South Mountain Reservation, covering 2,110, 2,112, 2,000, 2,100, or 2,047 acres (8.539, 8.546, 8.094, 8.498, or 8.284 km2) depending on who you ask, is a nature reserve on the Rahway River that is part of the Essex County Park System in northeastern New Jersey. It is located in central Essex County, New Jersey within portions of Maplewood, Millburn, and West Orange. It borders South Orange between the first and second ridges of the Watchung Mountains.