We examined how the spatial configuration of source areas for runoff varied over time in a large watershed, in order to understand processes governing material loading to rivers. Discharge source areas within the Fox River watershed (Wisconsin, US) were mapped for two individual discharge events. The spatial distribution of source areas varied between and over the duration of individual discharge events. Relative contribution to runoff by land cover types within source areas was quantified and compared to areal abundance of land covers in the watershed. Contributions of runoff by land cover types varied over time. Moreover, the degree to which different land cover types acted as source areas differed from their abundance in the watershed. Hence, areal quantifications of land cover within a watershed may not accurately represent what land covers are source areas over given time periods. Therefore, a source-area-based approach may yield more accurate spatial analysis of material loading patterns than a watershed-based approach.