The irregular spatial distribution and velocity independence of basal friction derived from Landsat measured surface velocity suggests that ice stream flow is not controlled by the properties of a deformable basal till alone. Rigid bedrock substrata may contact the base of the ice stream in small (<100 km2) areas where the velocity field displays strong vorticity and where the ice stream surface appears rumpled in Landsat images. ¿ American Geophysical Union 1992