Flood history and river flow variability recorded in tree rings on the Dhur River, Bhutan

Abstract During the 2018 WorldDendro fieldweek in Bhutan, we examined the flood history of the Dhur River. Most villages are located along streams, so knowing the flood history of the area will enable managers to prepare for future events. We collected scarred partial cross sections from 29 trees along a two km stretch of the Dhur River, and two cores per tree from 29 other trees from six species (Populus ciliata, Picea spinulosa, Tsuga dumosa, Quercus semecarpifolia, Pinus wallichiana, and Rhododendron arboreum). We identified large flood events in 2009, 1989, and 1967 from at least two trees with flood scars or traumatic rings. Our flood-scar chronology extends to 1940 with five cross-sectioned trees, and back to 1904 with core evidence from two trees. The oldest flood scar occurred in 1967. The 2009 flood scar was recorded in most of our streamside samples and is the result of heavy precipitation from Cyclone Aila at the end of May 2009. Two other storms and proceeding flood events occurred in 1989 and 1967 according to additional scars detected in several samples. This work demonstrates the successful use of density fluctuations in Pinus along with scarring in multiple species to reconstruct past flood events and identifies the effects of Cyclone Aila, as an extreme event for this area, which was unprecedented for the past hundred years.