USING TREE-RING WIDTHS AND CARBON ISOTOPES AS PROXIES FOR LARGE FLOOD EVENTS IN SCHOHARIE CREEK, CENTRAL NEW YORK

Three stands of white ash (Fraxinus americana L.) within the Schoharie Creek drainage basin in central New York were investigated to see if historically large floods were recorded in the rings. Two stands of trees were in the floodplain about 7 km apart, and the third was a control sample well above the floodplain about 15 km north of the others. Discharge through Schoharie Creek is generally controlled by a New York City reservoir dam above the test sites; however several large historic floods like the one on August 28^th 2011 associated with Hurricane Irene have had severe impacts in the basin. The 2011 flood was estimated to be a 500 year event and covered our test sites by more than 3m of floodwater.

To determine if we could tease out large-scale flood events from the tree-ring record we did comparisons of relative ring widths between the sites as well as δ¹³C of wood for every other year plus the years following each of the most significant floods since 1930. Ring-widths in floodplain trees are significantly smaller than their comparative upland counterparts for only half of the ten largest floods, however this includes the largest two floods in 2011 and 1996. In the year following Hurricane Irene the floodplain and upland rings have the most significant difference in δ¹³C and there is a large drop in floodplain δ¹³C following the consecutive September 2004 and April 2005 flood events which is only matched by the upland trees at the beginning of the sequence. It is likely that both are similarly affected by large precipitation events but the effect lags much longer in areas inundated by floodwater.

The implications of these findings are that about half of the severe flooding events are recorded in the ring growth in these trees and substantiated by their δ¹³C values which also reflect the severity of flooding.