Permafrost is widespread in the Swiss Alps and hidden in talus slopes and bedrock above the tree line. Frozen ground in mountain slopes is highly variable in its spatial distribution, can contain high amounts of ice and can creep downslope. Research on mountain permafrost has a short history and studies on hydrology interaction are still scarce. Permafrost ice may have different origins and can be several thousand years old. Its total volume in the Alps is roughly estimated to be one quarter of current glacier ice. Permafrost modifies mountain hydrology due to its low storage capacity and high impermeability. Ice melt contributes to stream runoff in small amounts. Permafrost observations in the Swiss Alps show generally increasing ground temperatures and active layer thickness, decreasing ice contents and rising rock glacier creep velocities. Rock fall activity from the active layer seems to be increasing during heat waves. Large rock avalanches are rare and occur year-round, with permafrost acting as a long-term catalyser.

By the end of the 21st century mountain permafrost in the Swiss Alps may retreat to 10 m depth at ice-rich sites and disappear completely below 3500 m asl. in steep rock slopes. In ice- rich ground, permafrost degradation is a slow process and subsurface temperature fields are projected to deviate strongly from steady state conditions in the future. Active layer thickening and permafrost degradation promote mass movements and hazardous process chains reaching low elevations are possible. Concerning hydrology, degradation of mountain permafrost can increase runoff during thaw and when permafrost has disappeared, it can de- crease flood peaks. Further, permafrost degradation can have adverse effects on water che- mistry downstream. Long-term measurements and complementing observations are funda- mental to better understand permafrost processes in high mountains, improve models and enhance awareness of the impacts of permafrost degradation. Intensified investigation of the processes linking permafrost degradation and slope instability is required. Knowledge gaps exist regarding the age, distribution and volume of permafrost ice stored in European moun- tains, its potential impact on future water resources and hydrological systems.