S.D. Codified Laws § 7-1-62

Beginning at the intersection of the third guide meridian west of the sixth principal meridian with the center of the main channel of White River, thence south and along said third guide meridian to the boundary line between the states of South Dakota and Nebraska; thence east on said state line to its intersection by the tenth guide meridian west of the fifth principal meridian; thence north on said last mentioned guide meridian to its intersection with the center of the main channel of White River; thence westerly and up the center of the main channel of said White River to the place of beginning.