Lake Hopatcong is the largest freshwater body in New Jersey, United States, about 4 square miles (10 km2) in area.

The glacier that shaped much of the New Jersey Highlands formed a long terminal moraine that forced water from the basin into the Musconetcong River.

In the early 1700s, the Great Pond stretched between Henderson Bay and today's Hopatcong State Park.

[2] More than 900 feet (270 m) above sea level, Lake Hopatcong served as a natural high point and primary water supply for the Morris Canal between the Delaware River and New York Harbor.

Other changes included the creation of Prospect Point, the backflowing of the River Styx in Hopatcong to create an inlet, and King Cove and the south end of the lake near Landing.

[6] In 1882, passenger rail service came to Lake Hopatcong, brought by the Central Railroad of New Jersey, which terminated at Nolan's Point.

Bertrand Island Amusement Park was open for the late spring, summer, and early fall seasons.

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.

Their presence attracted many famous visitors, including Babe Ruth, Bud Abbott, Bert Lahr,Lou Costello, and Milton Berle.

Doppler radar estimates of total rainfall for the 4-day period reached about 15 inches (38 cm) along the border between Sussex and Morris Counties.

[11] Every five years, starting the day after Labor Day, the New Jersey Department of Environmental Protection’s (DEP) water-level management plan allows the lake to be lowered five feet (1.5 m) to inspect the dam structure, repair lakeshore structures, control aquatic vegetation, and remove silt and drainage material from areas around the lake.

[12] From December 2008 to January 2009, the DEP left the dam open almost three weeks too long, which caused the water to drop almost a foot (0.3 m) below the normal drawdown level.

[14] However, with the large amount of rainfall that June, the lake returned to its proper level above the normal average depth.

In July 2014, there were reports of a 15 feet (4.6 m) boa constrictor in the lake,[15] but officials by August mostly figured it was a myth or a hoax.

[23] With the probable exception of the Delaware River, Lake Hopatcong is home to the greatest variety of gamefish species of any waterway in New Jersey.

Each spring, the New Jersey Division of Fish and Wildlife stocks the lake with rainbow trout, and in past years also stocked brook and brown trout, although they have not typically held over and survived the summers, due to an absence of cold, deep, oxygenated pockets of water in the lake.

The main forage is the abundant alewife herring, (Clupea vernalis), the basis of the lake's fish food chain.