Knowledge of key sources and biogeochemical processes that affect the transport of nitrate (NO₃^-) in streams can inform watershed management strategies for controlling downstream eutrophication. We applied dual isotope analysis of NO₃^- to determine the dominant sources and processes that affect NO₃^- concentrations in six stream/river watersheds of different land uses. Samples were collected monthly at a range of flow conditions for 15 mo during 2004-05 and analyzed for NO₃^- concentrations, ?? ¹⁵N_NO3, and ??¹⁸O_NO3. Samples from two forested watersheds indicated that NO₃^- derived from nitrification was dominant at baseflow. A watershed dominated by suburban land use had three ??¹⁸O_NO3 values greater than +25???, indicating a large direct contribution of atmospheric NO ₃^- transported to the stream during some high flows. Two watersheds with large proportions of agricultural land use had many ??¹⁵N_NO3 values greater than +9???, suggesting an animal waste source consistent with regional dairy farming practices. These data showed a linear seasonal pattern with a ??¹⁸O _NO3:??¹⁵N_NO3 of 1:2, consistent with seasonally varying denitrification that peaked in late summer to early fall with the warmest temperatures and lowest annual streamflow. The large range of ?? ¹⁵N_NO3 values (10???) indicates that NO ₃^- supply was likely not limiting the rate of denitrification, consistent with ground water and/or in-stream denitrification. Mixing of two or more distinct sources may have affected the seasonal isotope patterns observed in these two agricultural streams. In a mixed land use watershed of large drainage area, none of the source and process patterns observed in the small streams were evident. These results emphasize that observations at watersheds of a few to a few hundred km² may be necessary to adequately quantify the relative roles of various NO ₃^- transport and process patterns that contribute to streamflow in large basins. Copyright ?? 2009 by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. All rights reserved.