Iowa Code § 331.206: Supervisor districts.

1. One of the following supervisor district representation plans shall be used for the election of supervisors:

a. Plan “one”. Election at large without district residence requirements for the members.

b. Plan “two”. Election at large but with equal-population district residence requirements for the members.

c. Plan “three”. Election from single-member equal-population districts, in which the electors of each district shall elect one member who must reside in that district.

2. a. The plan used under subsection 1 shall be selected by the board or by a special election as provided in section 331.207. A plan selected by the board shall remain in effect for at least six years and shall only be changed by a special election as provided in section 331.207. However, a county with the main campus of an institution of higher learning governed by the state board of regents shall use plan “three” for the election of supervisors.

b. A plan selected by the board shall become effective on the first day in January which is not a Sunday or holiday following the next general election, at which time the terms of the members expire and the terms of the members elected under the requirements of the new supervisor representation plan at the general election as specified in section 331.208, 331.209, or 331.210 shall commence.