S.D. Codified Laws § 7-1-21
Beginning at the intersection of the township line between townships seventeen and eighteen, north Black Hills base line, with the center of the main channel of the Missouri River, thence west and along said township line to its intersection with the range line between ranges twenty-one and twenty-two, east Black Hills meridian; thence south and along said range line to its intersection with the forty-fifth parallel of north latitude; thence east and along said parallel to the intersection of the range line between ranges twenty-four and twenty-five, east Black Hills meridian and the said forty-fifth parallel of north latitude; thence south to the center of the main channel of the Cheyenne River; thence easterly and along the center of the main channel of said river to the center of the main channel of the Missouri River; thence north and along the center of the main channel of the Missouri River to the place of beginning.
SDCL 7-1-21