ICE VELOCITIES NEAR THE TERMINUS OF THE MATANUSKA GLACIER, ALASKA, DURING AN UNSEASONABLY WARM MELT SEASON

Horizontal ice motion in the western terminus region of the Matanuska Glacier, AK, was tracked over an 18-day period during June-July 2004. The movement of 23 stakes anchored into the ice was monitored using total station surveying equipment, which was set up over a temporary benchmark on an end moraine. The summer of 2004 was unique in that there was very little rain and higher than normal temperatures. Average air temperatures were more than 2° C above average and the precipitation was less than 50% of typical summer precipitation with no significant precipitation events during the duration of this study. These conditions provided a naturally controlled study in which ice velocity fluctuations could be attributed almost entirely to the daily temperature and its affect on meltwater influx to the subglacial drainage system. Although the subglacial drainage system is typically well-developed by this stage of the melt season, the above average meltwater influx we observed is hypothesized to result in greater water storage at the bed of the glacier. Water storage at the bed reduces basal drag, leading to faster ice velocities.

Despite the small time frame of this study, the results record higher than average ice velocities. The average velocity of 23 stakes over the entire study period was 22.1 cm/d. A similar study done by Ensminger, et al. in the summers of 1996 and 1997 showed an average velocity of 16.7 cm/d. Generally, in both Ensminger's and this study, average ice velocities mimicked average daily temperature, with velocity increases following temperature highs. There was a lag time of one to two days between high/low temperatures and high/low velocities respectively. This lag time likely reflects the time necessary for surface waters to reach the base of the glacier. The maximum discharge measured by a stream gauging station located just downriver from the ice margin shows an increased meltwater influx. Maximum discharge during the summer of 2004 was 4365 cfs, while in 1996, it was only 3053 cfs. This supports the hypothesis that increased meltwater influx accelerates ice motion over short periods of time.