Mo. Rev. Stat. § 234.170
If a bridge located wholly within any one county be necessary over any watercourse, and the county commission of such county shall determine that such bridge cannot be built, reconstructed or maintained at the expense of said county on account of the lack of funds and power to raise funds, then said county commission may grant the right to such person, partnership or corporation, as they shall think proper, to build or reconstruct such bridge as a toll bridge in such manner and upon such terms and conditions as shall be agreed upon, and empower such person, partnership or corporation to charge, collect and receive such tolls as shall be determined as maximum tolls by the said county commission at the time of empowering or granting said rights. Said person, partnership or corporation shall then build or reconstruct and maintain said bridge at their own expense; provided, this section shall not apply to bridges on state roads.
§ 234.170, RSMo