Mont. Code Ann. § 7-14-4635

The commission may provide that bond redemption and the interest payment shall constitute a first, direct, and exclusive charge and lien on all such fees, charges, and other revenues and interest thereon and sinking funds created therefrom received from the use and operation of the project for the acquisition, construction, or completion of which such revenue bonds were issued. All such fees and charges and other revenues, together with the interest thereon, shall constitute a trust fund for the security and payment of such bonds and shall not be used or pledged for any other purpose so long as such bonds (or any of them) are outstanding and unpaid.