Ob River Flood History From Tree Rings

Flooding of the Ob River impacts its floodplain ecology as well as the inflow of heat and fresh water to the Kara Sea and Arctic Ocean. A previous study showed that seasonal-total (December-July) volume discharge of the Ob could be reconstructed back to 1705 from ring widths of floodplain conifers growing above the inundation zone: growth is reduced by cool air temperatures driven by broad flooding in June. The relationship, while statistically significant, explains only about 30% of the variance of Ob discharge over a 1937-2009 calibration period. Here we we extend this work by examining whether additional information on flooding could be provided by rings of Salix and Populus trees growing in the the more immediate vicinity of the river channel in settings subject to periodic inundation. Microscopic examination of rings from sites collected in various expeditions along the lower Ob between 1997 and 2016 reveals anomalies in ring structure related to extended inundation. Duration of inundation at various levels is inferred from analysis of daily water levels at Salekhard. Extended periods of high water level in 1941 and 1979 are associated with suppressed ring-growth and changes in cell anatomy in Salix and Populus rings. The changes are synchronous at locations separated by hundreds of kilometers along the river. We are applying classification and regression trees (CART) to test whether consideration of these features can yield a useful categorical reconstruction of extreme flood events or improve on the accuracy of the seasonal-total reconstructed discharge. Results suggest potential of targeted tree-ring sampling of riparian and non-riparian floodplain trees for improved information on flood variability. Information could be useful for resource planners as well as for climatologists interested in hydroclimatic variability.