Bathymetry of Ashokan, Cannonsville, Neversink, Pepacton, Rondout, and Schoharie Reservoirs, New York, 2013–15

Drinking water for New York City is supplied from several large reservoirs, including a system of reservoirs west of the Hudson River. To provide updated reservoir capacity tables and bathymetry maps of the City’s six West of Hudson reservoirs, bathymetric surveys were conducted by the U.S. Geological Survey from 2013 to 2015. Depths were surveyed with a single-beam echo sounder and real-time kinematic global positioning system along planned transects at predetermined intervals for each reservoir. A separate quality assurance dataset of echo sounder points was collected along transects at oblique angles to the main transects for accuracy assessment. Field-survey data were combined with water surface elevations in a geographic information system to create three-dimensional surfaces in the form of triangulated irregular networks (TINs) representing the elevations of the reservoir geomorphology. The TINs were linearly enforced to better represent geomorphic features within the reservoirs. The linearly enforced TINs were then used to create raster surfaces and 2-foot-interval contour maps of the reservoirs. Elevationarea-capacity tables were calculated at 0.01-foot intervals. The results of the surveys show that the total capacity of the West of Hudson reservoirs decreased by 11.5 billion gallons (Ggal), or 2.3 percent, because of sedimentation since construction, and the useable capacity (the volume above the minimum operating level required to deliver full flow for drinking water supply) has decreased by 7.9 Ggal (1.7 percent). The available capacity (the volume between the spillway elevation and the lowest intake or sill elevation used for drinking water supply) decreased by 9.6 Ggal (2.0 percent), and dead storage (the volume below the lowest intake or sill elevation) decreased by 1.9 Ggal (11.6 percent). The elevation of the spillway at Schoharie Reservoir was changed because of reconstruction during 2015, resulting in an additional decrease of 0.1 Ggal in total, useable, and available capacity.