Direct Measurement of Basal Water Pressures: Progress and Problemss

TLDR

The sealed‑off bed areas are likely sub‑sole drift and permeability barriers, whose increased extent has reduced the probability of bore‑hole connection to the basal water system. The study drilled 24 bore holes in 1975–1977 using electrothermal and hot‑water drills, expecting that pressurization of inactive zones would eventually link them to the active basal water system, and that water levels in connected holes would represent conditions over a 100‑m‑scale bed area. Only two of 24 bore holes connected to the basal water system, a sharp drop from earlier years, indicating that roughly 90 % of the bed is hydraulically inactive due to an internal change in the subglacial hydraulic system, with active water pressures at 50–75 % of overburden and a clear correlation between bore‑hole water levels and glacier surface motion.

Abstract

Abstract In 1975 and 1977, 24 bore holes were drilled to the bed of South Cascade Glacier, Washington, U.S.A., using both electrothermal and hot-water drills. Only two holes connected directly with the basal water system, a significant decrease from the four to five such connections in 13 holes drilled in 1973 and 1974 (Hodge, 1976). Most of the bed, possibly as much as 90%, appears to be hydraulically inactive and isolated from a few active subglacial conduits. Bore holes which penetrate these inactive areas initially should connect eventually with the active basal water system due to bed pressurization by the water standing in the bore hole, provided there is a sufficient supply of water available to form and maintain the connection passageway. These sealed-off areas probably consist of the sub-sole drift and permeability barriers found recently at the bed of Blue Glacier by Engelhardt and others (1978); an increase in the area of bed covered by these features probably caused the decrease in chance of bore-hole connection. This apparently was not due to any external cause but rather was the result of a real internal change in the subglacial hydraulic system which occurred between 1974 and 1975. If most of the area of a glacier bed is hydraulically isolated sub-sole drift, or something similar, such features may well control large-scale glacier sliding changes, since changes in the amount of water having access to the glacier bed will take considerable time to affect the interstitial water pressure in the more widespread sub-sole drift. Water pressures in the active part of the basal water system of South Cascade Glacier are generally in the range of 50–75% of the ice overburden pressure. Water levels in a connected bore hole are probably representative over an area of the bed 100 m or more in extent. A correlation of bore-hole water levels with changes in surface motion supports the idea that the sliding of a temperate glacier is controlled largely by the basal water pressure.