S.D. Codified Laws § 7-1-57
Beginning at a point where the boundary line between the states of South Dakota and Nebraska intersects the fifth guide meridian west of the sixth principal meridian, thence north and along said guide meridian to the center of the main channel of White River; thence westerly and up the center of the main channel of said White River to its intersection with the north boundary of the Pine Ridge Indian Reservation; thence west and along the north boundary of said Indian reservation to its intersection with the south fork of the Cheyenne River; thence southwesterly and along the center of the main channel of the south fork of the Cheyenne River to its intersection with the one hundred third meridian of west longitude; thence south and along said meridian to the said boundary line between the states of South Dakota and Nebraska; thence east and along said state line to the place of beginning.
SDCL 7-1-57