We present results of an investigation of two jökulhlaups (glacial lake outburst floods) at Gornergletscher, Switzerland, using dye-tracer experiments and complementary hydrological measurements. Repeated dye injections into moulins showed that tracer transit speeds were larger after the lake had emptied, but when proglacial discharge was still high, than during the main phase of the jökulhlaups. This counter-intuitive finding was modelled by tracer retardation inside the injection moulin. This model, together with an estimate of the maximum time the tracer takes to transit the injection moulin, allows us to calculate bounds on the transit speed in the main drainage channel where the lake water flows. These results indicate that the main drainage channel transit speeds are indeed highest during the peak of the flood. Moreover, it is known from a previous study that water amounting to half of the lake volume is temporarily stored within the glacier during a Gornergletscher jökulhlaups. Our observations suggest that this process occurred via lateral spreading of water at the glacier bed.