Statistical Evaluation of Glacier Boreholes as Indicators of Basal Drainage Systems

ABSTRACT Between 1988 and 1992 closely spaced arrays of boreholes were drilled at Small River Glacier, British Columbia. The borehole arrays have been used to investigate the interannual and spatial consistency of patterns of basal hydraulics beneath the glacier. A simple robust classification was devised identifying unconnected, high standing, low standing and dry base water levels in boreholes. Spatial and interannual comparisons were made using a simple nearest neighbour statistic, corrected for differences in frequency of different borehole types and evaluated using Monte Carlo confidence intervals to compensate for array form. Arrays in the lower ablation zone showed spatial and interannual coherence, with three distinct areas characterized by low water pressure, till-associated non-connection and high pressure. There was no indication of a dominant conduit. Slightly higher up-glacier borehole patterns were less coherent, and varied from year to year, probably a result of subglacial karst capturing basal waters at a number of low pressure points at the bed. Therefore both the upper and lower arrays at Small River Glacier appear to encompass unusual drainage conditions. The nearest neighbour analysis provides valuable constraints on more specific interpretation.