DIRECT MEASUREMENT OF SLIDING AT THE GLACIER BED

Sliding at the base of Trapridge Glacier, Yukon Territory, Canada, was measured using a ''drag spool''. We describe this simple and inexpens ive instrument as well as its installation and operation. From 1990 to 1992 seven sites were instrumented with drag spools. At six of the si tes basal sliding, during the period of observation, accounted for 50- 70% of the total flow observed at the glacier surface. The contributio n from ice creep is known to be small, so most of the remaining surfac e motion must be attributed to subglacial sediment deformation. For th e seventh site the observed sliding rate was similar to 90% of the tot al now, an indication that the sliding contribution varies spatially a cross the bed. Diurnal variations in the response of one of our instru ments appear to be correlated to subglacial water-pressure fluctuation s and are interpreted in terms of changes in sliding velocity rather t han the opening and closing of basal cavities.