Thoughts on one of the left spans being moved to this location?
The 1913 date is evident of the rebuilding after the great flood of 1912, which destroyed the previous bridges on this island. The two trusses which are still seen were placed prior to this flood, although the through girder was placed in 1913.
Perhaps. 1913 for the TPG and a concrete encapsulation.
Plate girder span a replacement? I see a 1913 date on a pier, indicating alteration of some sort.
Here is a sheet for this bridge, scanned from the physical document located at the Chicago & North Western Railway Historical Society Archives. It sheds no information on the 110' truss or through girder, or the original location of the 160' truss. One note is that the bridge was built in 1911 and rebuilt in 1913, although it is unknown what was erected when.
In addition, I had been contacted by a fisherman who found the clear remains of a quadrangular truss on the river bottom next to the middle span.