Cal. Harb. & Nav. Code § 7225

The proceeds from the sale of the bonds shall be paid into the treasury of the district and placed to the credit of a special improvement fund and expended only for the purpose for which the indebtedness was created; however, when the purpose has been accomplished, any moneys remaining in the special improvement fund may be transferred to the fund to be used for the payment of the principal of and interest on the bonds.