Mo. Rev. Stat. § 81.210

Whenever the corporate limits of any city now coming or which may hereafter come under the operation of section 81.200 of this chapter shall hereafter be altered or extended, and whenever the population of any such city, or of any ward or wards thereof, has been or may be so increased or diminished as to render, in the opinion of the common council, a division or redistricting of the corporation into wards, or a change in the boundary of any ward or wards necessary, the same may be done by ordinance at some regular meeting of the common council.