Reconstructing subaqueous mass movements in Lake Grundlsee (Styria, Austria), a key record for understanding geohazards in a complex Alpine setting

Lake Grundlsee, located in the Northern Calcareous Alps, is one of three large, deep, elongate lakes in the southern Salzkammergut region in Austria. The lake is situated in a geologically complex setting characterized by widespread evaporites of the Haselgebirge Formation to its south, which are known to promote a wide range of mass movement processes in the region. Recent research from the adjacent Bad Aussee Basin, which hosts an at least 880-m-thick Pleistocene sedimentary succession, indicates that Quaternary salt-tectonic activity is an important but still underexplored influencing factor on regional landscape evolution. Understanding these dynamics requires long-term records that extend beyond historical documentation. Bathymetric data and lacustrine sediment archives from neighboring Lake Hallstatt and Lake Altaussee have demonstrated the potential of such records for reconstructing Alpine landscape evolution and mass movement activities. However, neither subsurface geology nor in-depth sedimentological studies were performed on the adjacent Lake Grundlsee so far.

In this study, we present a comprehensive geologic and hydrographic survey of Lake Grundlsee. It consists of a high-resolution bathymetric survey of the lake basin and a seismic reflection profile survey supplemented by sediment core extraction from the lake bottom.  

The bathymetric data was collected in July 2025 using a Teledyne T50 multibeam echosounder (MBES), operated at 400 kHz with lakebed backscatter registration for lake floor classification purposes. The resulting 3D bathymetric model of Lake Grundlsee shows a maximum water depth of approximately 63 m and a complex lake floor morphology.

The bathymetric data reveals numerous previously unknown subaqueous landslides distributed throughout the basin, with particular concentrations in the eastern sub-basin and along the northern slope. Multiple generations of mass movements are evident, including older, partially overprinted deposits. The largest landslides show runout lengths exceeding 600 m and scar widths of over 250 m.

Reflection seismic profiles of a total length of ~40 km were obtained in November 2025 using an Innomar parametric sub-bottom profiler operating at 12 kHz. The narrow low-frequency sound beam images the subsurface geometry of these deposits with high resolution and with penetration depths of up to 10m providing insight into deposit thickness, failure planes, and stratigraphic context. Associated turbidite deposits are traceable across the basin floor.

In order to investigate the lithological characteristics of the uppermost sedimentary sequences a total of 18 sediment cores were retrieved in November 2025 using a gravity corer with hammer system.

Hydrogeological and geomorphological mapping of the surrounding catchment, documenting the distribution of evaporites and mass movement deposits around Lake Grundlsee, contextualizes the subaqueous findings and helps identify potential source areas and predisposing geological conditions.

The causes of slope failure remain under investigation. Possible triggering mechanisms include spontaneous gravitational instability on steep subaqueous slopes, seismic shaking — as suggested by studies in neighboring lakes such as Lake Altaussee — and salt-related subsurface processes associated with the underlying Haselgebirge Formation as well as human-induced failures.

This study provides a baseline for future hazard assessment and demonstrates the value of integrating subaqueous and terrestrial geomorphological approaches in Alpine lake research.