Penacook Lake is a 362-acre (1.5 km2)[1] lake located in Merrimack County in central New Hampshire, United States, in the city of Concord. It has also been known as "Long Pond".[2] It is the largest lake in Concord and serves as the city's water supply. Water that is not captured by the city's water treatment plant flows two-thirds of a mile to the Merrimack River.