It would be a ca. 1920 bridge. It looks it might be one of Iowa's standard plan bridges for the period. I saw one similar, here: http://www.historicbridges.org/iowa/crt63/index.php
Thanks for sharing.
More likely it was moved to present location in the 1960s, if the photographs posted are indeed of the bridge in question. On the portal frame there are the remains of a builders' plaque with the date 1920. I have noticed one similar and have a likely candidate in mind. It could very well have been constructed by the American Bridge Company as their plaques from that time period differ from earlier, later, least what I have seen.
Compare to this, image number 4 >
Many counties liked to reuse bridges. As traffic grew in one area build a new, modern one and shift the old someplace else.