Mo. Rev. Stat. § 236.255

In any case where a dam has been erected for a period of thirty years or more, and property above the dam within one mile of the point where the dam is situated has been developed for use of the impounded waters above the dam, the owner of the land on either side of the stream at the point where the dam is located shall have the right and authority to repair, maintain or reconstruct the dam, even though the dam is not used for the operation of any mill, electric power and light works, or other machinery.