After Bentley (1987), I use Swithinbank's definition of an ice stream as: 'part of an inland ice sheet in which the ice flows more rapidly than . . . the surrounding ice.' A distinction can be drawn between ice streams, which are bounded laterally by ice, and outlet glaciers, which are confined in bedrock valleys. In general this is a useful distinction, although in nature there are many examples of transitional situations (Bentley, 1987,2003). Remote-sensing observations combined with calculations of balance velocities indicate that within modern polar ice sheets flow is organized into major drainage pathways, which channel most of the ice discharging from the ice sheets (Joughin et al., 1999; Bamber et al., 2000a,b). Ongoing efforts to map ice-surface velocity and bedrock topography will enable delineation of the specific sections of these pathways that can be categorized as either ice streams or outlet glaciers.
Recent applications of satellite radar interferometry provided surface velocity maps for parts of Greenland and Antarctica but the coverage represents still a small portion of the latter continent. Only one large ice stream has been identified in Greenland (Fahnestock et al., 1993). This ice sheet is ringed by mountain ranges and drained by about 30 large outlet glaciers (Bamber et al., 2000b). In Antarctica, there are about three dozen major drainage pathways with pure ice streams concentrated in the Ross Sea sector of the West Antarctic ice sheet (Bamber et al., 2000a; Rignot & Thomas, 2002). These are the Siple Coast ice streams characterized by fast motion, hundreds of metres per year, under low driving stresses, typically <50kPa (Bindschadler et al., 2001a; Blankenship et al., 2001). The surface slope of these ice streams decreases downstream, resulting in a concave-up shape, so that driving stresses drop to several kPa on the ice plain of the Whillans Ice Stream (Bindschadler et al., 1987). This fast motion under low driving stresses is possible because basal resistance to flow of these ice streams is low and much of the support for driving stress is shifted to the margin (Raymond, 2000; Tulaczyk et al., 2000b). Although these features characterize the 'classic' West Antarctic ice streams, they are not included in Swithinbank's (1954) definition of ice streams.
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