Isotopic hydrograph separation (IHS) to define sources of event and preevent water during hydrological episodes has greatly improved the understanding of water, solute, and contaminant transport to streams during recent decades. However, the large variation in snowmelt isotopic composition, caused by fractionation during melting, has impeded an accurate separation of streamflow during spring flood episodes. Here we present a method that greatly improves the separation of event and preevent water during snowmelt by accounting for both the temporal change in the snowmelt isotopic signal and the temporary storage of meltwater in the catchment. Comparison of results of this technique with previous results, using isotopic data from the 1997 spring flood on a small catchment in northern Sweden, suggests that earlier techniques significantly underestimate the preevent contribution. This paper also explores the importance of lateral mixing across the catchment of temporally varying event inputs for the IHS results.