Storm hydrographs in the Upper Kuparuk River basin (142 km2) in northern Alaska were separated into source components using a mixing model and by recession analysis. In non-Arctic regions, storm flow is commonly dominated by old water, that is, water that existed in the basin before the storm. We suspected that this may not be true in Arctic regions where permafrost diminishes subsurface storage capacity. Streamflow during the snowmelt period was nearly all new water. However, all summer storms were dominated by old water. Storms in a neighboring basin were dominated by new water but much less than was the snowmelt event. Thus a large increase in old water contributions occurred following the snowmelt period. This increase continued moderately through the summer in 1994 but not in 1995. We credit the seasonal changes in old water contributions to increased subsurface storage capacity due to thawing of the active layer.¿ 1997 American Geophysical Union