SDCL § 7-1-48: Mellette County boundaries.

Beginning at a point on the third guide meridian west of the sixth principal meridian, where the township line between townships thirty-nine and forty north crosses the same, thence north along the guide meridian to the center of the main channel of White River; thence west and along the center of the main channel of White River to the east boundary line of the Pine Ridge Indian Reservation; thence south and along the boundary line of the Indian reservation to the point where it intersects the line between townships thirty-nine and forty north; thence east and along the township line to the place of beginning.