Accurate forecasting of the Greenland Ice Sheet's contribution to global sea level change requires detailed knowledge of how ice ow responds to surface water inputs. Both ice velocities and surface melt have increased significantly over the last decade but recent research suggests that ice ow acceleration over the summer is regulated by the seasonal evolution of the subglacial drainage system. To investigate these and associated processes, a network of continuously-operating, dual-frequency global positioning system (GPS) receivers was deployed on a 140-km-long land-terminating transect in West Greenland, providing centimetre-precise, high-frequency records of ice motion. These data reveal that the enhanced summer ow regime is comprised of discrete, transient accelerations driven by the diurnal melt cycle, rapid in situ supraglacial lake drainage and rainfall/melt events. In 2010, a comprehensive array of instruments captured the rapid ( ~ 2 hour) drainage of a large supraglacial lake via a 3-km-long fracture, hydraulically-driven through km-thick ice. A further pronounced, widespread and sustained acceleration driven by rainfall and melt, observed in late August 2011, suggests that the predicted increase in cyclonic activity over Greenland may drive widespread off-season melt, rainfall and ow acceleration across the ice sheet. Together these events provide new insights into the basal hydrodynamic controls on ice sheet motion. Furthermore, observations of a persistent year-on-year acceleration in ice ow between 2009 and 2012 at a high elevation site located ~ 50 km inland of the equilibrium line support the hypothesis that the observed inland expansion of supraglacial lakes is driving faster ice ow at high elevations. These observations contrast with the prevailing self-regulation model and reveal that despite surface melt increasing water inputs to the bed are still insuffcient to develop effective subglacial drainage in the ice sheet's interior.