Rapid glacier sliding, reverse ice motion and subglacial water pressure during an autumn rainstorm

Measurements of basal water pressurefrom 15 boreholes located at both local (tens of meters) and regional (kilometers) length scales were used to elucidate the pressure/sliding relationship during an autumn rapid motion event on Bench Glacier, Alaska, USA. The 8 day event had two distinct phases, each with a ten-fold speed-up with respect to winter velocity. The water pressurein all 15 boreholes varied synchronously during the speed-up. The first phase of rapid sliding began after a peak in basal water pressureand continued while the pressurewas elevated and stable, or decreasing. The second phase of rapid sliding occurred when the basal water pressurewas low but increasing, and terminated before the pressurepeaked. Pressureand velocity do not appear unrelated, but the pressure/sliding relationship was not consistently linked to increasing, decreasing or a critical water pressure. The pressurevariations and sliding accelerations are a response to a warm rainstorm, although equally large input events occurred in weeks prior with no apparent response. Drainage system evolution therefore appears to play a key role in both the acceleration and the pressure/velocity relationship. Basal cavity dynamics are likely responsible for three episodes of reverse (up-valley) motion observed after enhanced sliding.