceed the NOAA’s Sediment Quality Guidelines. In contrast, the concentrations in the continental slope were nearly the same as 20 years ago, which may evidence that even most of these compounds were banned decades ago, their background concentrations associated to diffuse pollution have not decreased in the deep continental margin.

Miniaturised enzymatic conductometric biosensor with Nafion membrane for the direct determination of formaldehyde in water samples

A new conductometric enzyme-based biosensor was developed for the determination of formaldehyde (FA) in aqueous solutions. The biosensor was prepared by cross-linking formaldehyde dehydrogenase from Pseudomonas putida with bovine serum albumin in saturated glutaraldehyde vapours (GA) at the surface of interdigitated gold microelectrodes. Nicotinamide adenine dinucleotide cofactor (NAD(+)) was added in solution at each measurement to maintain enzyme activity. Addition of a Nafion layer over the enzyme modified electrode resulted in a significant increase of biosensor signal due to enhanced accumulation of protons generated by enzymatic reaction at the electrode surface. Different parameters affecting enzyme activity or playing a role in ionic transfer through the Nafion membrane were optimised. In optimal conditions (0.045 mg enzyme, 30 min exposure to GA, 0.3 μL of a 1% (v/v) Nafion solution deposit, measurement in 5 mM phosphate buffer pH 7 containing 20 μM NAD(+)), the biosensor signal was linear up to 10 mM FA, and the detection limit was 18 μM. Relative standard deviations calculated from five consecutive replicates of FA solutions were lower than 5% in the 1-10 mM range. The biosensor was successfully applied to the determination of FA in spiked water samples (tap water and Rhone river water), with recoveries in the 95-110% range.

The contribution of lateral aquatic habitats to insect diversity along river corridors in the Alps

River floodplains are among the most threatened ecosystems globally. Understanding the mechanisms that create and maintain biodiversity and ecosystem functioning of floodplains, is therefore a prerequisite for developing scientifically-sound management and conservation strategies. We quantified the spatial distribution of lateral aquatic habitats (i.e. tributaries, ponds, backwaters) and the associated insect assemblages (Ephemeroptera, Plecoptera, Trichoptera) along three Alpine river corridors (Tagliamento, Thur, and Rhone). Our objective was to assess the relative contribution of lateral habitats to river corridor diversity, and to identify the scale that contributed the most to regional (Alpine scale) species diversity. The number of lateral habitats decreased by 72 % from the near-natural Tagliamento (101) to the Thur river (42) and the regulated Rhone river (28). More than 50 % of the total species richness along each river was restricted to lateral habitats, which also exhibited higher taxon turnover rates (Whittaker’s beta diversity) than the associated main channel. Hierarchical diversity partitioning revealed that beta diversity among corridors was higher than expected, and accounted for 48 % of regional richness, partly reflecting biogeographical differences among catchments. However, diversity partitioning, excluding catchment-specific effects, showed that beta diversity among habitat types contributed significantly to regional richness (79 %). The present study is among the first to quantify the distribution and biodiversity of floodplain habitats along entire river continua. Our results demonstrated that biodiversity would be best preserved by protecting multiple catchments, and that lateral floodplain habitats contribute disproportionally to species richness at the river corridor and regional scale.

Long-term Changes and Drying Modality Affect Interstitial Assemblages of Alluvial Wetlands

Long-term changes (river incision, climate changes and groundwater overuse) induce increasing groundwater depletion and drought frequency in floodplains. To evaluate the effect of drought on functioning of wetlands and their interstitial zones, we studied the interstitial communities at seven stations located in five former channels of the Rhone River. Change in interstitial assemblages was assessed over a 32-year period in one wetland and over the entire floodplain between 1987 and 2010, at depths of 50 and 100 cm within the sediment. Over the 32-year period, we highlighted some links between environmental changes and variations in interstitial assemblages: a decrease in abundance and diversity of the hypogean fauna and a reverse pattern for Ephemeroptera and Cladocera. At the floodplain scale, several changes in habitat characteristics were observed and two drying modalities were identified: the formation of isolated pools and the complete disappearance of surface water. In isolated pools, the low turnover of surface water promoted a decrease in water quality, an increase in the abundance of benthic invertebrates in the interstitial habitat and a decrease in the abundance of hypogean invertebrates. In completely dried-out sites, the abundance and diversity of benthic taxa decreased, while the hypogean fauna remained dominant. These results demonstrate that interstitial assemblages are affected by long-term changes (river incision and climate changes). The local geomorphology and the resulting drying modalities control the responses of interstitial assemblages.

