Iowa constitution
Article III
Section 30

In all the cases above enumerated, and in all other cases where a general law can be made applicable, all laws shall be general, and of uniform operation throughout the state; and no law changing the boundary lines of any county shall have effect until upon being submitted to the people of the counties affected by the change, at a general election, it shall be approved by a majority of the votes in each county, cast for and against it.