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The Ob is the third largest Eurasian river supplying heat and freshwater to the Arctic Ocean. These inputs influence water salinity, ice coverage, ocean temperatures and ocean circulation, and ultimately the global climate system. Variability of Ob River flow on long time scales is poorly understood, however, because gaged flow records are short. Eleven tree-ring width chronologies of Pinus sibirica and Larix sibirica are developed from the floodplain of the Lower Ob River, analyzed for hydroclimatic signal and applied as predictors in a regression model to reconstruct 8month average (December-July) discharge of the Ob River at Salekhard over the interval 1705-2012 (308 yrs). Correlation analysis suggests the signal for discharge comes through air temperature: high discharge and floodplain water levels favor cool growing-season air temperature, which limits tree growth for the sampled species at these high latitudes. The Highlights • Tree-ring data from the Lower Ob River Basin has a signal for river discharge • Low growth reflects cool air temperatures, driven partly by high water levels • December-July discharge was reconstructed from tree rings back to 1705 • Extremes in discharge on the Ob occurred prior to the start of the gaged record • Reconstructed Ob River discharge is characterized by large multi-decadal swings

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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.

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