Oregon Revised Statutes Water Resources: Irrigation, Drainage, Flood Control, Reclamation § 547.615

The board of trustees shall cause to be assessed and levied each year upon the assessable property of the district, in addition to the levy authorized for other purposes, a sufficient sum to pay the interest on outstanding bonds issued in conformity with the provisions of ORS 547.605 to 547.620, accruing before the next annual levy, and such proportion of the principal as in their judgment will be for the best interests of the district, to create and raise a sinking fund to retire the bonds and to be used for no other purpose whatever.