There are 12 US Post Offices in Whiteside County serving a total of 59,747 residents equating to 4,979 residents per post office. It's estimated that approximately 95,363 packages are handled across the 12 county post offices annually. Whiteside County is a county located in the U.S. state of Illinois. According to the 2010 census, it had a population of 58,498. Its county seat is Morrison. The county is bounded on the west by the Mississippi River. Whiteside County comprises the Sterling, IL Micropolitan Statistical Area, which is also included in the Dixon-Sterling, IL Combined Statistical Area. Former U.S. President Ronald Reagan was born in the Whiteside County community of Tampico."