The natural and anthropogenic sources of nitrate (NO3) in the Bow River, Oldman River, and select tributaries of the Oldman River (OMR) in southern Alberta were determined by analyzing a combination of δ15NNO3, δ18ONO3 and δ11B values in surface water samples and in anthropogenic nutrient end-members. The δ15NNO3, δ18ONO3 and δ11B values of surface water revealed Bonnybrook WWTP effluent has the greatest influx of nutrient load to the Bow River and downstream of Calgary agricultural return flow water is an additional source of nutrients. Decreasing NO3 loads and NO3/B ratios downstream of Calgary indicated that there are also N-removal processes occurring in this reach. The δ15NNO3 and δ11B values of the surface water in the OMR indicated a significant nutrient input by the Lethbridge WWTP effluent and tributaries displayed δ11B values heavily indicative of cow manure. Hence, boron isotopes are a useful co-tracer of nutrients in surface water.