RETREAT, DOWNWASTING AND ENGLACIAL TUNNEL DISCHARGE, EASTERN SECTOR, BERING GLACIER, ALASKA

Field experiments and mapping procedures initiated in 1998 on the eastern sector of Bering piedmont glacier, Alaska, are part of a multi-phase, long-term investigation of ablation and retreat. The primary objective was to measure typical summer rates of downwasting and retreat, and investigate effective controlling factors.

Diurnal surface downwasting measurements at three ablation sites located approximately 1 km upglacier from the ice front varied from 5.7 to 8.5 cm/day for debris-free ice in June, 2005. Meter-long segments of PVC pipe were placed in hand-augered holes then monitored by repeated measurements of the amount of pipe exposed due to ice surface lowering.

Rates of annual downwasting were measured from ice surface profiles that extend 1 km upglacier from an ice front datum. These show bi-annual fluctuation between 10 and 16 m/year for debris-free ice since 1998. The effects of broadly disbursed debris resulted in accelerated ablation that reached limits of 12 and 21 m/year. The cover of a well defined debris band caused retarded melting of 2 to 4 m/year. The same profiles show that retreat on land and in shallow ponds averaged 50-75 m/year.

A calibrated cable installed in 2005 to a depth of 9.3 m at the piedmont apex 400 feet above msl will be checked in 2006 to assess the effects of cooler conditions at a higher elevation. The influence of other factors (i.e. foliation, crystal size, infiltration capacity, and other weather conditions) on rates of downwasting are currently under investigation.

Discharge through abandoned englacial tunnels was estimated by applying a technique used in limestone caves. Wave length and depth of shallow asymmetric cup-like indentations (scallops) on tunnel walls are used to estimate velocity of englacial water flow, which when multiplied by tunnel dimensions yields an estimated discharge at the time of tunnel formation.