A rare combination of natural circumstances permits assessment of current theories on water flow beneath glaciers. Outburst floods from the subglacial lake Grimsvotn in Iceland took place before, during and after surging of Skeloararjokull, the glacier beneath which the outburst floods drain. The observable drainage patterns associated with these floods show the different nature of the basal water conduit system of the glacier during surge and non-surge phases. During surge conditions, basal water is dispersed slowly across the bed in a distributed drainage system; but when the glacier is not surging, water is transported rapidly through a system of tunnels.