Variations in the Sliding of a Temperate Glacier

Abstract

Detailed measurements of the positions of stakes along the center-line of the lower Nisqually Glacier were made over a period of two years. Variations in the basal sliding speed were calculated from the measured changes in surface speed, surface slope, and thickness, using the glacier flow model of Nye (1952) and allowing for the effect of the valley walls, longitudinal stress gradients, and uncertainties in the flow law of ice. The flow is predominantly by basal sliding and has a pronounced seasonal variation of approximately ±25%. Internal deformation contributes progressively less to the total motion with distance up-glacier. Neither the phase nor the magnitude of the seasonal velocity fluctuations can be accounted for by seasonal variations in the state of stress within the ice or at the bed, and the variations do not correlate directly with the melt-water discharge from the terminus. A seasonal wave in the ice flow travels down the glacier at a speed too high for propagation by internal deformation or the pressure melting/enhanced creep mechanism of basal sliding. The rate of sliding appears to be determined primarily by the amount of water in temporary storage in the glacier. The peak in sliding speed occurs, on the average, at the same time as the maximum liquid water storage of the South Cascade Glacier. The data support the idea that glaciers store water in the fall, winter and spring and then release it in the summer. This temporary storage may be greatest near the equilibrium line. The amount of stored water may increase over a period of years and be released catastrophically as a jökulhlaup. Any dependence of sliding on the basal shear stress is probably masked by the effect of variations in the hydrostatic pressure of water having access to the bed.

Effect of extension rate on terminus position, Columbia Glacier, Alaska, U.S.A.

“…Kamb and others (1994) demonstrate that increases in basal water storage on the days-toweeks time-scale cause larger speeds on Columbia Glacier. It is likely that seasonal water-input changes on Columbia Glacier also cause speed cycles as they do on other glaciers (Hodge, 1974;Iken and others, 1983). Speed on Columbia…”

Implications of till deformation on glacier dynamics

“…It should be noted, however, that borehole drilling through that glacier encountered basal debris and not clean bedrock (Harrison and others, 1986), so till deformation may have actually played a key role that surge. Hodge (1974) related water storage to seasonal velocity changes. Kamb and others (1994) also found that water storage was a good control variable for basal motion on shorter time scales, such as the speedup events on Columbia Glacier.…”

Hydrology and dynamics of a polythermal (mostly cold) High Arctic glacier

“…The predominance of cold ice in such glaciers suggests that supraglacialsubglacial hydrological connections are limited, and therefore that the hydrologically induced flow instabilities commonly observed on temperate glaciers (e.g. Hodge, 1974;Iken and Bindschadler, 1986;Willis, 1995) and mostly warm polythermal glaciers (e.g. Etzelmüller et al, 1993;Jansson, 1996;Kavanaugh and Clarke, 2001) are unlikely.…”