Ice-penetrating radar and kinematic GPS observations from Subglacial Lake Whillans (SLW), West Antarctica, reveal a shallow lake that is confined by steep basal topographic features. Radar imaging of SLW, although indicating wet basal conditions, is consistent with a water column depth of only ~6m or less during the near low-stand state at the time of the survey. Kinematic GPS profiles reveal that SLW is generally defined by a ~15m surface depression centered at S 84.237° W 153.614°. This point coincides with the area of lowest hydropotential in the lake basin and also the largest surface elevation range in ICESat data. Therefore this location appears to be an opportune site for subglacial access drilling of this active subglacial lake. A distinct basal topographic ridge on the grid south side of the basin is coincident with a strong contrast in relative basal reflectivity (~6dB), which we interpret as the lake boundary. Mapped hydropotential (calculated assuming hydrostatic equilibrium) shows that water enters the lake from the upstream direction and drains downstream. We hypothesize that a lake-level rise of ~5m plus flexural effects is sufficient to overtop a drainage divide. Thus SLW acts as a temporary storage basin for water beneath Whillans Ice Stream. ► Radar and GPS surveys geophysically characterize a dynamic subglacial lake. ► Subglacial lakes store and release transient water flow impacting ice dynamics. ► A depression in hydropotential coincides with the greatest surface elevation change. ► Ice flexural effects may play a key role in lake drainage. ► Water flux during filling (or draining) is modest (∼10m3/s).