Polythermal glacier hydrology and ice flow dynamics

Measurements at polythermal John Evans Glacier, Ellesmere Island, indicate strong links between spatial and temporal variations in subglacial hydrology and changes in surface velocity. Radio-echo sounding was used to determine ice thicknesses and bed topography, which allowed reconstruction of likely subglacial flow routing. Spatial patterns in residual bed reflection power (BRPr) enabled identification of warm and cold areas of the glacier bed. Within warm areas, the detailed pattern of BRPr was similar to the pattern of predicted subglacial water flow.;Long-term ice motion measurements along a centre-line of 20 stakes indicate summer velocities slightly higher than overwinter levels in the accumulation area, but up to double overwinter levels in the ablation area. To assess the importance of basal motion in accounting for these patterns, expected ice deformation rates were calculated based on the likely range of ice temperatures and longitudinal coupling lengths at the glacier. In the accumulation and upper ablation areas, predicted motion due to ice deformation closely matched observations in both summer and winter. In the lower ablation area, basal motion was required in addition to ice deformation, with the largest basal velocities centred over the areas of predicted subglacial drainage. In addition, there was a rapid onset to summer basal motion directly below a set of moulins which provide the main route for surface meltwater to reach the glacier bed.;Short-term ice motion measurements record three short-term (two- to four-day) high velocity events in summer 1998 and 1999, with velocities >100% above overwinter levels. These events were associated with periods of rapidly increasing meltwater input to the subglacial drainage system. The first events in 1998 and 1999 occurred at the start of the melt season at the transition from a snow covered to an ice covered surface. A second event in 1999 occurred midway through the summer as surface melting resumed following a cold period, and was strongly localised along a predicted subglacial drainage axis.