Marine ice-sheet collapse can contribute to rapid sea-level rise 1 . Today, the West Antarctic Ice Sheet contains an amount of ice equivalent to approximately six metres of sea-level rise, but most of the ice is in the slowly moving interior reservoir. A relatively small fraction of the ice sheet comprises several rapidly flowing ice streams which drain the ice to the sea. The evolution of this drainage system almost certainly governs the process of ice-sheet collapse 2-5 . The thick and slow-moving interior ice reservoir is generally fixed to the underlying bedrock while the ice streams glide over lubricated beds at velocities of up to several hundred metres per year. The source of the basal lubricant-a watersaturated till 6,7 overlain by a water system 8 -may be linked to the underlying geology. The West Antarctic Ice Sheet rests over a geologically complex region characterized by thin crust, high heat flows, active volcanism and sedimentary basins 9-16 . Here we use aerogeophysical measurements to constrain the geological setting of the onset of an active West Antarctic ice stream. The onset coincides with a sediment-filled basin incised by a steep-sided valley. This observation supports the suggestion 5,17 that ice-stream dynamics-and therefore the response of the West Antarctice Ice Sheet to changes in climate-are strongly modulated by the underlying geology.We use satellite imagery to identify streaming ice in central West Antarctica 18 (Fig. 1). The 20-km-wide region, with distinct margins bordering featureless ice (Fig. 2a), is dominated by flow-parallel banding and surface undulations, characteristic of streaming ice 19,20 . Streaming has been confirmed by GPS (Global Positioning System) measurements of surface ice velocity along a seismic line crossing the southern margin (Fig. 2c) 21 . New aerogeophysical data support the ice-streaming identification. Airborne ice-penetrating radar profiles show disrupted internal layering between the identified margins (Fig. 3). In the satellite image, the flow-parallel bands begin downslope (west) of a prominent, dark 'S'-shaped feature. Airborne laser altimetry (Fig. 2b) shows that this 'S' is caused by a 100-m-high escarpment in the ice surface which marks a significant break in ice surface slope. Upstream (east) of the 'S', the ice surface has a mean slope over 25 km of 7 m km −1 which decreases to 3 m km −1 downstream. The corresponding driving stress for ice-sheet flow decreases from over 100 kPa upstream of the 'S' to 60 kPa downstream. We interpret the 'S' as the onset of streaming, and will refer to the portion of the ice stream downstream of this 'S' as the onset region.Ice-penetrating radar data (Fig. 2d) shows that the onset of streaming rests over low-lying southwest-dipping subglacial topography. The onset 'S' is bounded by a plateau to the north and a prominent ridge to the south. Two kilometres west (downstream) of the 'S', the ice stream flows in a subglacial valley bounded by steep

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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.

Bore-hole drilling techniques have been used to connect with the subglacial water system of the temperate South Cascade Glacier. The water level in a connecting bore hole probably represents a direct measurement of the basal water pressure over an area at least to m in extent. Fluctuations of up to 40 m in bore-hole water levels occur typically over periods of several days and often peak about 2 d after large changes in water input at the glacier surface. The long-term trend in bore-hole water levels supports the idea of seasonal storage and release of liquid water.