Orographic control of
glaciers and denudation in the Chugach/St. Elias Ranges
Andrew Meigs
Department of Geosciences
Oregon State University
Corvallis,OR
meigsa@geo.orst.edu
The Chugach/St. Elias Range in southern Alaska is the world’s only active
collisional orogenic belt dominated by glacial erosion. A strong
orographic control on climate is revealed by modern temperature and
precipitation gradients from the southern, windward flank across the range to
the northern, leeward flank. Differences in the magnitude of equilibrium
line altitude depression as revealed by the extent of occupation of glaciated
valleys and fjords during Holocene and Pleistocene glacial advances attests to
the long-term influence of this orographic effect on the mechanisms, magnitude,
and rate of denudation across the range. Changes in ice extent and thickness in
basins vary dramatically over glacial maxima-minima cycles and between the
windward and leeward flanks of the range. These variations affect the
proportion of landscape eroded at the bed of glaciers, base level for
hillslopes and rivers in unglaciated valleys, sediment production, routing, and
delivery out of the orogen, and modification of glacial valley forms by fluvial
processes. Glaciers expand and retract 10’s of km up- and down-valley on
10^3-yr timescales on the windward side of the range. Glaciers on the leeward
side of the range, in contrast, expand and retract at most a few km up- and
down-valley on 10^3-yr timescales. Thus the windward side of the range
experiences high frequency oscillations of ice distribution throughout the
landscape, which implies greater potential for erosion and transportation of
sediment. The windward side experiences similar changes in ice extent,
but with lower frequency of 10^4-yr timescales and longer. Given that the
tectonic influx of material is from the south-southeast on the windward flank
of the range, the dramatic effect of topography on glacier distribution between
the windward and leeward flanks may explain the long-term patterns of long-term
exhumation, the distribution and magnitude of crustal shortening, and rate of
the lateral growth of the orogenic belt.