Updated in September 2018 with rearranged photos and additional commentary.
The Eads Bridge is my favorite bridge across the Mississippi River in downtown, though the MacArthur is a close second. The oldest bridge across the river at St. Louis, it utilized structural steel on a massive scale for the first time. Interestingly, though, there is evidence that cast iron was still used, as well.
The giant tubes of steel or iron were revolutionary for the strength they provided for the arches.
The massive arches would be at home in a Roman aqueduct or basilica.