Mich. Comp. Laws § 120.21: Commodities; tolls, fees, rents; approval.

Sec. 21.

Each port district shall have power to fix and collect tolls, fees, rents and other charges for the use of the services, property, facilities and commodities furnished by it, subject to review and approval of a majority of the members present and voting of the board or boards of supervisors of the county or counties in which the port district is situated. The tolls, fees, rents and other charges shall at no time be less than necessary to satisfy the requirements of any statute, ordinance or resolution under which revenue bonds then outstanding shall have been issued by the port district.