A major meltwater body exists beneath a tributary of Bindschadler Ice Stream, West Antarctica, in a region where subglacial lakes have not been mapped but near where rapid vertical motion of the ice sheet surface has suggested shifting of a subglacial water body. The water is trapped by a local reversal in ice-air surface slope arising from ice flow over variable basal topography and from the positive feedback of basal lubrication from the trapped water. Strong variations in the water content of the sediments upglacier of the water body arise from a similar process. These results are revealed by a novel application of the amplitude variation with offset (AVO) seismic technique. The existence of such water bodies and of the strong spatial variation in subglacial sediment properties is not captured in current models of subglacial hydrology, lubrication of ice stream motion, and sediment transport.