[1] We report the spatial and temporal pattern of sliding on the 7-km-long Bench Glacier, Alaska. Using five continuously recording GPS antennas following motion of the surface ice, distributed at 1 km spacing along the glacier center line, we documented surface ice motion over 50 days during summer 2002. Surface speeds in two previous winters constrain the motion component associated with ice deformation, allowing isolation of the sliding speed history. We observed two speedup events bracketing 2 weeks of steady slow sliding. The first event was not associated with a meteorological trigger, was more subtle than the second, and propagated up-glacier at a rate of several hundred meters per day. The second event coincided with a warm up-valley wind, which triggered considerable melt of the glacier surface. Sliding speeds in this event reached 0.3 m d À1 and began almost simultaneously at all sites in the ablation area. Both the horizontal and vertical displacement time series can be explained by growth and collapse of cavities in the lee of bumps in the bedrock bed. Cavities grow during rapid sliding and decay by viscous creep. We posit that effective pressure, averaged over some large area of the bed, is inversely proportional to the sliding speed. This effective pressure then controls the collapse rate of cavities, whose dimensions are estimated from a plausible, stepped-bed geometry. This model explains well the horizontal and vertical surface displacement history through the first event and beginning of the second event. The vertical record demands a substantial and abrupt drop in water pressure that departs from the posited sliding-effective pressure relationship. We argue that this pressure drop reflects establishment of efficient subglacial drainage, also manifested in a nearly simultaneous step increase in water discharge in the exit stream. The establishment of an efficient conduit system terminates sliding; its maintenance inhibits further sliding over the remainder of the summer.

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