Mont. Code Ann. § 75-6-323

At or before the time of the issuance of any bonds under this part, the governing body of the authority shall by resolution or in the trust indenture provide for the creation of a sinking fund and for monthly payments into the sinking fund from the revenue of the water supply, wastewater transportation, or treatment system operated by the authority sums in excess of the cost of maintenance and operation of the properties that will be sufficient to pay the accruing interest and retire the bonds at or before the time that each will respectively become due and to establish and maintain reserves for the bonds. All sums that are or should be, in accordance with the provisions, paid into the sinking fund must be used solely for payment of interest and for the retirement of the bonds at or prior to maturity, as may be provided or required by the resolutions.