Background
The Ashokan Reservoir is located in the Catskill Mountains of New York State and is part of New York City’s (NYC) water supply system. The NYC water-supply system is operated by the NYC Department of Environmental Protection (NYCDEP) under a filtration avoidance determination (FAD) issued by the New York State Department of Health. The Ashokan Reservoir watershed is 255 mi2 and is one of two reservoirs in the New York City Catskill Reservoir system and one of six reservoirs in the West-of-Hudson Catskill-Delaware system. The upper Esopus Creek is the primary tributary to the Ashokan Reservoir, with a 192 mi2 watershed from the source, Winnisook Lake, to the Ashokan Reservoir near Boiceville, NY. Suspended-sediment concentration (SSC) and turbidity are primary water-quality concerns in NYC’s water-supply system (NYSDOH, 2017). High SSC and turbidity mobilized during large storms and transported to the Ashokan Reservoir from the Esopus Creek watershed can require operational changes to the water supply system or treatment with aluminum sulfate to prevent exceedance of turbidity regulatory criteria (NYCDEP, 2017). In the NYC water-supply system turbidity is largely caused by clay and silt rather than organic material (Effler et al. 1998, Peng et al. 2002, 2004).
Preliminary results from recent sediment and turbidity monitoring, sediment fingerprinting, and geomorphic assessments suggest the primary sources of chronic, low flow and post-flood suspended sediment loading are glacio-lacustrine silt and clay deposits in contact with or adjacent to the active stream channels. From 2009 to 2012, SSC and turbidity were measured at 14 monitoring sites throughout the upper Esopus Creek watershed by the United States Geological Survey (USGS) in cooperation with the NYCDEP. The Stony Clove Creek consistently produced higher SSCs and turbidity than any of the other Esopus Creek tributaries. The current FAD requires the NYCDEP to implement sediment and turbidity reduction projects (STRPs) in the Ashokan watershed and to conduct water-quality monitoring studies of suspended sediment and turbidity in the basin (NYSDOH, 2017). Six STRPs have been completed in the Stony Clove Creek watershed, one in the Beaver Kill watershed, and one in the Woodland Creek watershed since 2012. Some of the STRPs were designed and constructed in response to erosion into turbidity source sediment caused by tropical storm Irene and others were implemented to meet the FAD requirements. Four of the Stony Clove Creek watershed STRPs were monitored for SSC and turbidity pre and post construction from 2011 to 2014. Results showed SSC and turbidity were significantly reduced after the projects were completed (Siemion et al, 2016) and current monitoring in the upper Esopus Creek watershed suggests that the Stony Clove is no longer the greatest tributary source of suspended sediment and turbidity (NYCDEP, 2019a). At least three additional STRPs are planned for Stony Clove watershed through 2021 and the rest of the Esopus Creek watershed through 2026 (NYCDEP, 2019b).
In 2016 the NYCDEP and USGS entered into a collaborative agreement for the USGS to conduct a comprehensive study of suspended sediment and turbidity in the upper Esopus Creek watershed in response to the FAD requirement for sediment and turbidity monitoring. The study was planned as a 10-year study, divided into two contractual periods to allow for changes to the study design based on lessons learned during the initial 5-year period. A multi-tiered monitoring design was used to provide an analysis of the effect of the STRPs on stream water from the stream reach to the sub-basin and watershed scale. The general objectives of the study are outlined in the upper Esopus Creek Watershed Turbidity/Suspended Sediment Monitoring Study: Project Design Report (NYCDEP, 2017):
- Monitor SSC and turbidity through a range in streamflow at the sub-basin scale to evaluate how differences in streamflow, geology, and geomorphology affect SSC and turbidity and to evaluate the effectiveness of STRPs at reducing suspended sediment and turbidity.
- Monitor and characterize the variability of SSC and turbidity among several stream reaches within the Stony Clove Creek watershed and evaluate the effectiveness of STRPs using the reach-level suspended sediment and turbidity characterization.
Approach
The objectives of this project are accomplished by collecting discrete SSC and turbidity samples throughout a range in stream discharge conditions and monitoring in situ turbidity at a 15 minute time step during a 5 year period at 9 primary monitoring stations within the upper Esopus Creek watershed. At 1 secondary station monitoring will be confined to in situ turbidity. The monitoring stations were chosen in coordination with the NYCDEP and based on previous work in the basin (McHale and Siemion 2014). The data are used to quantify the contribution of each tributary to the total suspended sediment load of upper Esopus Creek, to compare loads among the tributaries, and to investigate patterns in SSC and turbidity along the main channel. Reach scale monitoring in the Stony Clove Creek subbasin is conducted with 2 mainstem and 4 tributary primary monitoring sites, and 14 secondary monitoring sites. The reach scale monitoring brackets existing and future STRP locations and is used to link water quality results to geologic and geomorphic measurements at the reach scale. A sediment fingerprinting component was added to the study in 2022. This component of the study will apportion the sediment load to alluvial, galcial lacustrine, glacial till, and near stream forest soil sources.
Benefit Statement
This project will address requirements in the NYCDEP proposal to fulfill the water quality study objectives in Section 4.6 of the 2007 Filtration Avoidance Determination; specifically, to monitor suspended sediment and turbidity levels within the upper Esopus Creek watershed and its tributaries, characterize SSC and turbidity through a range in discharge at each monitoring station, and provide data to inform stream management strategies. This project will help to evaluate the effectiveness of stream restoration projects in the Stony Clove watershed and put those results into context within the larger upper Esopus Creek watershed. This project addresses several of the priorities identified in the Upper Esopus Creek Management Plan including: 1) comprehensive monitoring and surveillance of the Upper Esopus for suspended sediment, 2) information that can be used to protect, restore, enhance, or reduce impairments to aquatic habitat by helping prioritize tributaries for their contributions to suspended sediment loadings and 3) data to aid general non-point source program implementation. The project will also address the two major water quality impairments identified in the upper Esopus Creek watershed, turbidity and sedimentation within the Ashokan Reservoir watershed (CCE 2007).
This research is part of a larger evaluation of upper Esopus Creek geomorphology and stream bank stability being conducted by the New York City Department of Environmental Protection (NYCDEP) and Ulster County Soil and Water Conservation District (UCSWCD). This project fulfills the USGS Water Resources mission to provide reliable, impartial, timely information that is needed to understand the Nation’s water resources and effectively manage surface-water resources for domestic, agricultural, commercial, industrial, recreational, and ecological uses.
