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is the largest freshwater body in the state of New Jersey, USA, approximately 4 square miles (10 km²) in area. The lake is located in the mountains of northern New Jersey, north of Netcong and along the border between Sussex and Morris counties.
The lake is within the borders of four municipalities, Hopatcong, Jefferson Township, Roxbury Township, and Mount Arlington. In the eighteenth century, the lake existed in two separate parts known as the "Great Pond" on the south and "Little Pond" on the north.
A natural body of water, the area of the lake has been enlarged by the damming of the Musconetcong River, its natural outlet, on the southern side of the lake.
Before the arrival of Europeans, the current lake area contained two separate lakes that were the sites of Lenape villages. The waters of the lakes were important to the traditions of the matrilineal clans of Native American who had lived in the area for thousands of years. They farmed, fished, and hunted here. Following their traditions, many Amerindians came to this lake to drink before significant ceremonies.
In 1750, a dam was built on the Musconetcong River where it empties from the lake, raising the water level by approximately six feet (2 m) and joining the two lakes into one.
In 1831, a feeder channel was constructed from the lake to supply water to the Morris Canal. The dam on the outlet of the lake has been maintained for flood control.
With the arrival of passenger rail service in 1883, the lake became a popular summer resort. Bertrand Island Amusement Park, which flourished until the 1930s, was a major attraction that was open for the late spring, summer, and early fall seasons. After World War II the park was open only during the summer. The amusement park closed completely in 1983, but the lake continues to be a popular recreational destination for the region.
Lake Hopatcong hosted several strong chess tournaments in the 1920s, including the Ninth American Chess Congress in 1923, which was won by Frank Marshall and Abraham Kupchik, and another tournament in 1926, which was won by world champion José Raúl Capablanca.
Lake Hopatcong, unlike most lakes in New Jersey (other than Greenwood Lake on the New York border and Lake Lackawanna in Byram) has bars and restaurants that are accessible directly by boat. Popular tourist attractions on the lake include the Hopatcong State Park, The Lake Hopatcong Historical Museum, and The Lake Hopatcong Yacht Club.
On November 22, 2005, the EPA gave the Lake Hopatcong Commission a federal grant of $745,000 to improve the water quality and prevent phosphorus from entering the lake.