Oxygen-18 is a stable, naturally occurring isotope of oxygen, whose concentration in a particular rainfall event may be quite different from its concentration in groundwater. It is on the basis of this difference that storm hydrographs recorded at seven stream gauges in the Big Creek and Big Otter Creek basins of Southern Ontario were separated into their components as to source. The storm hydrographs considered in this study were the result of a severe spring storm that triggered devastating floods in the adjacent, heavily populated, Grand River drainage area.Using the oxygen-18 technique, it was found that groundwater was the major component of storm runoff, providing more than 50% of the peak discharge. These results are consistent with natural tracer studies on basins one to two orders of magnitude smaller and in different hydrogeologic environments.By comparing oxygen-18 and specific conductivity data, storm runoff was divided into groundwater, direct rainfall, and direct runoff components.On the basis of the oxygen-18 and specific conductivity data, it was concluded that storm runoff in the study area was generated primarily through increased groundwater discharge. It is believed that increased hydraulic gradients, near or at the stream channel, which develop quickly after a rainfall, are responsible for the increase in groundwater discharge.