Water flow in steep high-mountain rock walls plays an important role in landscape evolution and slope stability, yet its timing, magnitude, and sources remain poorly constrained. This is especially true in permafrost-affected terrain, where ice within fractured bedrock can strongly modify subsurface flow paths. At the Aiguille du Midi (3842 m a.s.l., Mont Blanc massif), underground tunnels provide rare direct access to near-vertical rock walls, enabling in situ hydrological observations.
Between May 2022 and October 2023, water discharging from fractures was monitored using a real-time system measuring flow rate, temperature, electrical conductivity, and tracer fluorescence, together with meteorological data and ground surface temperatures. Results show strong surface–subsurface connectivity, with snowmelt as the dominant water source and additional contributions from late-summer rainfall. Electrical conductivity, stable isotopes, and recession analyses further indicate the contribution of older subsurface ice.
Flow onset closely followed atmospheric warming, with clear diurnal cycles emerging once rock surface temperatures exceeded 0 °C. Lag times of hours between temperature and flow peaks indicate rapid, largely unsaturated infiltration through fractured rock. Adjacent fracture systems exhibited contrasting flow regimes, highlighting strong small-scale heterogeneity, including delayed responses in sediment-filled fractures. Measured flow rates frequently exceeded 10 L h⁻¹, and water temperatures often surpassed 5 °C, implying substantial advective heat transfer capable of accelerating permafrost degradation.
These observations provide new constraints on hydrological processes in steep permafrost rock walls and have implications for hydro-thermal modeling, climate sensitivity of mountain water systems, and permafrost-related natural hazards.