INFLUENCE OF SUBGLACIAL DRAINAGE CONDITIONS ON THE VELOCITY DISTRIBUTION WITHIN A GLACIER CROSS-SECTION

Substrate properties and hydrological conditions at the base of Haut G lacier d'Arolla, Switzerland, are nonuniform. The thickness and grain size of subglacial sediment vary on length scales less than one ice th ickness,,whereas hydrological conditions vary seasonally and on length scales of about one ice thickness transverse to ice motion. There is a close relationship between the annually averaged velocity field and this nonuniformity of bed conditions. Basal motion dominates in an are a with large water-level variations in boreholes, whereas ice deformat ion contributes significantly to total movement elsewhere. Localized e nhanced basal motion occurs primarily in summer, especially during a J uly ''spring event,'' but this motion is barely discernible in annuall y averaged surface velocity measurements, because transverse coupling suppresses surface expression of the basal motion discontinuity. These results highlight the need to include representations of bed nonunifo rmity in models of glacier flow and to consider ice deformation and ba sal motion as interdependent processes.