BOREHOLE WATER-LEVEL VARIATIONS AND THE STRUCTURE OF THE SUBGLACIAL HYDROLOGICAL SYSTEM OF HAUT GLACIER DAROLLA, VALAIS, SWITZERLAND

Late-summer subglacial water pressures have been measured in a dense array of boreholes in the ablation area of Haut Glacier d'Arolla, Switzerland. Interpolated sur faces of minimum diurnal water pressure and diurnal water-pressure variation suggest the presence of a subglacial channel within a more widespread, distributed drainage system. The channel flows along the centre of a variable pressure axis (VPA), some tells of metres wide, that is characterized by low minimum diurnal water pressures (frequently atmospheric) and high diurnal water-pressure variations. These characteristics are transitional ol-er a lateral distance of c. 70 m to higher and more stable subglacial water pressures in the adjacent distributed system. Water-pressure variations recorded in boreholes located close to the centre of tile VPA reflect the delivery of surface-derived meltwater to the glacier bed and result in a diurnally reversing, transverse hydraulic gradient that drives water out from the channel into the distributed system during the afternoon and back to the channel overnight. Subglacial observations suggest that such flow occurs through a vertically confined sediment la)er. Borehole turbidity records indicate that the resulting diurnal water flows are responsible for the mobilization and transport of fine debris in suspension. Analysis of the propagation velocity and amplitude attenuation of the diurnal pl es;sure waves suggests that the hydraulic conductivity of the sediment layer decreases exponentially with distance from the channel, falling fr om c. 10(-4) m s(-1) at tile channel boundary to c. 10(-7) m s(-1) 70m au-av. These apparent hydraulic conductivities are consistent with Darcian flow through clean sand and typical glacial till, respectively. We suggest that fine material is systematically flushed from basal sediments located adjacent to large, melt-season drainage channels beneath warm-based glaciers. This process ma! have important implications for patterns of glacier erosion, hydrochemistry and dynamics.