Iowa Code § 40.1: Congressional districts.

The state of Iowa is hereby organized and divided into four congressional districts, which shall be composed, respectively, of the following counties:

1. The first district shall consist of the counties of Cedar, Clinton, Des Moines, Henry, Iowa, Jackson, Jasper, Jefferson, Johnson, Jones, Keokuk, Lee, Louisa, Mahaska, Marion, Muscatine, Scott, Van Buren, Warren, and Washington.

2. The second district shall consist of the counties of Allamakee, Benton, Black Hawk, Bremer, Buchanan, Butler, Cerro Gordo, Chickasaw, Clayton, Delaware, Dubuque, Fayette, Floyd, Grundy, Hardin, Howard, Linn, Mitchell, Poweshiek, Tama, Winneshiek, and Worth.

3. The third district shall consist of the counties of Adair, Adams, Appanoose, Cass, Clarke, Dallas, Davis, Decatur, Greene, Guthrie, Lucas, Madison, Monroe, Montgomery, Page, Polk, Ringgold, Taylor, Union, Wapello, and Wayne.

4. The fourth district shall consist of the counties of Audubon, Boone, Buena Vista, Calhoun, Carroll, Cherokee, Clay, Crawford, Dickinson, Emmet, Franklin, Fremont, Hamilton, Hancock, Harrison, Humboldt, Ida, Kossuth, Lyon, Marshall, Mills, Monona, O’Brien, Osceola, Palo Alto, Plymouth, Pocahontas, Pottawattamie, Sac, Shelby, Sioux, Story, Webster, Winnebago, Woodbury, and Wright.