Subaqueous Morphology of Lake Lucerne (Central Switzerland): Implications for Mass Movements and Glacial History
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Eawag_06696 Swiss J Geosci (2011) 104:425–443 DOI 10.1007/s00015-011-0083-z Subaqueous morphology of Lake Lucerne (Central Switzerland): implications for mass movements and glacial history Michael Hilbe • Flavio S. Anselmetti • Raymond S. Eilertsen • Louise Hansen • Walter Wildi Received: 1 October 2010 / Accepted: 3 August 2011 / Published online: 25 November 2011 Ó Swiss Geological Society 2011 Abstract Bathymetric data available for Swiss lakes have document mass-movement activity on steep slopes above typically only low to moderate resolution and variable the lake. Six transverse moraines, visible as subaqueous quality, making them insufficient for detailed underwater ridges, as lake-floor lineaments, or only imaged on reflec- geomorphological studies. This article presents results of a tion seismic profiles, indicate a complex glacial-inherited new bathymetric survey in perialpine Lake Lucerne using morphology. As many of the documented features result modern hydrographic equipment. A digital terrain model from potentially catastrophic events, high-resolution (DTM) of the lake floor (raster dataset with 1 m cell size) bathymetry can significantly improve natural hazard covering the Chru¨ztrichter and Vitznau basins documents assessment for lakeshore communities by extending clas- signatures of major Holocene mass movements and relics sical hazard maps to the subaqueous domain. from the glacial history of the lake. Combining the bathymetry data with reflection seismic profiles and an Keywords Swath bathymetry Á Perialpine lakes Á existing event chronology allows investigating the mor- Natural hazards Á Subaqueous moraines Á phology in its geological context. Subaqueous sediment Soft-sediment deformation slide scars with sharp headwalls cover large areas on moderately inclined slopes. The particularly large Weggis slide complex, correlated with an historical earthquake (AD 1 Introduction 1601), features a *9 km long and 4–7 m high headwall and covers an area of several square kilometers. Large Knowledge of the landscape morphology on different debris cones of prehistoric rockfalls and the deposits of scales is essential in order to track and understand the recent rockfall events imaged on the almost flat basin plain various processes shaping the Earth’s surface. For the subaqueous environment, where direct observation is difficult, terrain models derived from water-depth Editorial handling: D. Ariztegui and A.G. Milnes. measurements (bathymetry) are the basic form of mor- phological data. In Switzerland, early studies on the & Á M. Hilbe ( ) F. S. Anselmetti submerged morphology of the perialpine lakes have been Department of Surface Waters, Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Du¨bendorf, Switzerland conducted more than 100 years ago, with the first sys- e-mail: [email protected] tematic bathymetric surveys in the framework of the federal ‘‘Siegfried Map’’. They documented the general R. S. Eilertsen shape of these deep, often fjord-like basins and also led to Geological Survey of Norway (NGU), 9296 Tromsø, Norway the discovery of features on the lake floor such as channel- L. Hansen levee complexes on delta slopes in Lake Constance and Geological Survey of Norway (NGU), 7491 Trondheim, Norway Lake Geneva (von Salis 1884; Forel 1892) or sublacustrine moraine ridges in Lake Lucerne (Ho¨rnlimann 1886). Early W. Wildi Institut F.-A. Forel, Universite´ de Gene`ve, 1290 Versoix, theories on the age of the last glaciation (Heim 1894) or the Switzerland existence of turbidity currents originating from river 426 M. Hilbe et al. mouths (Forel 1892) were essentially based on such during a pilot project carried out in cooperation with various observations. Subsequent process-oriented investigations Swiss federal agencies, exploring the usability of high- have relied on the topographic information provided by the resolution bathymetry data for scientific and applied pur- bathymetry data, for instance the direct measurement of the poses in Swiss lakes. Two surveys covered parts of Lake proposed turbidity currents in a submerged channel on the Lucerne (September–October 2007) and Lake Geneva Rhone delta slope in Lake Geneva (Lambert and Giovanoli (January–February 2008). The survey on Lake Geneva is 1988). The first bathymetric surveys used line soundings outlined in Sastre et al. (2010). The article presented here taken along profiles, with a density of only a few dozen summarizes results of the first survey on Lake Lucerne soundings per km2 (Ho¨rnlimann 1886). In later years the (Fig. 1), a perialpine lake with variable basin morphologies, data density was increased to several hundred soundings reflecting tectonic predisposition, glacial