Streamflow generation in boreal catchments remains poorly understood. This is especially true for snowmelt episodes, which are the dominant hydrological event in many seasonally snow covered regions. We examined the spatial and temporal aspects of flow pathways by linking detailed oxygen 18 observations of stream, melt, soil, and groundwater with hydrometric measurements in a small catchment in northern Sweden during the snowmelt period. The results demonstrate that soil horizons below 90 cm were hardly affected by the approximately 200 mm of snowmelt water infiltrating into the soil during the spring. The approximately sixtyfold increase in runoff, from 0.13 mm d-1 to 8 mm d-1, was generated by a 30--40 cm rise of the groundwater level. The total runoff during the snowmelt period from late April to late May was 134 mm, of which 75% was preevent water. Mass balance calculations based on hydrometric and isotopic data independently, both using upscaling of a hillslope transect to the entire 13-ha catchment, provided similar results of both water storage changes and the amount of event water that was left in the catchment after the snowmelt. In general, groundwater levels and runoff were strongly correlated, but different functional relationships were observed for frozen and unfrozen soil conditions. Although runoff generation in the catchment generally could be explained by the transmissivity feedback concept, the results suggest that there is a temporal variability in the flow pathways during the spring controlled by soil frost during early snowmelt.