DIAGNOSTIC ASSESSMENT OF WARNER CREEK'S FLUVIAL GEOMORPHIC RESPONSE TO 2010-2012 HYDROLOGY IN THE CATSKILL MOUNTAINS OF NEW YORK STATE

The Ashokan Reservoir watershed in the Catskill Mountains of New York State supplies New York City (NYC) with over 40% of its water. Due to a recent increase in frequency of high magnitude floods, turbidity from suspended sediment entrainment poses a threat to the unfiltered water supply. Stream erosion into the glaciated mountain landscape is the dominant process that leads to occasionally excessive turbidity conditions in the higher-relief, eastern half of the Catskill Mountains. Warner Creek, a 12.5 km-long tributary of Stony Clove Creek in the Ashokan watershed, is a documented source of chronic suspended sediment loading. In 2010, fluvial geomorphic features along Warner Creek were mapped along the entire length of the stream to provide a baseline for subsequent investigations. The lower 3.2 km were remapped in 2011 and 2012 with high resolution GPS technology and ArcGIS to evaluate the channel’s geomorphic response to several high magnitude flow events that occurred during the study period. Measurements indicate that Warner Creek is currently in a state of disequilibrium. Headcut formation into underlying glacial deposits and subsequent migration is destabilizing the stream, increasing bank erosion, and sediment load. Trends from 2010 to 2012 show a 6% increase in erosion, with a subsequent 7% increase in deposition between 2011 and 2012 as the channel changes laterally following vertical adjustments. Even with increasing bank erosion, in-stream Large Woody Debris (LWD) deposits decreased in 2012, indicating that large flood events not only bring LWD into the stream but also play a key role in its removal. Assuming a stable hydrologic regime, Warner Creek will continue to approach quasi-equilibrium through time. Continued annual assessment of Warner Creek will be necessary to confirm the apparent trend in channel adjustment and flood hydrology. This research can inform management decisions to accommodate future channel response to a changing hydrologic regime.