Surface melting can alter ice sheet sliding by supplying water to the bed, but subglacial processes driving ice accelerations are complex. We examine linkages between surface runoff, transient subglacial water storage, and short-term ice motion from 168 consecutive hourly measurements of meltwater discharge (i.e. moulin input) and GPS-derived ice surface motion for Rio Behar, a ~60 km² moulin-terminating supraglacial river catchment the southwest Greenland ablation zone. Short-term accelerations in ice speed correlate strongly with lag-corrected measures of surface mass loss, specifically supraglacial river discharge (r= 0.9; p<0.001). Though our 7-day record cannot address seasonal-scale forcing, diurnal ice accelerations align with normalized differenced supraglacial and proglacial discharge, a proxy for subglacial storage change, better than GPS-derived ice surface uplift. These observations counter theoretical steady-state basal sliding laws and suggest that moulin- and proglacially induced fluctuations in subglacial water storage, rather than absolute subglacial water storage, drive short-term ice accelerations.