The Rahway River rises in Essex County as two separate branches. The West Branch begins at spring-fed Crystal Lake in West Orange and flows south through the South Mountain Reservation in the valley between first and second ridges of the Watchung Mountains. It runs directly through downtown Millburn.

The East Branch rises as a surface stream in West Orange and forms part of the boundary between West Orange and Orange, then travels through the towns of South Orange and Maplewood. The two branches meet at Hobart Gap near Interstate 78, continuing south through the Union County communities of Springfield, Union, Cranford and Clark.

In Rahway the river receives the Robinson’s Branch and South Branch, which are approximately 10 miles (16 km) long. The South Branch starts in Roosevelt Park in Edison behind the Menlo Park Mall, and flows through Edison, Iselin, and Rahway. The river’s mouth is at the Arthur Kill between Carteret (on the south) and Linden (on the north), opposite Port Mobil on Staten Island. Source: Wikipedia.