S.D. Codified Laws § 7-3-3
Whenever the area of any county in this state has been altered by division or by the changing of its boundaries, it shall be the duty of the board of county commissioners of such county, at the first regular meeting thereafter, to establish commissioner districts in such county and fix the boundaries thereof in the manner provided by law for establishing commissioner districts and changing the boundaries thereof insofar as the same is applicable. Such commissioner districts so established and the boundaries so fixed shall remain as established and fixed until the same may be changed as provided by law. Whenever the organization of a new county results from such alteration, the same duty shall devolve upon the commissioners of the new county.
SDCL 7-3-3