Antarctic ice shelves play a key role in regulating the rate of ice flow in tributary ice streams. Temporal variations in the associated ice-shelf buttressing stress are observed to impact ice flow in glaciers and ice streams. Ephemeral grounding induced by tides is an important mechanism for modulating the buttressing stress. Here, we develop an approach to inferring variations in 3-D surface displacements at an ice-shelf-stream system that explicitly accounts for ephemeral grounding. Using a temporally dense nine-month-long synthetic-aperture radar image acquisition campaign collected over Rutford Ice Stream, West Antarctica, by the 4-satellite COSMO-SkyMed constellation, we infer the ephemeral grounding zones and the spatiotemporal variation of the fortnightly ice-flow variability. We find ephemeral grounding zones along the western ice-shelf margin as well as a few prominent ephemeral grounding points in the central trunk and in the vicinity of the grounding zone. Our observations provide evidence for tide-modulated buttressing stress and the temporally asymmetric response of ice-shelf flow to tidal forcing. Long-term oceanic warming and ice-shelf thinning will cause the loss of ephemeral grounding and decrease in ice-shelf buttressing stress.