Moneys in the designated construction fund of each district may be invested by the board, subject only to the limitations contained in an indenture providing for the issuance of revenue bonds. All securities or other investments made under the provisions of this chapter shall be held by the county treasurer as custodian thereof. All interest or other earnings received pursuant to such investments shall be collected by the county treasurer, and, on order of the county auditor, shall be deposited in the county treasury to the credit of the fund from which such interest or other earnings are derived.