S.D. Codified Laws § 7-1-43
Beginning at the intersection of the center of the main channel of the Missouri River with the base line forming the southern boundary line of the state of Minnesota extended across the Missouri River, thence west and along said base line to the tenth guide meridian west; thence north and along said tenth guide meridian to its intersection with the center of the main channel of White River; thence west and along the center of the main channel of said White River to its intersection with the west line of the fifth principal meridian survey, being the west range line of range seventy-nine west of said survey; thence north and along said west range line to the northern township line of township one hundred eight north of range seventy-nine west; thence east and along said north line of township one hundred eight north to the center of the main channel of the Missouri River; thence southeasterly down the center of the main channel of the said Missouri River to the north end of the island known as American Island; thence along the west bank of said island to the south end of said island to the point at or near the center of the main channel of said river; thence down the center of the main channel of said river to the place of beginning.
SDCL 7-1-43