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 a rray of boreholes in the ablation area of Haut Glacier d'Arolla, Switz erland. Interpolated sur faces of minimum diurnal water pressure and d iurnal water-pressure variation suggest the presence of a subglacial c hannel within a more widespread, distributed drainage system. The chan nel flows along the centre of a variable pressure axis (VPA), some tel ls of metres wide, that is characterized by low minimum diurnal water pressures (frequently atmospheric) and high diurnal water-pressure var iations. These characteristics are transitional ol-er a lateral distan ce 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 deliver y of surface-derived meltwater to the glacier bed and result in a diur nally reversing, transverse hydraulic gradient that drives water out f rom the channel into the distributed system during the afternoon and b ack to the channel overnight. Subglacial observations suggest that suc h flow occurs through a vertically confined sediment la)er. Borehole t urbidity records indicate that the resulting diurnal water flows are r esponsible for the mobilization and transport of fine debris in suspen sion. Analysis of the propagation velocity and amplitude attenuation o f the diurnal pl es;sure waves suggests that the hydraulic conductivit y 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 a re consistent with Darcian flow through clean sand and typical glacial till, respectively. We suggest that fine material is systematically f lushed from basal sediments located adjacent to large, melt-season dra inage channels beneath warm-based glaciers. This process ma! have impo rtant implications for patterns of glacier erosion, hydrochemistry and dynamics.