Fast-flowing glaciers and ice streams are pathways for ice discharge from the interior of the Antarctic Ice Sheet to ice shelves, at rates controlled by conditions at the ice-bed interface. Using recently compiled high-resolution data sets and a standard inverse method, we computed basal shear stress distributions beneath Pine Island and Thwaites Glaciers, which are currently losing mass at an accelerating rate. The inversions reveal the presence of riblike patterns of very high basal shear stress embedded within much larger areas with zero basal shear stress. Their colocation with highs in the gradient of hydraulic potential suggests that subglacial water may control the evolution of these high-shear-stress ribs, potentially causing migration of the grounding line by changes in basal resistance in its vicinity.