Iowa Code § 468.561: Liens unimpaired.

When drainage refunding bonds are issued, nothing in this part shall be construed as impairing the lien of any unpaid drainage assessments or installments in the drainage district, the time of payment of which is not extended, nor shall this part be construed as impairing the priority of the lien of any unpaid drainage assessments or installments nor the right, duty, and power of the officers authorized by law to levy, collect, and apply the proceeds of the assessments or installments to the payment of outstanding drainage bonds issued in anticipation of the collection of the assessments or installments.