Location of Mechanical Controls on Columbia Glacier, Alaska, U.S.A., Prior to its Rapid Retreat

Abstract

Data collected in 1977, before the start of the major retreat in 1981, are used to compute stresses acting on Columbia Glacier. Contrary to the situation in many other glaciers, where the driving stress is a good measure of basal drag, horizontal strain rates are large and spatially variable and the associated viscous forces are large. These result in important force transmissions along and across the glacier. Basal friction is more constant than is the driving stress, except near the terminus where basal resistance is concentrated at certain sites. The use of a valley shape factor as used in many one-dimensional models is tested against the present, more thorough analysis. It is found to be valid for longitudinal averages over more than about three ice thicknesses (about 1500 m). The average shape factor is 0.78, meaning that 78% of the flow resistance to a flowline at the center of the glacier is due to drag at the glacier base.