Bathymetry changes due to delayed basal ice melt at the proglacial lake Obersulzbachsee, Hohe Tauern, Austria – Implications for sediment budgeting

Recent glacier lake formation in mountain areas is a consequence of temperature increase and subsequent glacier melt. These new lakes affect the sediment cascade by collecting great parts of the sediment input by the meltwater streams. A quantification of these trapped sediments can be achieved by assessing changes in the lake bottom surface at different periods in time. Bathymetry changes can be affected by the delayed melting of buried glacier ice, preserved at the lake floor, which lead to an overestimation of sediment volumes.

We analysed bathymetry changes within the proglacial lake Obersulzbach, Hohe Tauern range, Austria over a period of 13 years. Lake floor and delta sediments were investigated using high resolution, multi-temporal geophysical data derived from sub bottom profiling (SBB), echo sounding and ground penetrating radar (GPR). We compared three instances of bathymetry data that document changes of the lake floor attributed to ice melt and sedimentation. SBB and GPR data were applied to detect buried ice underneath the sediments in order to assess the sediment and the ice volume in the lake and delta. 

The proglacial lake Obersulzbach formed in 1998 when the tongue of the Obersulzbach glacier in the Hohe Tauern Range, Austrian Alps, retreated behind a bedrock barrier. The glacier lake evolved in a former confluence zone of four glacier parts that originate in the valley head of the Obersulzbach valley. The lake has a maximum depth of 40 meters and a size of 170,000 m². The glacier ice retreated from the lake area in 2010 to a distance of more than 500 meters from the lake in 2021. Since 2009, a delta started to build up at the distal part of the lake fed by two meltwater streams. Parts of the delta started to sink below lake level in 2019, forming localised depressions. This process continued in 2020 and 2021 when large parts of the delta sunk into the lake increasing the lake area by 30%. In the delta area, the surface sunk by up to 20 m within 2 years. We attribute these changes to a delayed, but rapid melting of buried glacier ice at the lake floor and within the delta more than 10 years after the retreat of the glacier tongue.