Iowa Code § 357A.9: Members divided into classes.

The initial board of each district shall divide its members by lot into three classes of as nearly equal size as possible. The terms of the directors in the first, second, and third classes shall expire on the dates of the annual meetings in the first, second, and third years, respectively, following the year in which the district is incorporated, or as soon thereafter as their respective successors are elected and have qualified. At the annual meeting in each year after the year in which the district is incorporated, a director shall be elected to succeed each director whose term of office expires on that date, and each director so elected shall hold office for a term of three years and until a successor is elected and has qualified. Vacancies shall be filled by appointment by the remaining directors, for the unexpired term.