The subglacial hydraulics of the surge-type Black Rapids Glacier, Alaska: a schematic model

Abstract

The subglacial hydraulic system of the surge-type Black Rapids Glacier was studied in 1993 by comparing glacier velocity and seismicity with the stage, electrical conductivity, and turbidity of its proglacial streams. Brief events of increased velocity and seismicity occurred at the beginning and end of the measurement season. Five events coincided with drainages of supraglacial lakes and potholes. During events, water was stored englacially or subglacially and released subsequently, as indicated by a dye tracing experiment. Conductivity-stage-seismicity relationships suggest a model wherein daily storage and release of water depended on variations in subglacial pressure, which were reflected by daily variations in seismicity. Heavy precipitation and increases in stage preceded late-season pothole drainages. We hypothesize that precipitation triggered pothole drainages by enlarging drainage conduits, thus lowering subglacial pressure. Differences between the drainage systems in 1993 and 1986-89 may reflect mechanisms of surge evolution.