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.
The old Chain Of Rocks Bridge was built in 1929, long before the canal was built. When the canal was dug in 1949, a bridge had to be built in this spot to provide access to the Illinois side of the Chain Of Rocks Bridge.
While this crossing is lightly used today, it still has to be high enough and long enough to allow riverboat traffic to pass without being a navigation hazard. The solution was to build a steel truss bridge to stand up to the long span, and a pair of trusses handling the approaches at either end of the bridge.
This bridge is called the Single Chain Bridge given that there is only one structure in the bridge, as opposed to the Double Chain Bridge just upstream, which has two bridges in parallel. The Chain Of Rocks Bridge was abandoned in 1970, so the Single Chain Bridge was largely ignored. It deteriorated to the point where it required major renovation in 1999. Today, it looks like a brand new bridge.
Did US-66 ever cross the Single Chain Bridge? As it turns out, US-66 crossed the Chain Of Rocks Bridge from 1936 to 1955. From 1955 to 1965, the Chain Of Rocks Bridge was officially Bypass US-66. So, the Single Chain Bridge hosted US-66 from the time it was built in 1949 to when US-66 was moved in 1955, a total of 6 years. It also carried the Bypass US-66 designation for 10 years.