Mo. Rev. Stat. § 82.110: City may be redistricted, when.

Whenever the corporate limits of any such city shall be so extended, and whenever and as often as 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 lawmaking authorities of such city, a revision 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.