S.D. Codified Laws § 46A-10A-2
The board of county commissioners of each county may appoint a commission of three or more members, the total membership of which shall always be an uneven number and at least one member of which shall be a member of the board, to be known as the county drainage commission. If a commission member resigns his or her position, is unable to fulfill the duties of the position, or is removed for cause under the provisions of § 46A-10A-3, the board shall appoint a new member to the commission within thirty days. A commission may not conduct official business unless all memberships on the commission are filled and unless a majority of the members are present at a meeting of the commission. Administrative officials of the county may be appointed as ex officio members of the commission, but have no vote in commission matters.
SDCL 46A-10A-2