The Rahway River drainage basin encompasses approximately 41 square miles in Union, Essex, and Middlesex counties in the northeastern part of the U.S. state of New Jersey.
The Rahway River consists of four separate branches that converge in Rahway, from whence it flows as a single waterway to its mouth at the Arthur Kill.
The longest, or West Branch, courses for 24 miles from Verona.
The South Branch, which originates in Woodbridge, and the Robinson's Branch, which begins in Scotch Plains, join the main stem in Rahway.
The upper portion above Rahway consists of floodplains, woodlands and freshwater marshes; the lower portion includes saltwater marshes and tidal flats.