If a county bridge has been destroyed by fire, flood, or any other cause, the board of county commissioners may cause to be established a free ferry in lieu thereof. The board may also cause to be established a free ferry over any of the interior streams of the state where it is not expedient to build a bridge, and may cause it to be maintained and operated. The cost of maintaining and operating such ferry shall not exceed the interest on the probable cost of constructing a bridge across the stream.