Supraglacial fluvial landscape evolution on the Greenland Ice Sheet

Abstract:

In the ablation zone of the Greenland Ice Sheet, melting during the summer drives drainage development in which flow is routed downslope through a network of supgraglacial streams and lakes until it is sequestered by the englacial system or flows off of the glacier. This supraglacial drainage network sets the efficacy by which melt water is transport into the glacier and thus has important implications for coupling between ice sheet sliding and surface melt. Thermal erosion in supraglacial streams is rapid compared to other fluvial environments, raising the possibility that supraglacial topographic evolution is to some extent set by local fluvial incision rather than by underlying bedrock or iceflow.

We study a series of supraglacial drainage basins on top of the West Greenland Ice Sheet between 1000-1500 m elevation using a combination of high-resolution images, and concurrent (2 m resolution) DEMs constructed from World View Imagery. Although large-scale topography correlates well with underlying bedrock topography, spectral filtering of the surface also reveals broad, low relief valleys that suggest fluvial modification at all elevations. We extract several hundred supraglacial stream longitudinal profiles per drainage basin, finding many channel segments that are clearly out of equilibrium but also numerous concave up channel segments that are not well correlated with underlying bedrock. These concave up segments have a similar power law exponent, suggesting similarities to equilibrium bedrock and alluvial rivers (although the exponent is different in this setting). We develop a stream-power model to predict equilibrium longitudinal profiles where erosion is due to melting driving by viscous dissipation of heat within streams. We speculate that fluvial erosion driven by viscous dissipation is in part responsible for shaping the Greenland Ice Sheet ablation zone annually, superimposed on long wavelength bedrock control of surface topography and basins.