There are four bridges as part of the Chain of Rocks crossing, two on the new I-270 alignment, and two on the old US-66 alignment. This bridge, or rather, pair of twin spans, is on the new I-270 alignment, and they cross the Chain Of Rocks Canal.

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 the canal carries riverboat traffic, these bridges have to be very high above the water. There could also be no piers in the navigation channel, so the main span had to relatively long. The solution was to build a pair of massive steel truss bridges.

These bridges are often called the Double Chain Bridge in that there are two spans, and they cross the Chain Of Rocks Canal. They are the first of the big metal monster bridges that you find as your head south on the Mississippi River.