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

<p>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.</p> <p>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.</p> <p>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 &#8216;mobility area&#8217; is advected downstream even beyond the moulins where the supraglacial channels drain to the bed. Further downstream, neighbouring &#8216;mobility area&#8217; 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.</p>