Exploring short‐term rockfall inventories in deglaciating catchments: From evidencing glacial history to modelling rockfall runout

Rockfalls are an efficient agent of landscape denudation and a crucial but poorly quantified component of the glacier debris supply cascade. Climate change is driving increased rockfall generation as rising air temperatures cause glacier thinning and thawing of permafrost. These processes alter rock slope stress profiles and thermal regimes, leading to greater sediment fluxes in cryospheric systems as landscapes adjust to ice-free conditions. We used repeat terrestrial laser scans combined with change detection during the summer of 2019 to quantify rockfall activity over a 0.7 km ² rock wall area along the ablation zone lateral margins of the debris-covered Miage Glacier, Italy. We detected 2,581 rockfalls spanning eight orders of magnitude (10 ⁻³–10 ⁴ m ³; median 0.021 m ³) including an event of about 28 × 10 ³ m ³ from a newly deglaciated slope. Large rockfalls (≥10 m ³) on lower, glacier-proximal slopes, whilst infrequent (<1% by count), achieved the most geomorphic work. Most (79%) rockfalls originated within <75 m above the glacier surface (mAG; representing 29% of the survey area); a boundary that corresponds with the Little Ice Age trimline. Some rockwalls exhibited a secondary zone of higher rockfall activity at about 125–150 mAG, revealing a second trimline with a millennial-scale signal of elevated rock damage possibly associated with ice surface dynamics during or immediately after the Younger Dryas Stadial. Modelled rockfall runout distances were determined in part by path topography: rockfalls originating from lower slopes travelled <100 m horizontally whilst those originating higher could travel up to 650 m, approaching the glacier centreline, reflecting a spatial differential in hillslope-glacier connectivity that will evolve concurrently with cryospheric degradation in the wider catchment. We show that detailed, short-term monitoring campaigns can yield novel and useful descriptions of mass movement fluxes and spatial patterns in alpine regions. Expanding our dataset by observing rock walls near the equilibrium line altitude could help bridge the longitudinal gap to existing high elevation inventories to provide a more unified picture of rockfall dynamics in deglaciating catchments.