Iowa Code § 468.54
The taxes when collected shall be kept in a separate fund known as the county drainage or levee fund and shall be paid out only for purposes properly connected with and growing out of the county drainage and levee districts on order of the board. The auditor shall continue to keep a record of each of the drainage and levee district's funds so as to accurately reflect the financial condition of each district account. The county treasurer, on order of the board of supervisors, shall invest such funds not immediately needed for current operating expenses in United States government bonds, in time certificates of deposit, in savings accounts in banks as the board shall approve, in the interest-bearing obligations of the drainage and levee districts of the county, or as provided by chapter 12C. Interest collected by the treasurer on the funds invested shall be deposited in the county drainage or levee fund, and on July 1 of each year the auditor shall apportion and credit the interest to each drainage or levee district account in the proportion which the average credit balance of each district bears to the average balance of the county drainage or levee fund. The averages to be ascertained shall be the averages of the balances existing on the first of each month during the fiscal year immediately preceding. Interest collected on drainage or levee district taxes shall be credited to the district for which the taxes are being collected. This section does not permit expenditures in behalf of any district in excess of its share of the county drainage or levee fund. This section does not apply to drainage and levee districts under trustee management unless the trustees consent to its application, and in the absence of such consent, section 468.528 applies.
Iowa Code § 468.54
89 Acts, ch 126, §2 92 Acts, ch 1016, §35