The relationship between the evolution of subglacial drainage system morphology and spatial patterns of glacier surface velocity was investigated using dye tracing experiments and ground surveying throughout the 1995 melt season at Haut Glacier d'Arolla, Switzerland. With the onset of high and variable melt season discharges, subglacial drainage changed from a predominantly distributed system to a predominantly channelized system. The change occurred later farther up glacier. During the period of drainage evolution the glacier was subjected to three periods of rapidly rising meltwater discharge. The magnitude and spatial pattern of the glacier's velocity response differed between these periods and can be explained in terms of the impact of the evolving drainage system morphology on the amplitude and spatial distribution of basal hydrological forcing. Increasing discharge through a distributed drainage system caused widespread basal forcing and high glacier velocity. Increasing discharge through incipient channels below moulins, not yet connected to the main channel, caused more localized basal forcing and slightly increased glacier velocity. Increasing discharge through a fully channelized drainage system caused no significant basal forcing and glacier velocity was not significantly different from the annual deformation flow pattern. Empirical orthogonal function analysis of flow patterns defined two distinct spatial modes of surface velocity which corresponded closely with the drainage system morphologies inferred to be present during each event. The relative importance of these modes changed through the melt season, suggesting a temporal change in the spatial pattern of hydrologically induced basal forcing.