Rates, processes and morphology of freshwater calving at Miage Glacier (Italian Alps)

The rates and processes of freshwater calving at Miage Glacier (Mont Blanc Massif, Italy) are described. Calving at Miage Glacier has occurred for two centuries on its right-lateral side, into a small ice-marginal lake (Miage Lake). Field surveys identified the main processes leading to iceberg production and quantified the calving losses over a summer season. Calving losses were compared (1) with the surface ablation of the debris-covered tongue, evaluated through a simple model based on measured ablation rates at different altitudes and debris cover thicknesses, and (2) with other inputs to the lake (stream inflow discharge) and with the lake volume. Results show that thermal undercutting by warmer surface water plays an important role in driving ice-cliff evolution. Thermal notches grow at ∼30-35m year^-1 and cause a similar amount of cliff retreat. Calving contributes ∼2% of the estimated summer runoff from the debris-covered part of the ablation zone, but this is equivalent to ∼38% of the lake volume, and is of the same magnitude as the mean discharge from the inflow streams. These data indicate that calving of the ice cliff is one of the main water sources for maintaining the lake at the maximum summer volume, with the surface at the level of the subaerial outlet stream. A survey of Italian calving glaciers shows that calving is becoming more widespread, and that debris covers are present at all calving ice margins. The lake-ice interactions described in this study can, therefore, be considered to have wider representativeness.