Concentrations and isotopic compositions of NO3−from the Oldman River (OMR) and some of its tributaries (Alberta, Canada) have been determined on a monthly basis since December 2000 to assess temporal and spatial variations of riverine NO3−sources within the OMR basin. For the OMR sites, NO3−-N concentrations reached up to 0.34 mg L−1, δ15N-NO3−values varied between −;0.3 and +13.8%, and δ18O-NO3−values ranged from −10.0 to +5.7%. For the tributary sites, NO3−-N concentrations were as high as 8.81 mg L−1, δ15N-NO3−values varied between −2.5 and +23.4%, and δ18O-NO3−values ranged from −15.2 to +3.4%. Tributaries in the western, relatively pristine forested part of the watershed add predominantly NO3−to the OMR with δ15N-NO3−values near +2% indicative of soil nitrification. In contrast, tributaries in the eastern agriculturally-urban-industrially-used part of the basin contribute NO3−with δ15N-NO3−values of about +16% indicative of manure and/or sewage derived NO3−. This difference in δ15N-NO3−values of tributaries was found to be independent of the season, but rather indicates a spatial change in the NO3−source, which correlates with land use changes within the OMR basin. As a consequence of tributary influx, δ15N-NO3−values in the Oldman River increased from <+3% to >+6% in the downstream direction (W to E), although [NO3−-N] increased only moderately (generally <0.5 mg L−1). This study demonstrates the usefulness of δ15N-NO3−and δ18O-NO3−values in identifying the addition of anthropogenic NO3−to riverine systems.