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PFEFFER, W TAD  University of Colorado.
O'Neel, Shad  University of Colorado.
Krimmel, Robert M.  US Geological Survey.

In the long term, glacier changes are ultimately driven by climate changes through mass balance, but flow processes both in conditions of equilibrium and disequilibrium also control glacier changes. Under ‘ordinary’ conditions, glacier flow process act to establish a glacier geometry and flow pattern in equilibrium with mass balance, but circumstances exist where glacier flow processes act independently of mass balance, and dominate climate in the determination of the evolution of glacier geometry and volume. These ‘extraordinary’ circumstances include glacier surges and tidewater instability.

Tidewater glaciers, defined as outlet glaciers with termini grounded below sea-level, retreat cyclically, with retreat generally starting abruptly and proceeding irreversibly until the terminus retreats into shallow water. During retreat, the discharge flux greatly exceeds the mass balance flux, and climate plays a nearly negligible role in the rate of volume change. The phenomenon of tidewater glacier retreat is well-documented in coastal central and southeast Alaska, where tidewater glaciers occupying major coastal fjords, with the exception of Columbia Glacier, retreated from their fully extended positions during the 19th and early 20th centuries. Columbia Glacier, in Prince William Sound, has retreated approximately 13 km since the abrupt onset of retreat in 1982. During 2001, seasonal terminus velocities exceeded 10 km per year, and discharge flux of ice into the ocean was as great as 12 cubic km per year.

We review the ongoing retreat of Columbia Glacier, and examine the processes leading to a transition to abrupt and irreversible retreat by considering the behavior of a sliding law which depends inversely upon effective pressure (Pice – Pwater) in the context of classical kinematic wave theory as applied to glacier flow by John Nye. In that theory, kinematic waves are seen to propagate downstream if flux increases with increasing thickness and to propagate upstream if flux increases with decreasing thickness. For an effective pressure-dependent flow law, flux can increase with decreasing thickness if the loss of bed traction dominates the effect of loss of driving stress. A simple analysis shows the conditions for which this is true, and those conditions are seen to obtain for Columbia Glacier at the time of onset of rapid retreat in 1982. It is also shown that in contrast to down-stream propagating kinematic waves, diffusion does not eliminate the propagating effect under the conditions of interest, where surface slope and ice thickness are not small.

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