We use surface velocity derived from sequential Landsat imagery and a
control method to estimate the basal-friction distribution of a major
West Aatarctic ice stream. The area-averaged basal stress is approxima
tely 1.4 x 10(4) Pa, or about 29% of the area-averaged driving stress
of 4.9 x 10(4) Pa. Uncertainty of the derived area-averaged basal stre
ss is difficult to assess and depends primarily on spatial variation o
f the flow-law rate factor in the constitutive law. Spatial variation
associated with depth-averaged temperature variation gives an uncertai
nty of approximately +/-10(3) Pa. Approximately 60% of the ice stream
has a basal-stress magnitude less than 10(4) Pa, and approximately 30%
has less than 10(3) Pa. These characteristics suggest the presence of
a mechanically weak, water-charged subglacial till. Small-scale stick
y spots where basal friction exceeds the area-averaged driving stress
are scattered irregularly across the subglacial regime and comprise ap
proximately 15% of the ice-stream area. Sticky spots cluster in region
s where Landsat imagery suggests structural features in the underlying
bedrock.