The river is so shallow here that it is impassible to boats most of the year. In addition, the river has a hard rock bottom, so it is not possible to dredge a deeper channel. To enable boats to navigate past the Chain Of Rocks area, a 8.4 mile long canal was built from just north of downtown Saint Louis to just below the confluence with the Missouri River.
The reason for two bridges on each alignment is that the highways have two waterways to cross, the Mississippi River main channel, and the Chain Of Rocks Canal.
Since there is no navigation traffic on the main river in this area, bridge designers did not have to worry about making the bridge high enough for boat traffic, nor did they worry about where they put the piers. The result is a totally nondescript bridge that would look at home on any 1960's era urban freeway. That is rather ironic given that the Double Chain Bridges just to the east of here are such spectacular structures.