Morphology and recent history of the Rhone River Delta in Lake Geneva (Switzerland)

Abstract

The current topographic maps of the Rhone Delta-and of Lake Geneva in general-are mainly based on hydrographic data that were acquired during the time of F.-A. Forel at the end of the nineteenth century. In this paper we present results of a new bathymetric survey, based on single-and multi-beam echosounder data. The new data, presented as a digital terrain model, show a wellstructured lake bottom morphology, reflecting depositional and erosional processes that shape the lake floor. As a major geomorphologic element, the sub-aquatic Rhone Delta extends from the coastal platform to the depositional fans of the central plain of the lake at 310 m depth. 9 canyons cut the platform edge of the delta. These are sinuous (''meandering'') channels formed by erosional and depositional processes, as indicated by the steep erosional canyon walls and the depositional levees on the canyon shoulders. Ripples or dune-like morphologies wrinkle the canyon bottoms and some slope areas. Subaquatic mass movements are apparently missing on the delta and are of minor importance on the lateral lake slopes. Morphologies of the underlying bedrock and small local river deltas are located along the lateral slopes of Lake Geneva. Based on historical maps, the recent history of the Rhone River connection to the sub-aquatic delta and the canyons is reconstructed. The transition from three to two river branches dates to 1830-1840, when the river branch to the Le Bouveret lake bay was cut. The transition from two to one river branch corresponds to the achievement of the correction and dam construction work on the modern Rhone River channel between 1870 and 1880.

The role of mass-transport deposits and turbidites in shaping modern lacustrine deepwater channels

“…Both canyons have smooth morphologies and their sinuosity index (SI; calculated as the ratio of the distance along the channel axis to the straight-line valley length) is~1, classifying them as "straight" channels according to Clark et al (1992). The canyons C3 (5.87 km-long, SI-1.08) and C4 (5.42 km-long, SI-1.14) show complex canyon heads and share a common distal canyon at~140 m water depth (Sastre et al, 2010). The westernmost canyons are longer, more incised and display steeper slopes.…”

“…High-resolution time-lapse bathymetric datasets in modern subaquatic channels in lacustrine basins provide an excellent tool to investigate these processes. In particular, the availability of highresolution bathymetric datasets Kremer et al, 2015b;Sastre et al, 2010) and the well-known boundary conditions of the Rhone subaquatic channels in Lake Geneva (Switzerland/France) make this site a prime research location to investigate the depositional patterns of MTD and turbidites operating in deep-water channels.…”

“…In recent years, multidisciplinary research efforts in the Rhone Delta in Lake Geneva have been undertaken to explore the active Rhone subaquatic canyons Girardclos et al, 2012;Kremer et al, 2015aKremer et al, , 2015bLambert and Giovanoli, 1988;Loizeau, 1991;Loizeau et al, 1997;Sastre et al, 2010;Stark et al, 2013). Overall we synthetize the resulting time-lapse bathymetric dataset incorporating new sedimentological and geotechnical analyses using innovative sampling techniques via manned submersibles and research vessels.…”

Characterization of fecal indicator bacteria in sediments cores from the largest freshwater lake of Western Europe (Lake Geneva, Switzerland)

“…150-100 mg of ground and homogenized sample was subject to a pyrolysis step. A flame ionization detector Sastre et al, 2010) showing the location of the two sediment cores studied: core V4, located at $ 500 m from the current WWTP outlet, and core V7 located in between the present WWTP outlet and the former one used from 1964 to 2001.…”

Into the abyss of Lake Geneva: the elemo interdisciplinary field investigation using the MIR submersibles

“…With its contiguous twobasin structure and the substantial exposure to wind arising from the channelling effect of the parallel mountain ranges, the water body experiences basin-scale gyres, strong internal seiches, and especially, large-amplitude Kelvin waves (Bouffard and Lemmin 2013b). Other special features are the spectacular surrounding mountains and an underwater topography that in certain areas is enriched by the underwater canyons (Sastre et al 2010), shaped by erosion and deposition dynamics of the particle-rich discharge of the Rhône River (Girardclos et al 2012), which is strongly affected by the upstream hydropower operations (Loizeau and Dominik 2000).…”