Subglacial drainage characterization on a small surge type alpine glacier on the St. Elias range, Yukon Territory, Canada.

Abstract:

Subglacial drainage is known to play an important role in glacier dynamics trough its influence on basal sliding. However, drainage is also the most poorly understood dynamic process due to the difficulties of observing, identifying and modeling the physics involved.

In an effort to improve understanding of subglacial hydrological processes, we have monitored a small, approximately 100 m thick surge-type alpine glacier for seven years. 225 boreholes, positioned to capture the spatial structure of the drainage system over a 0.4 km² study area in its upper ablation area, were instrumented with permanent pressure transducers, in addition to a permanent GPS array and weather station installed on the glacier.

Our principal results are

i) Each year the drainage system experiences a relatively brief (1-2 month) period of activity driven by diurnal surface melt, with a spring event and a spatial reorganization of the drainage system.

ii) The evolution of the drainage system can lead to very narrow main drainage axes, poorly connected laterally but traceable over long downslope distances (>500 m) with very little attenuation of the pressure signal, indicating that leakage into englacial or subglacial storage is relatively small. In at least one instance, a borehole have reached directly a channel with highly turbulent water flow.

iii) Our data also indicates the occurrence of distributed drainage over extended spatial regions, co-existing in close spatial proximity with channelized areas showing little evidence of hydraulic connection between them.

v) Crevasses appear to have a significant influence on the pattern of drainage.

vi) While most neighboring boreholes exhibit uncorrelated winter water pressures, there is strong evidence for winter-time drainage activity in the form of pressure oscillations along main drainage axes.

vii) There is strong evidence for interannual variability in the distributed versus channelized character of drainage, with drainage axes shifting in space.

Some of these results are consistent with established ideas about drainage physics, while others suggest refinements to existing understanding . We discuss how our results fit into current efforts to model subglacial drainage and alterations to drainage model physics that are suggested by our observations.