Effects of a large flood on sediment and turbidity reduction projects in the Esopus Creek watershed, NY

Abstract

On December 24-25, 2020, 7.3 to 14.6 cm of rain fell on a large snowpack in the upper Esopus Creek (UEC) watershed in the Catskill Mountains of New York. The resulting flood had an annual exceedance probability (AEP) of 4 to 20% (recurrence intervals of 25 to 5 years) in streams across the watershed, resulted in substantial geomorphic adjustments in some stream channels, and transported the highest sediment concentrations observed since stream restoration projects in the UEC began in 2012. The largest flooding occurred in the Stony Clove Creek subbasin of the UEC which contains 8 sediment and turbidity reduction projects. The UEC is the primary water source for the Ashokan Reservoir, part of New York City’s unfiltered water-supply system. A network of 16 turbidity-only and 13 suspended sediment and turbidity monitoring stations has been in operation within the UEC since October 2016. One of the primary purposes of this monitoring network is to investigate changes in suspended-sediment concentrations (SSC) and turbidity resulting from sediment and turbidity reduction projects (STRPs) implemented in tributaries to the UEC between 2012 and 2018. During the 2 to 8 years following the installation of the projects and prior to the 2020 flooding, declines in SSC and turbidity were measured at all monitoring sites although there were no flows that exceeded a 50% AEP flood. The flood of December 2020 had a 4-percent AEP at the subbasin outlet (Stony Clove Creek below Ox Clove at Chichester NY, USGS station number 01362370) and provided an opportunity to assess the effectiveness of the STRP following a large flood. An order of magnitude increase in suspended-sediment concentration per unit discharge was measured at the outlet of the Stony Clove Creek subbasin following the flood. Increased SSC persisted for 3 months throughout the range in discharge and for at least 1 year at high discharges following the flood. The concentration-discharge relation returned to near pre-flood levels at low discharges but continued to remain above pre-flood levels at high discharges for more than 1 year. Mapped bank erosion increased in all Stony Clove subbasins following the flood and increases in stream contact with clay-rich glacial till and lacustrine sediments were greater relative to increases in contact with alluvium. Large increases in sediment concentration were observed where contact with glacial lacustrine material also increased. Minor increases in sediment concentration per unit discharge were measured from stream reaches where STRP were constructed and substantially less erosion was noted within those reaches relative to non-STRP reaches, though some breaches in revetments were noted.