There are 7 US Post Offices in Dewey County serving a total of 6,224 residents equating to 889 residents per post office. It's estimated that approximately 9,934 packages are handled across the 7 county post offices annually. Dewey County is a county located in the U.S. state of South Dakota. As of the 2010 census, the population was 5,301. Its county seat is Timber Lake. The county was created in 1883 and organized in 1910. It was named for William P. Dewey, Territorial surveyor-general from 1873 to 1877. Almost the entire county lies in the Cheyenne River Indian Reservation. The balance of the county, along its extreme northern county line, lies in the Standing Rock Indian Reservation. It is one of five South Dakota counties that are contained within Indian reservations."