Ice dynamics at the mouth of ice stream B, Antarctica

Field data collected at the mouth of Ice Stream B show that the flow dynamics of this region are distinctly different than either the major portion of the ice stream upstream or the ice shelf downstream. Surface slopes in this region are as low as ice shelf surface slopes, yet with the exception of patches of ice which may be floating, the ice is grounded. Basal shear stress is negligible, the resistance to flow being partitioned between shear at the sides and longitudinal gradients of longitudinal and transverse stress. The surface is generally crevasse‐free. Features similar to ice rises are observed upstream of the grounding line. Their origin is uncertain, but they move at velocities comparable to the surrounding ice. The flow is laterally extensive and longitudinally compressive, but there are large local variations of the strain rate from the regional trends. The boundary between the two major tributaries to Ice Stream B, followed with the radar, is characterized by a band of strain rates much smaller than average. Detailed measurements at the downstream B network highlight this local variability of strain rates but confirm that there is a strong correlation between surface topography and strain rates. The strain rates indicate that the undulating topography is locally generated. The lower‐elevation ice is thicker and moves faster. A velocity profile across the crevassed northern margin shows that the decrease of velocity toward the edge is nearly linear. A calculation of ice stream discharge at this location agrees closely with two rather rough estimates of balance flux and is considerably larger than a third estimate. The discharge of Ice Stream B does not appear to be significantly out of balance with published estimates of total ice accumulation within the present catchment basin.