| Summary: | Measurements of glacier flow velocity and basal water pressure at two sites on Columbia Glacier, Alaska, are combined with meteorological and hydrologic data to provide an observational basis for assessing the role of water storage and basal water pressure in the rapid movement of this large glacier. During the period from July 5 to August 31, 1987, coordinated observations were made of glacier surface motion and of water level in five boreholes drilled to (or in one case near to) the glacier bed at two sites, 5 and 12 km from the terminus. Glacier velocities increased downglacier in this reach from about 4 m d^−1 to about 7 m d^−1. Three types of time variation in velocity and other variables were revealed: (1) Diurnal fluctuation in water input/output, borehole water level, and ice velocity (fluctuation amplitude 5 to 8%); (2) Speed-up events in glacier motion (15–30% speed up), lasting about 3 days, and occurring at times of enhanced input of water, in some cases from rain and in others from ice ablation enhanced by strong, warm winds; (3) "Extra-slowdown" events, in which, after a speed-up event, the ice velocity decreased in about 3 days to a level consistently lower than that prior to the speed-up event. All of the time variations in velocity were due, directly or indirectly, to variations in water input to the glacier. The role of basal water in causing the observed glacier motions is interpreted by Kamb et al. (this issue). © 1994 American Geophysical Union. Manuscript Accepted: 13 January 1994; Manuscript Received: 16 June 1993. Financial support for this work was provided by NSF grants DPP-8619348 to the University of Colorado and DPP-8619352 to the California Institute of Technology. The automated EDM system was developed by Bruce Vaughn. Help in the field work was provided by Carolyn Driedger, Mark Wumpkes, and Matthias Blume. To our helicopter pilot, Roger Kramer, we owe a special debt of gratitude. Published - jgrb9508.pdf
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