Effects of A 4-Percent Annual Exceedance Probability Flood On Stream Restoration Projects In The Esopus Creek Watershed, Ny

On December 25, 2020, 5 inches 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 percent (recurrence intervals of 25 to 5 years) in streams across the watershed, resulted in substantial geomorphic changes in some stream channels, and transported the highest sediment loads observed since stream restoration projects in the UEC began in 2012. The lowest AEP and highest discharge were observed in the Stony Clove Creek subbasin of the UEC. 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 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 9 sediment and turbidity reduction projects (STRPs) implemented in tributaries to the UEC between 2012 and 2018. Seven of the STRPs were constructed in the Stony Clove Creek subbasin. During the 2 to 8 years following the installation of the projects, declines in SSC and turbidity were measured at all monitoring sites although there were no flows that exceeded a 20-percent AEP flood. The flood of December 2020 had a 4-percent AEP at the watershed outlet (Stony Clove Creek near Chichester, 01362370) and provided an opportunity to assess the effectiveness of the STRPs following a large flood. Data from the Stony Clove Creek near Chichester stream gage suggest a 10-fold increase in the mass of sediment transported for a given flow relative to pre-storm conditions, and that increase persisted for months following the flood. Sediment transport returned to near pre-storm levels at low streamflows after 4-6 months, but continued to remain above pre-storm levels at high streamflows for more than a year. The STRPs were bracketed by upstream-downstream turbidity monitoring stations. Minimal increases in turbidity from upstream to downstream of the STRPs during and after the flood were observed; however, reach-scale monitoring in tributaries to Stony Clove Creek and field observations suggest that the large increases in post-flood sediment transport were due to the Stony Clove intersecting new sources of sediment during and after the flood rather than failure of the STRPs.