Located in Ulster County, about 13 miles west of Kingston and 73 miles north of New York City. Formed by the damming of the Esopus Creek, which eventually flows northeast and drains into the Hudson River. Consisting of two basins separated by a concrete dividing weir and roadway, it holds 122.9 billion gallons at full capacity and was placed into service in 1915.
The Ashokan is one of two reservoirs in the City's Catskill Water Supply System. The other is the Schoharie, located 27 miles to the north, whose water flows into the Ashokan via the Shandaken Tunnel and the Esopus Creek. Including the water it receives from the Schoharie Reservoir, the Ashokan supplies about 40% of New York City's daily drinking water needs in non-drought periods. Water enters the Ashokan's West Basin and, after a settling period, is withdrawn from its East Basin. It is carried southeast under the Hudson River via the 92-mile Catskill Aqueduct, which has a maximum depth of 1,114 feet. It ordinarily enters the Kensico Reservoir in Westchester for further settling, where it mixes with Delaware system water and then travels south in two aqueducts before entering New York City's water supply distribution at the Hillview Reservoir in Yonkers, just north of the City line.
The Ashokan watershed's drainage basin is 255 miles and includes parts of 11 towns: Denning, Hardenburgh, Hurley, Kingston, Marbletown, Olive, Shandaken and Woodstock in Ulster County; Hunter and Lexington in Greene County and Middletown in Delaware County.