Mo. Rev. Stat. § 242.160
In the same month of each year after the election of the first board of supervisors, the board of supervisors shall call a meeting of the owners of land and other property in the district, in the same manner as is provided for in section 242.150, and such owners shall meet at the time and place fixed by the board of supervisors and elect one supervisor therefor in like manner as prescribed in section 242.150, who shall hold his office for five years or until his successor is elected and qualified; and in case of a vacancy in any office of supervisors the remaining supervisors may fill such vacancy until the next annual meeting, when a successor shall be elected for the unexpired term; provided, that after the report of the commissioners has been confirmed by the court under the provisions of section 242.280, only the land and other property having benefits assessed against it shall be entitled to vote at the annual meetings held under the provisions of this section.
§ 242.160, RSMo