Yankton, city (1990 pop. 12,703), seat of Yankton co., extreme SE S.Dak., on the Missouri River; inc. 1869. A railroad and trade center in an agricultural region, it has grain elevators, creameries, and plants manufacturing metal and marble products, electrical equipment, and feeds. Settled 1858 as a fur-trading post, Yankton was the Dakota territorial capital from 1861 to 1879; the old capitol building still stands. Mount Marty College and a federal prison are there. Nearby Lewis and Clark Lake, formed by Gavins Point Dam (completed 1956), is part of the Missouri River basin project.