SDCL § 7-1-60: Sully County boundaries.

Beginning at the intersection of the third standard parallel north with the tenth guide meridian west of the fifth principal meridian, thence north and along said tenth guide meridian to the fourth standard parallel north; thence west and along said fourth standard parallel to the center of the main channel of the Missouri River; thence down the center of the main channel of said river to the said third standard parallel north; thence east and along said third standard parallel to the place of beginning.