A decade of multibeam bathymetric mapping: Implications and lessons learned from (peri-)Alpine lakes in Switzerland

Studying the morphology of the landscape is crucial for understanding the processes that shape the Earths surface. In subaqueous environments, where direct observations are challenging, bathymetry-derived terrain models are the fundamental form of geomorphological data. Over a century ago, Switzerland initiated systematic bathymetric surveys under the federal "Siegfried Map" project, marking an effort to explore the subaquatic morphology of perialpine lakes. These early studies laid the groundwork for subsequent research on the subaqueous landscape in Switzerland. The early bathymetric surveys in Switzerland focused on documenting the general shape of deep basins and discovering features on lake floors such as channel levee complexes and sublacustrine moraine ridges. These observations formed the basis for early theories on the age of the last glaciation and the existence of turbidity currents in lakes. Recent advances in multibeam swath bathymetry systems combined with differential GNSS location services dramatically improved survey efficiency as well as spatial and vertical resolution by several magnitudes, generating new findings with every surveyed lake. Apart from the reconnaissance of the overall basin shape, the detailed geomorphologic mapping led to the discovery of various subaquatic features, such as landslides and rockfalls, glacial features, pockmarks, channel and canyon systems, fault structures, and prehistoric and historic human impact. These findings had significant implications for evaluating natural hazards caused by earthquakes, floods, and tsunamis. Detailed glacial imprints became suddenly visible in high-alpine proglacial lakes, revealing the recessional behavior of glaciers. Mapping the source area of mass movements on the lakes slopes represents the base for understanding lacustrine tsunamis and their modeling. Subaquatic canyons of deltaic systems often extend further into deeper waters than anticipated, promoting accelerated transport of coarse-grained sediments into the deepest parts, typical target areas for major drilling campaigns. Submerged traces of prehistoric settlements revealed unexpected chapters of human activities. Therefore, no drilling effort should be planned without a detailed lake floor map. In this contribution, we will outline lessons learned from these surveys of 22 lake systems across Switzerland since 2007, summarize key findings, and review the implications of the technology on the limnogeological community. We will also glimpse the future and explore what to expect from ongoing 4D-bathymetric mapping campaigns.