Stony Clove Basin Sediment and Turbidity Monitoring

Problem

Suspended-sediment concentration (SSC) and turbidity are primary water-quality concerns in New York City’s (NYC) water-supply system (U.S. Environmental Protection Agency, 2007). 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). Sediment can originate from the watershed land surface and the active stream corridor (the stream bed and its adjacent banks and hillslopes) (Walling 2005). In the upper Esopus Creek watershed, the main source of water to the Ashokan Reservoir, the active stream corridor is the primary source of sediment and turbidity to the stream. Terrestrial sources of sediment and turbidity are created when areas of erodible sediments coincide with areas of transport to the stream (Church 2002). In some cases the sources are in contact with the stream itself. A process-level understanding of sediment sources and transport pathways is required to develop effective strategies to reduce stream sediment and turbidity. The source areas and transport pathways must be identified and the source stabilized or the transport pathway disconnected from the source—or both of these issues must be addressed. In cases where the streambed or stream bank is the primary source of sediment, stream stabilization projects are required to mitigate the problem (Rosgen 1997). Without a process-level understanding of sediment and turbidity sources and transport pathways, remediation efforts will likely produce only short-term benefits or may even further exacerbate the problem by enabling other sources to make contact with the stream (Rosgen 1997).

Previous sediment and turbidity studies in the upper Esopus Creek watershed (McHale and Siemion 2014) have identified Stony Clove Creek as the largest contributor of suspended sediment and turbidity to the upper Esopus Creek. Several bank failures and hill slope mass failures adjacent to and in contact with the stream have exposed glacial and glacial lacustrine sediments to the stream creating a chronic source of suspended sediment and turbidity to Stony Clove Creek. New York City Department of Environmental Protection (NYCDEP) has begun to address this problem by cataloging stream bank erosion, slope failures, exposed geology and collecting other geomorphic data to create stream feature inventories for the watershed. The geomorphic assessments have been used to identify priority stream reaches for stream restoration projects and 4 projects intended to reduce

turbidity and suspended sediment loading were completed between 2012 and 2014. Longitudinal water quality monitoring could help to identify stream sections that contribute disproportionately to turbidity levels and suspended sediment load in the watershed. Identifying those problem sections will in turn improve project site identification and prioritization as well as evaluation of project effectiveness at reducing turbidity levels and suspended sediment loads.

Section 4.6 of the most recent Filtration Avoidance Determination (FAD) agreed upon by NYCEP, the United States Environmental Protection Agency (EPA), and the New York State Department of Health (NYS-DOH), requires NYCDEP to evaluate the effectiveness of stream restoration work in reducing turbidity. To address this requirement existing stream restoration projects need to be monitored and new projects need to be evaluated pre-, and post implementation. As per the FAD schedule of deliverables NYCDEP submitted a proposal in November 2014 outlining a set of studies through a 10 year period intended to improve the characterization and understanding of stream corridor suspended sediment sources in the Stony Clove Creek watershed. These studies are also intended to help evaluate the effectiveness of stream restoration projects to reduce turbidity and suspended sediment based on the improved characterization of sources (NYCDEP 2014).

Water quality monitoring at the watershed and stream reach scale, in combination with stream feature inventories will help provide the process-level understanding necessary to (1) characterize the longitudinal variability in turbidity sources and suspended sediment loads; (2) prioritize stream reaches for turbidity reduction projects; and (3) design effective stream restoration projects that will result in long-term stream stabilization and improvements in water quality. Post-implementation monitoring is necessary to assess the short and long-term effectiveness of stream restoration projects over a range of hydrologic conditions. Monitoring suspended sediment and turbidity at the Stony Clove Creek watershed outlet will provide a measure of the collective effect of all the stream restoration projects on water quality within the watershed. However, stream reach scale monitoring is necessary to evaluate the effectiveness of individual stream restoration projects. This project-scale monitoring can be used to assess the relative benefits of specific stream restoration projects in specific geomorphic and geologic settings. The NYCDEP Stream Management Program can use those project-scale assessments to identify the most cost effective stream management practices and target the highest priority stream reaches with those practices. This study is designed to improve turbidity source characterization at the reach scale, to evaluate turbidity reduction achieved by specific stream restoration projects, and provide data to evaluate the effectiveness of stream management practices at the watershed scale.

