South Mountain Reservation, covering between 2,047 and 2,112 acres, depending on the source, 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.

The Rahway River is a river in Essex, Middlesex, and Union Counties, New Jersey, United States, The Rahway, along with the Elizabeth River, Piles Creek, Passaic River, Morses Creek, the Fresh Kills River, has its river mouth at the Arthur Kill.