Riverdale is a borough in Morris County, New Jersey, United States. As of the 2010 United States Census, the borough's population was 3,559, reflecting an increase of 1,061 from the 2,498 counted in the 2000 Census, which had in turn increased by 128 from the 2,370 counted in the 1990 Census.Riverdale was incorporated as a borough by an act of the New Jersey Legislature on March 12, 1923, from portions of Pequannock Township, subject to the results of a referendum passed on April 17, 1923.