Watchung Reservation is the largest nature reserve in Union County, New Jersey, United States. Covering 1,945 acres, it is bounded by the city of Summit, the borough of Mountainside, and the townships of Berkeley Heights, Scotch Plains, and Springfield. The reservation consists mainly of the upper valley of Blue Brook, between the ridges of First Watchung Mountain and Second Watchung Mountain. A dam near the headwaters of the creek creates Lake Surprise.