The contract provisions for the payment of construction charges to the United States and the bonds securing the payment of the same, if any be issued and deposited, may be of such denominations and may call for payment of such interest not exceeding six percent per annum, and may provide for such installments and for repayment of the principal at such times as may be required by the federal laws and as may be agreed upon between the board and the Secretary of the Interior. The contract indebtedness to the United States shall be a prior lien to any subsequent bond issue of the district.