Seasonal variability in the Greenland Ice Sheet’s (GrIS) sliding speed is regulated by the response of the subglacial drainage system to meltwater inputs. However, the importance of channelization relative to the dewatering of isolated cavities in controlling seasonal ice deceleration remains unsolved. Using ice velocity, moulin hydraulic head, and glaciohydraulic tremor measurements we show the passing of a subglacial floodwave following the drainage of an up-glacier supraglacial lake slowed minimum sliding speeds to wintertime background values without increasing the hydraulic capacity of the moulin-connected drainage system. We interpret these results to reflect a persistent basal traction increase consistent with the dewatering of isolated cavities exert the dominant control on seasonal ice velocity decreases. Current predictions of the GrIS’s ice-dynamic response to increased surface melting hinges on the subglacial drainage system’s ability to increase its capacity to offset sustained meltwater influxes, which our results demonstrate may not be the case.