Using CryoSat-2, we observe monthly changes in the Pine Island Glacier (PIG) ice shelf surface and derive oceanic melt at its base. Basal channels, kilometres wide, are reflected in the ice surface and captured in our observations. We demonstrate that melt is concentrated on the western walls of channels, that channels play a role in grounding pinning points, and that PIG's main channel geometry is inherited upstream of the grounding line. These results highlight the importance of channels to ice shelf stability.