Watchung Reservation is the largest nature reserve in Union County, New Jersey, United States. The Watchung area is located on a ridge within northern-central New Jersey, as it consists mainly of the upper valley of Blue Brook, between the ridges of the First Watchung Mountain and Second Watchung Mountains. A dam near the headwaters of the creek creates Lake Surprise.

Mountainside is a borough in Union County, in the U.S. state of New Jersey. The borough is located on a ridge in northern-central New Jersey, within the Raritan Valley and Rahway Valley regions in the New York metropolitan area. As of the 2020 United States census, the borough's population was 7,020, an increase of 335 (+5.0%) from the 2010 census count of 6,685, which in turn reflected an increase of 83 (+1.3%) from the 6,602 counted in the 2000 census.