Iowa Code § 468.538: Scope.

This part applies when the board of trustees of an overlying district accepts all improvements and rights-of-way surrendered by a board of supervisors or board of trustees of a contained district, in accordance with sections 468.256 through 468.259. In addition, after such acceptance, the overlying district must include at least thirty-five thousand acres with a pumping station, regardless of whether the drainage or levee district is located in more than one county. Such a district shall continue to be controlled and managed by a board of trustees elected as provided in this part.