HR: 1330h
AN: H52A-1165    [PDF]
TI: Interpretation of fine and coarse sediment yield from Bench Glacier, Alaska
AU: * Riihimaki, C A
EM: cariihim@colby.edu
AF: Colby College, Dept. of Geology, Waterville, ME 04901 United States
AU: MacGregor, K R
EM: macgregor@macalester.edu
AF: Macalester College, Dept. of Geology, St. Paul, MN 55105 United States
AU: Anderson, R S
EM: andersrs@colorado.edu
AF: University of Colorado, INSTAAR, Boulder, CO 80309 United States
AU: Anderson, S P
EM: spanders@colorado.edu
AF: University of Colorado, INSTAAR, Boulder, CO 80309 United States
AU: Loso, M G
EM: mloso@es.ucsc.edu
AF: UC-Santa Cruz, Earth Sciences Dept., Santa Cruz, CA 95064 United States
AB: Quantifying the rate of bedrock erosion by glaciers is crucial to our understanding of alpine landscape evolution. We document basin-averaged sediment evacuation for three summers at the 7-km-long Bench Glacier, Alaska, based on 15-minute gaging of stream stage and turbidity, together with manual sampling of sediment concentrations and bedload transport in the exit stream. Sediment evacuation was equivalent to 1-2 mm/yr of bed lowering each summer, with bedload accounting for 30% of the total output. Simultaneous time-series of ice surface displacements each summer yielded consistent basal sliding of only 1-2 m/yr.ÿ The ratio of bed-lowering to sliding is 10$^{-3}$, an order of magnitude higher than found by Humphrey and Raymond (1994). A simple interpretation of glacier sediment yields is confounded by unknown changes in subglacial sediment storage. We consistently observed a decline of the sediment discharge over the summer for a given water discharge, indicating seasonal sediment exhaustion within the subglacial channelized network. Does this reflect complete removal of stored sediment from the entire glacier bed? High-resolution GPS measurements at several locations in the ablation zone show episodic surface ice trajectories that can best be explained by sliding on up-glacier inclined bedrock ledges during rapid motion events. We therefore infer that a large portion of the bed in the ablation zone is sediment free. However, sediment may be retained in the accumulation zone, possibly because the subglacial channelized network does not extend this far upglacier. We suggest that efficient removal of sediment from the ablation zone may enhance erosion by increasing ice-bedrock contact, whereas basal sediment storage within the accumulation zone may inhibit bedrock erosion. We therefore predict that over long timescales, bedrock steps should form near the upglacier extent of the channelized network.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1827 Glaciology (1863)
SC: Hydrology [H]
MN: 2003 Fall Meeting