Sediment Resuspension in Cannonsville Reservoir

Abstract

ABSTRACT The operation of the sediment resuspension process in Cannonsville Reservoir, NY, a eutrophic flow-augmentation and water supply impoundment for New York City, during the major drawdown year of 1995 is documented. Methodologies used in the assessment included transmissometer profiling; electron microscopy-based physical and chemical characterization of individual particles from the water column, sediments and the principal tributary; analysis of sediment trap collections; and mass balance calculations for total suspended solids (TSS). The resuspension process had several pronounced manifestations. First was the development of a conspicuous benthic nepheloid layer (BNL) which, by mid-summer, extended nearly 10 m above the bottom at one location. Second was the increase in the concentration of inorganic particles in the upper waters as the reservoir was drawn down. These particles, ultimately derived from the watershed, caused increases in turbidity. Third was the measurement of higher downward fluxes in a near-bottom sediment trap compared to a below-thermocline trap deployment and throughout the water column during the fall mixing period. Finally, the TSS deposition rate greatly exceeded estimates of TSS retention for the reservoir. The resuspension phenomenon has important management implications for this and other water supplies because it represents a source of turbidity, and because the resuspended particles interfere with widely adopted signatures of phytoplankton production. Sediment resuspension is probably promoted in this reservoir by the drawdown of the water surface.