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Variations in Time and Space of the Velocity of Lower Columbia Glacier, Alaska

Published online by Cambridge University Press:  30 January 2017

Mark F. Meier
Mark F. Meier*
Affiliation:
U.S. Geological Survey, Suite 850, 1201 Pacific AvenueTacoma, Washington 98402U.S.A.
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Abstract

Ice velocity, acceleration, and strain-rate are being measured at intervals of about 45 d using automated processing of aerial photographs. Preliminary results for the lowest 6 km of Columbia Glacier for the period July 1976 to November 1977 show the following:

  • (1) High rates of flow (2–6 m/d) and longitudinal extension (occasionally exceeding 1/a).

  • (2) A smooth, almost sinusoidal change in velocity 6 km above the terminus from about 2 m/d in August 1976 and 1977 to about 3 m/d in February–March 1977.

  • (3) Very rapid increases in velocity near the terminus in October 1976 and September–October 1977 which follow embayment formation (increased iceberg calving).

  • (4) A suggestion that the rapid velocity increases propagate up-glacier for 3–4 km before becoming unrecognizable.

  • (5) Seasonal changes in velocity (which presumably relate to basal sliding) ranging from about 1 m/d 6 km above the terminus to about 4 m/d near the terminus.

Type
Reports on Current Work
Information
Journal of Glaciology , Volume 23 , Issue 89 , 1979 , pp. 408
Copyright
Copyright © International Glaciological Society 1979

The rapid increase of velocity near the terminus appears to be caused by the transient increase in effective ice slope due to ice-cliff retreat. The high rate of basal sliding and the velocity peak in mid-winter imply unusual conditions at the bed. If one assumes that the basal water is in the form of a layer punctured by roughness elements (Weertman–Robin), calculation suggests a reversed generalized pressure gradient in this reach, resulting in water storage (unfortunately one cannot be sure of this because the basal shear-stress gradient is difficult to compute near the terminus). If one assumes that the basal water pressure is determined by a Röthlisberger conduit, the pressure in the conduit approaches, or perhaps even exceeds, the basal ice pressure. Thus an unusually thick water layer appears likely, but the seasonal variation remains to be explained.