history and pres- per km2, most notably through the use of single-beam ent-day lake floor processes. Previous investigations of its acoustic echosounders in the second half of the twentieth sedimentary record as well as historical accounts document century (Bertschmann 1953; Lambert 1988). Nevertheless, repeated rockfalls and subaqueous mass movements within bathymetric surveys remained labor-intensive and the and around the lake during the Holocene (Siegenthaler et al. spatial resolution of the data limited, allowing the sys- 1987; Siegenthaler and Sturm 1991; Schnellmann et al. tematic exploration of only the most prominent 2006). While the signatures of these events have been morphological features. To date, no programme for sys- described in sediment cores and in reflection seismic pro- tematic hydrographic surveys of Swiss lakes exists, and files, their morphological expression on the lake floor was available bathymetric data are of variable age and quality, only poorly known. The new bathymetry data presented providing at most moderate, often even unknown, data here allow precise mapping and characterization of lake densities and resolutions. floor features related to mass movements in two basins of In the past 30 years, new kinds of hydrographic survey the lake (Chru¨ztrichter and Vitznau basin). In addition, they equipment capable of substantially higher data density, document the morphology of three large moraine ridges resolution and accuracy than the traditional sounding separating the central basins of the lake (Chru¨ztrichter, methods have become available for marine science (e.g. Vitznau, Gersau and Treib basins). The combination of the Allenou and Renard 1978). The bathymetry data obtained bathymetry data with newly acquired reflection seismic with these hydroacoustic instruments (‘‘swath bathymetry profiles across the Vitznau basin reveals previously systems’’ or ‘‘multibeam echosounders’’ sensu latu) allow unknown features in the deeper subsurface and their relation underwater geomorphological studies with a significantly to the present lake floor morphology, providing new evi- higher level of detail and the identification of numerous dence on the glacial history of the basins. smaller-scale features. With respect to traditional methods, current standard swath bathymetry instruments improve the lateral resolution by more than one order of magnitude, 2 Setting and previous studies with similar or better vertical accuracy. Such high-resolu- tion surveys have become a standard in seafloor research Lake Lucerne is a perialpine, fjord-type lake of glacial and their widespread use in combination with other meth- origin located in Central Switzerland (Fig. 1). Its sur- ods—including imaging of the sediment subsurface and roundings are situated within the extents of Alpine glaciers sampling—has contributed to an improved understanding at the Last Glacial Maximum, with ice surface elevations of seafloor processes in many areas, ranging from volcanic reaching 600–1,200 m above the present lake surface (Bini and hydrothermal activity to glacial landforms, delta sed- et al. 2009). Today, seven main basins with depths ranging imentation and mass movements (e.g. Fornari et al. 1988; from 35 to 214 m cover a total surface area of 114 km2 McAdoo et al. 2000; Laberg et al. 2007; Ottesen et al. (BAFU 2008). Two of the subaqueous sills separating the 2008; L’Heureux et al. 2009; among many others). While basins are of deltaic origin (present Muota delta near these tools have been developed and become standard in Brunnen; paleodelta near Stansstad). The remaining four the marine environment, their application in lacustrine sills have been interpreted as moraine ridges based on environments, using portable systems, slowly started only morphological criteria (e.g. Penck and Bru¨ckner 1909). during the past few years, with first case studies in the Four proximal basins (Uri, Treib, Gersau, Alpnach) are United States (Lake Tahoe, Crater Lake; Gardner et al. entirely located within the Helvetic Alpine nappes and in 2000; Bacon et al. 2002) and recently also in perialpine many places surrounded by steep lateral slopes, with lakes in Italy and France (Bini et al. 2007; Fanetti et al. neighbouring peaks reaching elevations of more than 2008; Ledoux et al. 2010). 1,500 m above lake level. These basins are fed by four The operation of such state-of-the-art hydrographic major inflows that account for about 80% of the lake’s equipment was tested for the first time in Switzerland water supply (BAFU 2008), resulting in dominantly clastic Subaqueous morphology of Lake Lucerne (Central Switzerland) 427 Fig. 1 Lake Lucerne at the northern margin of the Alps in Central numbered I to IV. Areas with available high-resolution bathymetry Switzerland, with main rivers, neighbouring lakes and tectonic units. data