Iowa Code § 9F.6
Whenever the population of any county, township or city is referred to in any law of this state, it shall be determined by the last preceding certified federal census unless otherwise provided. Whenever a special federal census is taken by any city, the mayor and council shall certify the census as soon as possible to the secretary of state and to the treasurer of state as otherwise herein provided, and upon the failure to do so, the treasurer of state shall, after six months from the date of the special census, withhold allocation from the state to the city of any moneys the amount of which is based on the population of the city, and shall continue to do so until such time as certification by the mayor and council is made, or until the next decennial federal census. If there be a difference between the original certified record in the office of the secretary of state and the published census the former shall prevail.
Iowa Code § 9F.6
Similar provision, §4.1(22)