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MacGregor, K.R., 2002

Modeling and field constraints on glacier dynamics, erosion, and alpine landscape evolution (Alaska)

Bibliographic Reference

MacGregor, K.R., 2002, Modeling and field constraints on glacier dynamics, erosion, and alpine landscape evolution (Alaska): University of California, Santa Cruz, Ph.D. dissertation, 277 p.


Glacial erosion is an important but poorly understood agent of alpine landscape evolution. Development of the glacial longitudinal profile was examined with a numerical model, while glacier sliding and its control were the subjects of a field project. Seasonal changes in meteorology, ice dynamics, and hydrology were documented at the Bench Glacier, Chugach Range, Alaska. A wave of elevated sliding velocity traveled from the glacier terminus into the accumulation zone at a rate of ?250 m/day. GPS-measured vertical velocities demonstrated divergence of the glacier surface from the bed, with maximum uplift rates coincident with maximum sliding velocities. Apparent bed separation was approximately 15 cm. Both the sliding wave and surface uplift occurred during a time of positive water storage in the glacier. The data suggest that upglacier propagation of a linked cavity network may explain the observed sliding event. The effect of glacial erosion over 105 -10 6 year timescales was addressed using a numerical model that incorporates the relevant glaciological processes that operate to produce hanging valleys, bedrock steps, overdeepenings and cirques. Simulations always show rapid flattening of the longitudinal profile from a fluvial initial condition. Inclusion of a tributary glacier creates a step in the main valley below the tributary junction that persists over multiple glaciations and generates a hanging valley. Steps result from increased ice discharge below tributary junctions, accommodated by increased ice thickness and sliding. The height of the hanging valley reflects the difference in the time-integrated discharge of ice in the tributary and the trunk valleys. Addition of a plateau allowed incorporation of blowing snow, avalanches, and headwall backwearing processes. In both steady and sawtooth climate scenarios, headwalls increase in length, steepen, and retreat over time. Bedrock cirques form in steady climate simulations only at the end of the model runs; the equilibrium line altitude (ELA) is hundreds of meters above the cirque floor. However, the time-averaged location of the ELA corresponds with the down-glacier cirque position. While the final profiles are relatively insensitive to the erosion rule used, quarrying is most effective near the upper glacier, whereas abrasion reflects the instantaneous pattern of integrated ice discharge.

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