Subglacial water beneath the Antarctic Ice Sheet is often funneled via subglacial channels, which inject freshwater into ice-shelf cavities where it interacts with ocean water. The temporal variability of this system has been poorly observed, but its importance for ice dynamics is well recognized. Airborne radar data show a subglacial channel evolving within a decade near of the grounding zone of the Roi Baudouin Ice Shelf (East Antarctica), while topographic signatures on the ice shelf indicate prior inactivity for 60 years. Combining our observations with subglacial hydrological modeling, we suggest that the interplay between episodic subglacial water pulses and ocean water intrusion drive the opening and closing of the channels. Our findings illuminate the short-term transient nature of subglacial channel activity. This impacts ice-shelf–ocean processes, which are important for constraining increasing ocean warming onto ice-shelf basal mass balance, but pose significant challenges for subglacial hydrological modeling at the grounding zone.