MODES OF DRAINAGE BENEATH ICE STREAMS

The nature and efficiency of drainage beneath ice streams is fundamental to their operation. Contrasts are demonstrated between the drainage systems of major lobes of the LGM British ice sheet and the modern Rutford ice stream in West Antarctica. The former had deduced gravitational shear stresses in the range 4–24 kPa, and horizontal velocities in the terminal zone in the range of 400–950 ma^-1. An esker system in the area of the ice stream represents locations of subglacial drainage tunnels that are suggested to be key drainage elements where excess surface water penetrates to the bed. It is suggested that channel-groundwater coupling self-organises in response to the rate of water recharge and the transmissivity of the subglacial bed. In the Rutford ice stream, the bed is lubricated by basal meltwater alone, and detailed surveys have failed to find evidence of channel flow. A groundwater model is used to simulate subglacial hydraulic pressures and the zones of subglacial deformation, which are implicated in the highly drumlinised form of the glacier bed.