Characterization of Stream Turbidity in the Catskills, New York: Insights into Environmental Controls

Elevated turbidity poses a threat to water quality, which is especially problematic in unfiltered water supply systems such as New York City's (NYC). The Catskills Region of New York, which supplies NYC with the majority of its drinking water, is especially prone to chronically elevated turbidity due to the erosion of glacial till in Catskill streams. Here, we characterize turbidity and streamflow in the Catskills to understand the drivers of turbidity in this region. To accomplish this, we examined over a decade's worth of observed turbidity and streamflow data (2010-2022, n = 88,255) at 20 United States Geological Survey (USGS) monitoring sites. We investigated the seasonal and temporal trends in turbidity and streamflow, as well as the potential underlying causes for extreme turbidity events. Our results indicate that turbidity peaks during January through April across sites, which suggests that earlier timings of spring snow melt may contribute to elevated turbidity during these months. The turbidity baseline conditions also differ across sites, along with several sites frequently exceeding the Environmental Protection Agency (EPA) turbidity regulatory limit of 5 NTU, suggesting that certain areas of the Catskill Watershed are more susceptible to higher turbidity. Examination of extreme floods in the Catskills, such as a severe flood in December 2020 that affected the entire region, reveals that there is a characteristic process that can explain turbidity dynamics after severe flooding in this region. The December 2020 flood elevated turbidity above baseline conditions for approximately three months at several Catskill sites. There was an intermediate flood in March 2021 that could flush the easily erodible sediment that had been deposited in the channels as a result of the first flood event. However, this intermediate flood did not produce enough energy to overwhelm the system and keep turbidity above baseline conditions. Overall, our analysis proposes potential mechanisms to explain elevated turbidity events throughout the watershed and highlights the extent of the turbidity problem in the Catskills, which has important implications for water resources management of this water supply system.

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