The bridge was designed by I.F. Stern of Chicago. It is unusually shaped, because the swing span is asymmetrical. According to local legend, the original bridge design had equal length spans on either side of the center pivot. Once the bridge was completed, the owner of the property on the south side of the river did not want the swing span crossing his land. In response, the owner removed most of the south end of the swing span and added a concrete counterweight to compensate for the lost material.
Legends aside, the reason for the "bobtail" configuration of the bridge is that the section of the river deep enough for navigation at that point is not wide enough to accommodate a full-sized swing bridge. The counterweight is necessary to keep both ends of the bridge in balance over the center pier, which is near the right descending bank. While the legend may be an appealing one, it cannot withstand the fact that no qualified bridge engineer would lavish the material and effort required to build a swing span over land.