Mo. Rev. Stat. § 68.110
Any county of the third classification with a population greater than four thousand four hundred but less than five thousand, any county of the third classification greater than six thousand nine hundred but less than seven thousand, any county of the third classification with a population greater than seven thousand five hundred but less than seven thousand six hundred, any county of the third classification with a population greater than eight thousand four hundred seventy but less than eight thousand five hundred fifty, any county of the third classification with a population greater than nine thousand but less than nine thousand two hundred, any county of the third classification with a population greater than ten thousand but less than ten thousand five hundred, any county of the third classification with a population greater than fifteen thousand six hundred but less than sixteen thousand, and any county of the third classification with a population greater than twenty-seven thousand six hundred but less than twenty-eight thousand shall be included in the jurisdiction of the Mid-America port commission agreement.
§ 68.110, RSMo