REPONSE OF TRANSPORT AND STORAGE OF WOOD AND SEDIMENT IN WARNER CREEK DUE TO HIGH MAGNITUDE HYDROLOGICAL EVENTS

A primary goal of the New York City Department of Environmental Protection is to provide high quality, unfiltered, surface-based drinking water to millions of NYC residents. Warner Creek, a tributary to Stony Clove Creek in the Ashokan Reservoir watershed of the Catskill Mountains of New York State, presents a difficult challenge to this goal through excessive suspended sediment loading attributable to erosion into Pleistocene glacial deposits. This study looks at stream features that reflect transport of wood and sediment, which can help determine the factors influencing channel stability and thus the processes producing turbidity from suspended sediment.

This study is confined to the lower 10,500 feet of Warner Creek, which was predicted by a 2010 assessment to be most sensitive to channel response. To evaluate these findings, a GPS-based Stream Feature Inventory (SFI) was compared with baseline SFI information from the 2010 assessment. The Stony Clove Creek watershed experienced two 25-year recurrence (~10,000 cfs) interval floods between assessments. An ArcGIS geodatabase of SFI findings and orthorectified aerial photos from 2001 and 2009 allowed analysis of the spatial distribution of Large Woody Debris (LWD), depositional features, and headcuts from both years. Further investigation included channel longitudinal profiles and bar sampling. Overall, the floods introduced more wood than the stream could transport, and most LWD jams were located at channel splits, at forest avulsions, in overflow channels, or in the channel margin of a boulder reach.

Similarly, the majority of depositional features, mostly cobble bars, have expanded since 2009. Areas of aggradation and the disappearance of a number of 2010 mapped headcuts also suggest that an increased supply of coarse sediment has helped arrest an apparent systemic degrading condition. Based on our assessment we cannot conclude that large woody debris and sediment are correlated to instability in these reaches of the stream. Continued monitoring will help determine if this flux is part of an ongoing trend toward excess aggradation from increased supply due to upstream bank erosion and decreased transport capacity or if it is part of an ongoing dynamic balance in a mountain stream.