Effects of abrupt changes in basal conditions on ice-streams: Application to sub-ice-stream lakes

The recent discovery of a large number (241) of subglacial lakes under fast flowing ice streams (Smith et al., 2009) indicates that ice streams experience many abrupt spatial transitions in their flow regimes - from regimes that are strongly coupled to the bed (i.e., bed following basal flow) to regimes that involve an ice/water interface at the bed. The 'traditional' grounding line, where ice becomes afloat at the seaward terminus of the ice stream, represents a special case of only one of these transitions. At the boundaries of the many sub-ice-stream lakes, the transition from bed coupled to water coupled vise versa could be thought of as a "double grounding line problem". In this study we investigate such transitions by treating the nature of the ice-stream response to basal transitions as resulting from viscous (described by Glen's flow law) and elastic (described by Hook's law) rheologies. Two-dimensional idealized models are designed to study the ice stream surface expressions as well as internal deformations associated with the abrupt transitions. We also investigate sensitivities of an ice stream to subglacial lake boundaries and to various parameters of the system - the size and geometry of a subglacial lake, the bed topography and basal traction, and the thickness and flux of ice.