S.D. Codified Laws § 7-1-12
Beginning at a point where the base line between townships one hundred and one hundred one north intersects the center of the main channel of the Missouri River, thence southeasterly and along the center of the main channel of said river to its intersection with the boundary line between the states of South Dakota and Nebraska; thence southeasterly and along said state line to the center of the main channel of Choteau Creek; thence northerly and along the center of the main channel of said Choteau Creek to the northeast corner of the former Yankton-Sioux Indian Reservation; thence northwesterly and along the northern boundary of said former Indian reservation to the northwest corner thereof; thence northerly and along the range line between ranges sixty-six and sixty-seven west of the fifth principal meridian to the base line between townships one hundred and one hundred one north; thence west and along said base line to the place of beginning.
SDCL 7-1-12