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The partially semi-arid Oldman River basin (OMRB), located in southern Alberta (Canada), has an area of 28200 km(2), is forested in its western headwater part, and is used for agriculture in its eastern part. Hydrometric measurements indicate that flow in the Oldman River has decreased by similar to 34% between 1913 and 2003, and it is predicted that water withdrawals will increase in the next 20 years. The objective of this study was to determine whether isotope ratio measurements can provide further insight into the water dynamics of the Oldman River and its tributaries. Surface water samples were collected monthly between December 2000 and March 2003. Groundwater samples were taken from 58 wells during one-time sampling trips. Runoff within the OMRB is currently about 70 mm year(-1), with a corresponding runoff ratio of 0.18. Seasonal flow characteristics are markedly different upstream and downstream of the Oldman River reservoir. Upstream, sharp increases in flow in late spring and early summer are followed by a rapid decrease to base flow levels. Downstream, a prolonged high flow peak is observed due to the storage effect of the Oldman River reservoir. The seasonal variation in the isotopic composition of surface water from upstream sites is small. This suggests that peak runoff is not predominantly generated by melting snow accumulated during the preceding winter, but mainly by relatively well-mixed young groundwater. A significant increase in the delta O-18 and delta H-2 values in the downstream part of the basin was observed. The increase in the isotopic values is partly due to surface water and groundwater influx with progressively higher delta O-18 and delta H-2 values in the eastern part, and partly due to evaporation. Hence, the combination of hydrometric data with isotope measurements yields valuable insights into the water dynamics in the OMRB that may be further refined with more intensive measurement programmes in the future. Copyright (c) 2006 John Wiley & Sons, Ltd.