The role of hydraulically disconnected areas on basal sliding: Insights from small alpine glacier on the St. Elias range, Yukon Territory, Canada.

Subglacial drainage is known to play an important role in glacier dynamics through its influence on basal sliding. However, drainage is also one of the most poorly understood processes in glacier flow due to the difficulties of observing the physics involved. Here we use a borehole and surface motion data set obtained from a small, approximately 100 m thick surge-type alpine glacier over nine years to shed new light on the relationship between subglacial hydrology and basal sliding. Over 300 boreholes were instrumented with pressure transducers over 0.5 km² in its upper ablation area, while a GPS array consisting of 16 dual-frequency receivers within the same area provides a continuous time series of surface velocity. During the melt season, subglacial water pressures frequently fluctuate with a dominant diurnal frequency. The pattern of diurnal pressure fluctuations over several days allows us to cluster boreholes into groups that are likely to be mutually hydraulically connected, in the sense that they typically exhibit very similar pressure fluctuations that are distinct from those in other clusters. Using this approach, we identify hydraulically connected and disconnected parts of the bed, and compute a representative effective pressure for each cluster. We then correlate variations in surface velocity to effective pressure and "connectivity", defined as an estimate of the fraction of the bed that is hydraulically connected. This reveals that the degree of hydraulic connectivity is a significantly better predictor of surface velocity variations at a seasonal scale than the effective pressure within the connected areas. While effective pressure displays the expected power-law relationship with surface speed during a relatively short period when a well-developed drainage system is in place, we observe changes in connectivity during all the main surfaces speed transitions identified throughout the melt season.