Hydrologic data were combined with analysis of a natural isotope, oxygen 18, and solutes (Ca, Mg, K, Na) to examine saturation overland flow and stream base cation dynamics during storms in a headwater swamp. Laboratory experiments were also used to study the interaction of mixtures of throughfall and groundwater with swamp substrates in zones of saturation overland flow. Increased streamflow was associated with decreased concentrations of Ca, Mg, and Na, whereas K concentrations remained constant, or increased in some storms. A comparison of chemical and isotopic hydrograph separation indicated that Ca, Mg, and Na were conservative in storms with instantaneous event water contributions 40% throughfall (event water). Stream K concentrations appear to be controlled by the mixture of throughfall and groundwater in saturation overland flow during occasional storms with high throughfall K concentrations. Laboratory data suggested that for most storms which have throughfall K concentrations of <1 mg L-1, K was released as overland flow interacts with swamp substrates, resulting in constant stream K concentrations. ¿ American Geophysical Union 1993