This is Lake Surprise, located at the center of the Watchung Reservation.
It’s kind of a weird name, but here’s what Union County has to say about it:
In 1845, David Felt built a secondary water source for powering his mill at Feltville by damming the Blue Brook. Feltville Lake was renamed Ackerman Lake when the summer resort of Glenside Park opened. By the time the Union County Park Commission bought the property for inclusion in the Watchung Reservation, the lake was called Silver Lake and was a popular place for swimming and boating. The park planners laid out a road to the lake that wound its way down through the woods such that the lake could not be seen until you were right next to it. Hence the new name – Lake Surprise.
According to the same reference, Lake Surprise was restored in 2000, and all the crud that was dredged from the bottom got scattered over a nearby field (previously used by Boy Scouts).
County of Union, New Jersey. “Watchung Reservation History Trail.” http://ucnj.org/community/parks-community-renewal/parks-facilities/watchung-reservation-history-trail/.