Mo. Rev. Stat. § 91.050: Cities owning waterworks may supply other cities.

Any city in this state which owns and operates a system of waterworks may, and is hereby authorized and empowered to, supply water from its waterworks to other municipal corporations for their use and the use of their inhabitants, and also to persons and private corporations for use beyond the corporate limits of such city, and to enter into contracts therefor, for such time, upon such terms and under such rules and regulations as may be agreed upon by the contracting parties.