Annual values of carbon isotope ratios for the period 1898--1990 are reported for Scots pine (Pinus sylvestris) growing in the valley of the lower river Ob in western Siberia. The data show highly significant inverse correlation to river flow over the period of instrumental records (1934--1990), although the strength of the relationship shows some dependence on the distance of the tree from the edge of the river. The predictive power of a linear regression model derived from the second half of the data set is validated by comparing the river flow for the first half of the period with values predicted by the model. Agreement is highly significant (r=0.80, P<0.001). The results demonstrate the potential of carbon isotope ratios to predict river flow in this region. ¿ 2000 American Geophysical Union