Dewey County is the Eastern half of the Cheyenne River Reservation. It is bounded by Corson county to the north, Walworth, Potter, and Sully to the east, Stanley to the south and Ziebach to the west. The major towns and the population, in this county are Timber Lake, pop.517. Isabel, pop. 319. Green Grass, Whitehorse, La Plant, Ridgeview, Parade, Eagle Butte, pop. 489, and Lantry. The Missouri River flows along the entire eastern and southern boundaries of the county.