Glacier-permafrost relations in a high-mountain environment: 5 decades of kinematic monitoring at the Gruben site, Swiss Alps

Abstract. Digitized aerial images were used to monitor the evolution of perennially frozen debris and polythermal glacier ice at the intensely investigated Gruben site in the Swiss Alps over a period of about 50 years. The photogrammetric analysis allowed for a compilation of detailed spatio-temporal information on flow velocities and thickness changes. In addition, high-resolution GNSS (Global Navigation Satellite System) and ground-surface temperature measurements were included in the analysis to provide insight into short-term changes. Over time, extremely contrasting developments and landform responses are documented. Viscous flow within the warming and already near-temperate rockglacier permafrost continued at a constant average but seasonally variable speed of typically decimetres per year, with low average surface lowering of centimeters to decimetres per year. This quite constant flow causes the continued advance of the characteristic convex, lava stream-like rockglacier with its over-steepened fronts. Thawing rates of ice-rich perennially frozen ground to strong climate forcing are obviously very low (centimetres per year) and the dynamic response strongly delayed (time scale decades to centuries). The adjacent cold debris-covered glacier tongue remained an essentially concave landform with diffuse margins, predominantly chaotic surface structure, intermediate thickness losses (decimetre per year) and clear signs of down-wasting and decreasing flow velocity. The former contact zone between the cold glacier margin and the upper part of the rockglacier with remains of buried glacier ice embedded on top of frozen debris exhibits complex phenomena of thermokarst in massive ice and backflow towards the topographic depression produced by the retreating glacier tongue. As is typical for glaciers in the Alps, the clean glacier part shows a rapid response (time scale years) to strong climatic forcing with spectacular retreat (> 10 meters per year) and mass loss (up to > 1 meter water equivalent specific mass loss per year). The system of periglacial lakes shows a correspondingly dynamic evolution and had to be controlled by engineering work for hazard protection.