Iowa Code § 468.108: Bridges.

1. When a levee, ditch, drain, or change of any natural watercourse crosses a public highway, necessitating moving or building or rebuilding any secondary road bridge upon or ditch or drain crossing the road, the board of supervisors shall move, build, or rebuild the bridge, ditch, or drain, paying the costs and expenses, including construction, maintenance, repair and improvement costs, from county funds.

2. If the bridge or crossing is upon or across a primary or interstate road, the moving, building, or rebuilding work shall be done by the state department of transportation and paid for out of the primary road fund.