SDCL § 7-1-16: Corson County boundaries.

Beginning in the center of the main channel of the Missouri River where the boundary line separating the states of South Dakota and North Dakota crosses the same, thence west and along said state line to the one hundred second meridian west longitude; thence south and along said meridian to the township line between townships seventeen and eighteen, Black Hills base line; thence east and along said township line to the center of the main channel of the Missouri River; thence northerly and along the center of the main channel of said Missouri River, including all the islands in said river, to the place of beginning.