Drainage of ice-contact MiageLake (Mont Blanc Massif, Italy) in September 2004

Abstract

Drainage of ice-marginal Miage Lake (36,000 m²; maximal depth = c. 30 m) in early September 2004 occurred in two stages. Initial rapid loss (c. 325,000 m³) over two days was due to increased outflow along the boundary between the glacier sole and the till substrate at a rate of up to 2 m³/s. This lowered the lake surface until separate ponds were isolated from the subglacial drainage route. Subsequent loss over weeks took place at slower rates by leakage through permeable till, most rapidly in the deepest (SE) basin where a sealing matrix of fines was absent. This point is identified as a site of continuous leakage at all lake stages. Lake volume and water balance considerations indicate a low turnover of lake water. The exposed lake bed revealed an extensive ice foot connecting the subaerial ice cliff to the subaqueous continuation of the right-lateral moraine. The ice foot showed evidence of buoyant calving from its surface prior to lake drainage. The lake floor largely consists of moraine ridges covered by a sandy-silt drape which thickens downslope, from centimetres under shallow water to > 1 m in lake-bottom hollows. The innermost moraine relates to the latest «Little Ice Age» and the early 20th Century fluctuations of the glacier in the eastern basin. Drainage events are unlikely to pose a hazard, and occur infrequently (and seemingly randomly) whenever marginal crevasses link the perched ice-marginal lake to the subglacial drainage network.