Development of supraglacial meltwater streams and their influence on the morphology of debris-covered glacier surfaces.

Debris-covered glacier tongues are widespread in high-relief mountains and are characterised by highly undulated surfaces with supraglacial ponds and circular cliffs in hummocky topography. These features are known to strongly enhance mass loss on debris-covered surfaces and have been widely mapped, but their formation mechanisms and underlying controls have not been studied in detail.

Here we aim to investigate the role of supraglacial streams on the morphological development of debris-covered glacier surfaces and related supraglacial features such as ice cliffs based on high-resolution DEMs and orthophotos (Pleiades and UAV) from two debris-covered glaciers of contrasting spatial scales: the Satopanth Glacier located in the Indian Himalaya and the Zmuttgletscher in the European Alps. We systematically analyse the morphological development of the debris-covered surface along the glacier from the onset zone of debris cover and supraglacial channels down to the hummocky and sunken tongue surfaces. We perform this using a semi-automated approach that includes meltwater flow routing, the extraction of surface roughness, profiles and extents of supraglacial channel-influenced valleys, as well as the mapping of ice cliffs.

Based on this analysis, we find a clear and coherent succession of morphological developments along both glaciers that seems initiated through erosion from supraglacial streams. On the initially smooth debris-covered surface, locally incised and meandering channels initiate ice cliffs that progressively backwaste, creating a downstream-widening supraglacial valley with an undulated surface. This ‘mobility area’ is advected downstream even beyond the moulins where the supraglacial channels drain to the bed. Further downstream, neighbouring ‘mobility area’ valleys laterally merge and create the quasi-chaotic highly undulated surfaces typically observed on tongues of debris-covered glaciers. We integrate these interpretations into a conceptual model that links the downstream morphological development of debris-covered surfaces and explains the genesis of related features such as ice cliffs.