This bridge was actually over the Blue River. The South Fork Blue River flows into the Blue just downstream from here. You can see on the map where the South Fork is towards the right side.
I found this info on the Frederickburg bridge on indianatrails.org..... "A Transportation Enhancements grant in 2005 will help move this "Triple Intersection Warren Through Truss" bridge from its original location in Washington County over the Blue River, to its new home as a trail bridge across White River. The 149-foot bridge, constructed in 1898, will connect multi-use trails in Hamilton County's Koteewi Park with the White River Campground"