Ultrasonic phased array contribution to the knowledge of the flow fusion welding process used to make the Roman large bronze statues

Recent archeometallurgical studies have shown that Greek and Roman large bronze statues were not cast in one piece, in spite of an exceptional mastering of the lost-wax techniques, but resulted from the welding of several parts previously cast. The ancient welding process proved to be very spectacular: liquid bronze (secondary casting) of the same composition as the base metal (primary casting) was directly poured between the two parts to be joined. We have recently shown that the mechanism involved is similar to the one involved in some modern fusion welding techniques such as arc, laser beam or oxy-fuel, which demonstrates the very high level of knowledge achieved by Greek and Roman foundrymen in the control of high-temperature processes. Twenty-one centuries later, high level of non-destructive testing (NDT) expertise is particularly being needed for archeometallurgical examinations of the statues, not only to describe the welding mechanism, but also to identify several variants of the process (particularly concerning the welding preparation). This paper explains how NDT, and particularly ultrasonic phased array, have brought decisive clues to understand the flow fusion welding process applied to the “Captive Gallic”, a statue found in 2007 during excavation in the Rhone River in Arles (Southern France).

Radionuclide Contents in Suspended Sediments in Relation to Flood Types in the Lower Rhone River

The aim of this work was to study the influence of watershed heterogeneity on the radionuclide contents of suspended particulate matter (SPM) in a large Mediterranean river. As the Rhone River catchment is characterized by a high climatic and geological heterogeneity, floods can be distinguished according to their geographic origins. Long-term time series of particles associated with radionuclides acquired in the framework of radiological surveillance provide a relevant dataset to investigate the variability of radionuclide contents. The SPM exported during Mediterranean floods differ from other floods as they display higher 238U and 232Th contents and the lowest activity ratio 137Cs/(239+240)Pu; these properties could be related to bedrock type and erosion process characteristics and/or to source term differentiation.

Searching the Rhone delta channel in Lake Geneva since François Alphonse FOREL

In the late 19th century, F.-A. Forel led investigations of the Rhone River delta area of Lake Geneva that resulted in the discovery of a textbook example of a river-fed delta system containing impressive subaquatic channels. Well ahead of the marine counterparts, scientific observations and interpretations of water currents shaping the delta edifice for the first time documented how underflow currents carry cold, suspension-laden waters from the river mouth all the way to the deep basin. These early investigations of the Rhone delta laid the basis for follow-up studies in the 20th and 21th centuries. Sediment coring, water-column measurements, manned submersible diving, seismic reflection profiling and bathymetric surveying eventually provided a rich database to unravel the key erosional and depositional processes, further documenting the impact of human-induced changes in the catchment. With the merging of old and new scientific knowledge, today a comprehensive understanding prevails of how a delta changes through time, how its channels are formed, and what potential natural hazards may be related to its evolution. New and efficient bathymetric techniques, paired with novel coring operations, provided a time-series of morphologic evolution showing and quantifying the high dynamics of the delta/channel evolution in an unprecedented temporal and spatial resolution. Future investigations will continue to further quantify these dynamic processes and to link the evolution of the subaquatic domain with changes and processes in the catchment and with natural hazards. Its size, easy access, and large variety of states and processes will continue to make the Rhone delta area a perfect ‘laboratory’ in which general processes can be studied that could be upscaled or downscaled to other marine and lacustrine deltas

Mixing of Rhône River water in Lake Geneva (Switzerland–France) inferred from stable hydrogen and oxygen isotope profiles

Summary Depth profiles were sampled at different locations throughout Lake Geneva on a monthly and seasonal basis over the course of 2 years and analysed for their stable hydrogen and oxygen isotope compositions. The isotopic compositions indicate an isotopic stratification in the metalimnion during summer and fall. This is related to mixing of Rhone River water, which in summer is dominated by snow and glacier melt waters, and lake water, with the latter having a homogenous isotopic composition. The observed interflow layer is 7–15 m thick and can be traced by the distinct stable isotope composition of the water for about 55 km throughout the lake as well as into shallow bay regions. Depth of the interflow layer close to the Rhone River mouth is similar to those previously described based on echo-soundings and turbidity profiles of sediment dispersion. In contrast to previous descriptions of the interflow within Lake Geneva, the stable isotope compositions allow for direct, natural tracing of the Rhone River water even in cases where the turbidity and conductivity measurements do not indicate such an interflow. In addition, the method allows for a quantification of the Rhone River and lake water in the interflow with the fraction of Rhone River water within the interflow estimated to be up to 37% in summer. The isotopic composition further indicates different vertical mixing processes within the two lake basins of Lake Geneva, related to the density gradients and local stability within the water column. The method may be applicable to other lakes in catchments with large differences in the topography as water that originates from high altitudes or glaciers has a distinct oxygen and hydrogen isotope composition compared to other sources of water originating at lower altitudes and/or from direct precipitation over the lake. Stable isotope measurements thus improve the understanding of the circulation of water within the lake, which is fundamental for an evaluation of the water residence times, dissolved pollutant and nutrient transport as well as oxygenation.