There are 5 US Post Offices in Corson County serving a total of 1,308 residents equating to 262 residents per post office. It's estimated that approximately 2,088 packages are handled across the 5 county post offices annually. Corson County is a county located in the U.S. state of South Dakota. As of the 2010 census, the population was 4,050. Its county seat is McIntosh. The county was named for Dighton Corson, a native of Maine, who came to the Black Hills in 1876, and in 1877 began practicing law at Deadwood. The county is encompassed within the Standing Rock Indian Reservation, which extends into North Dakota. The Lakota people reside primarily in the South Dakota part of the reservation; the Yanktonai and Dakota live in that part in North Dakota. The Grand River, a tributary of the Missouri River, runs through the reservation."