NEW INSIGHT ON THE CHARACTER OF THE HYDROLOGIC SYSTEM OF THE MATANUSKA GLACIER, ALASKA, BASED ON DRILLING, BOREHOLE MONITORING AND DYE TRACING STUDIES

Twelve boreholes were drilled into an overdeepening near the terminus of the Matanuska Glacier, Alaska, late in the melt season. Dye tracer experiments, water-level measurements and observations made while drilling show that nearly all the water passing through this part of the glacier moves through interconnected conduits in a complex karst-like system, in and beneath the thick zone of basal ice and along the base of the glacier. Ten of the holes intersected a hydrologic system at the top of the basal ice, while two of the holes intersected the system before basal ice was reached, most likely through basal crevasses. Two dye tracer tests were performed, where dye injected into a single borehole was recovered at multiple vents discharging at the terminus of the glacier. The velocity of the subglacial water varied, with a maximum velocity of 0.13 m/s over a straight line distance of 198m and a minimum velocity of 0.03 m/s over a 40m straight line distance. These results indicate that water in this part of the glacier moves through a distributed system, consisting of both major and minor conduits, that are connected by the porous basal ice. Borehole water levels remained near the ice surface indicating that the piezometric pressure is near that of the ice overburden pressure and that the water table followed the ice surface profile. However, the ice topography in the study area is highly variable, leading to local areas where the piezometric surface is above the ice surface. Several boreholes discharged large amounts of water from the subglacial system for several days following drilling, indicating high pressure, perhaps due to the local hydraulic resistance of the porous basal ice. Water levels fluctuated slightly on a diurnal cycle, indicating that the subglacial system is fed by surface meltwater.