The gross revenues derived from the operation of each unit of development comprising a part of the inland waterways facilities shall be applied for the following purposes in the following order: (1) Payment of the expenses of operating and maintaining in good operating condition the unit of development from which such revenues are derived; and (2) payment of the expenses of operating and maintaining in good condition any other unit of development or units of development comprising a part of the inland waterways facilities. Whenever the gross revenues received by the department from the inland waterways facilities during a fiscal year shall be greater than the expenses of operating and maintaining in good operating condition the inland waterways system during that fiscal year, then the revenues from the inland waterways system remaining at the end of that fiscal year after compliance with the first sentence of this section shall be applied for that one or more or all of the following purposes that may be designated by the department, the sums to be so applied to be in such amounts and to be applied in such manner as the department may designate: (1) Retention of such amounts, for use as working capital for the inland waterways facilities as the department may determine to be reasonably necessary for such purpose; (2) payment of the cost of capital improvements and additions to the inland waterways facilities; and (3) payment at their respective maturities of the principal of or interest on any bonds theretofore issued and at the time outstanding under the 1957 Docks Amendment; provided, that the revenues from the operation of any unit of development the acquisition or construction of which was financed wholly or in part with bonds issued under the 1957 Docks Act shall, to such extent as may be required by the provisions of the 1957 Docks Act, be applied for payment at their respective maturities of those bonds issued under the 1957 Docks Act and for payments into the sinking fund created in that act, and the revenues from the operation of any unit of development the acquisition or construction of which was financed wholly or in part with bonds issued under the 1959 Docks Act shall, to such extent as may be required by the provisions of the 1959 Docks Act, be applied for payment at their respective maturities of the principal of and the interest on those bonds issued under the 1959 Docks Act and for payments into the sinking fund created in that act.