Temporal variations in the concentration and stable isotope composition of dissolved phosphate and nitrogen in stormwater can be linked to potential sources. Stormwater quality was monitored by sampling at the outlets of two mainly residential watersheds located in Lincoln, Nebraska. Nutrient isotope analysis, when combined with the storm hydrograph and other monitoring data, helped characterize major contributors of nutrients during runoff. The δ18O-PO4 compositions of the runoff from three storms were widely varied early in the storm hydrographs, but converged during the declining limbs of the hydrographs to a composition shared by soil and vegetation source material. A strong correlation was observed for all but the largest storm events between nitrate in rainfall and in the stormwater event mean concentration. Isotopic analysis suggests that atmospheric nitrogen deposition provides a significant contribution to runoff from these watersheds, especially through impervious surface runoff throughout smaller storm events and early in larger events.