Objectives

The objectives of this project are to: 1) Characterize the variability of suspended sediment concentration and turbidity levels among several stream reaches within the Stony Clove watershed, and 2) Evaluate the effectiveness of stream restoration projects using the reach-level suspended sediment and turbidity characterization. The goal of this research is to provide 5 years of data that can be used with stream feature inventories to gain a process-level understanding of the sources and transport pathways of turbidity and suspended sediment in the Stony Clove watershed. This project will also evaluate the effectiveness of previous stream restoration projects within the Stony Clove watershed and determine how well stream management practices can affect suspended sediment yields and turbidity levels at the stream reach and watershed scale. Finally the results of this project will be used to help identify other potential turbidity reduction project sites.

Approach

The objectives of this project will be accomplished by monitoring SSC and turbidity throughout a range of discharge conditions at 2 main stem locations on Stony Clove Creek and at 4 tributary locations during a 5-year period. An additional 8 main stem and 6 tributary sites will be monitored for turbidity only using in situ probes (fig. 1). All proposed monitoring locations were chosen in coordination with the NYCDEP personnel and based on results from previous work in the watershed. These locations bracket known and probable sources of suspended sediment and turbidity and existing and potential future stream restoration projects.

This approach will allow us to estimate suspended sediment loads at all major tributaries to Stony Clove Creek and evaluate in-stream sources of sediment and turbidity along the main channel. The 5 year monitoring period should allow us to capture a wide range of flow conditions as well as characterize differences in turbidity levels and suspended sediment concentrations and loads as they are affected by season, discharge, and antecedent moisture conditions. Turbidity levels and suspended sediment concentrations and loads will be integrated with stream feature inventory data (including channel morphology, geology, and geometry) to evaluate how specific stream features affect turbidity and suspended sediment. The stream feature inventory data will be provided by the Ashokan Watershed Stream Management Program (AWSMP) and data interpretation of stream feature inventories will be completed in cooperation with NYC-DEP personnel.

Discharge, suspended sediment concentration, and turbidity have been collected at the U.S. Geological Survey (USGS) monitoring station at Stony Clove Creek at Chichester (USGS Gaging Station Number: 01362370) for the past 13 years. The Stony Clove station will allow the data collected during this study to be placed into context within that longer record. During this study period the Stony Clove Creek monitoring station will be funded through a separate agreement that focuses on the larger upper Esopus Creek watershed.

Turbidity will be measured every 15 minutes with in situ probes at existing and future stream restoration projects. The probes will be located along the stream above and below restoration projects. This approach is intended to identify which projects are most effective at reducing stream-water turbidity. Measurements will be taken for 3-4 years before construction of new projects and 1-2 years after those projects have been completed. We will evaluate the cumulative effect of all stream restoration projects constructed prior to and during this 5 year study period using data from the long term monitoring station at Stony Clove Creek at Chichester. USGS will also evaluate the effects of specific stream restoration projects with turbidity data collected upstream and downstream from the project sites before and after stream restoration projects and suspended sediment data collection at 6 locations throughout the watershed (table 1). Ideally a minimum of 3-4 years post-construction monitoring is needed before the evaluation for a specific project is considered sufficient (NYC-DEP, 2014). Therefore some of the evaluation would need to extend beyond the time period of this study.

Channel morphology and sediment sources vary throughout the watershed; as a result the methods used to stabilize those sources also vary depending on stream reach and slope failure characteristics. This project will relate those physical characteristics to SSC and turbidity levels. Information describing the stabilization methods used, channel morphology, and sediment/turbidity sources at the existing and future projects will be provided by NYCDEP and AWSMP.

Benefit

This project will help address specific parts in the NYCDEP proposal intended to satisfy requirements in Section 4.6 of the 2007 Filtration Avoidance Determination (NYCDEP 2014); specifically, to improve turbidity and suspended sediment source characterization for stream management implementation and to evaluate the effectiveness of stream restoration projects at reducing turbidity and thereby improve water quality. Results from the study will be used by the NYCDEP Stream Management Program to prioritize areas of stream bank instability and target those areas with the most cost effective practices to reduce stream turbidity. This project also addresses several of the priorities identified in the upper Esopus Creek Management Plan including: 1) comprehensive monitoring and surveillance of suspended sediment in the upper Esopus Creek, 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 watershed sources of sediment and turbidity as a function of geomorphology and stream bank stability. The results of this study will also be used in that larger effort. This project fulfills the USGS Water Resources mission to provide reliable, impartial, and 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.

Project Location by County

Catskill Region: Delaware County, NY, Greene County, NY, Schoharie County, NY, Sullivan County, NY, Ulster County, NY