South River is a borough in Middlesex County, New Jersey, United States. As of the 2010 United States Census, the borough's population was 16,008, reflecting an increase of 686 from the 15,322 counted in the 2000 Census, which had in turn increased by 1,630 from the 13,692 counted in the 1990 Census.What is now South River was originally formed as the town of Washington within East Brunswick Township on February 23, 1870. South River was incorporated as an independent borough by an act of the New Jersey Legislature on February 28, 1898, replacing Washington town. It was named after the South River, which marks the borough's eastern and northeastern boundary.