This study utilized the stable isotope composition of river water, alongside flow and precipitation data, to provide insight into the water sources that supply flows to the upper Bow River in southern Alberta. The annual mean delta 18O values of the Bow River increased from -20.7‰ in the headwaters to -19.1‰ at Calgary. It was shown that this downstream increase in delta 18O values was predominantly caused by the influx of tributary water with higher delta18O values, not by in-stream evaporation. In the headwaters region, isotopic analyses showed that winter precipitation (November to April) contributed 55-74% of river water, while it contributed 32-42% in downstream regions (e.g. Calgary), resulting in an increase in the delta 18O values of river water. Additionally, it was shown that the delta 18O and delta2H values of river water were not representative of those of regional precipitation due to evaporation that occurred during either groundwater recharge or snowpack sublimation.
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