The supraglacial snowpack plays an important role in mediating the delivery of meltwater produced at the snowpack surface to the rest of the glacier system, with potential implications for both proglacial hydrograph form and the timing and magnitude of glacier movement induced by changes in subglacial water pressure. However, there remain few field observations of the hydrological behaviour of supraglacial snowpacks and its seasonal evolution during the melt season. Data collected during the 2004 melt season at Haut Glacier d'Arolla, Valais, Switzerland, uses both qualitative and quantitative dye tracing techniques to enhance our understanding of supraglacial snowpack hydrology. Observations of the movement of dye-stained water show the complexity of flow patterns and the influence of ice layers and preferential flow zones in controlling percolation through the snowpack, and fluorometric techniques yield average flow rates for percolation through the snowpack of between 0·13 and 0·49 m h−1. The changing form of dye-return curves and increasing percolation rates reflect an increase over the course of the melt season in the efficiency with which the snowpack transmits meltwater. Snowpack permeability was found to be significantly lower (1·67 × 10−10 m2 on average) than that assumed in previous modelling studies of glacial hydrology, showing the need for improved information about snowpack hydrology if its importance in controlling runoff is to be fully appreciated. Copyright © 2006 John Wiley & Sons, Ltd.