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 Chain Of Rocks Bridge predates the Chain Of Rock Canal, which means that the main river channel carried boat traffic during periods of high water. This influenced the design of the bridge in two ways. First, the piers are relatively high for a bridge with so many short truss sections. The bridge is made up of 11 Warren Truss Sections, three longer sections that cross the water, 3 shorter sections on the Missouri side, and 5 shorter sections on the Illinois side.
The interesting feature of the Chain Of Rocks Bridge is the 24 degree bend about halfway across the river. The promoters had purchased land on each side of the river, but the parcels were not directly across the river from each other. The plan was to build the bridge on a diagonal to the river. The US Army Corps of Engineers objected to having a bridge cross the navigation channel at an angle. As a result, the section of the bridge crossing the navigation channel was built straight across the river, and then the remaining part of the bridge was built on the diagonal to meet up with the Missouri shore. The result is the 24 degree bend where the straight and diagonal sections meet.
The bridge was built in 1929 as a private venture. While promoters thought that there would be sufficient traffic to pay both the interest and upkeep, that traffic did not materialize, largely due to the fact that it was not easy or convenient to get to downtown Saint Louis after crossing the bridge. The bridge went bankrupt in 1931. Bridge ownership reverted to the City of Madison, Illinois, in 1939.
The Chain Of Rocks Bridge because part of US-66 in 1936. US-66 was moved to a new alignment in 1955, so the bridge became part of Bypass US-66. The Bypass US-66 designation moved to I-270 in 1965. The federal government demanded that tolls be removed from all highways designated as US highways. The City of Madison removed the toll from the Chain Of Rocks Bridge in August, 1966, to avoid having to appear in court. The bridge closed for repairs in February, 1970, and never reopened.
The bridge slated to be demolished in the mid-1970's. Due to a drop in the market price for scrap iron, it turned out that the demolition would not pay for itself, but rather, would cost several million dollars. As a result, the bridge was left as-is, largely forgotten until it was rediscovered in the mid-90's after a major flood. The city made a deal with a regional group called TrailNet to renovate the bridge and open it as part of a regional bicycle trail. After the bridge has been cleaned up and safety features were added, it reopened June 1, 1999, as a bicycle trail and hiking path. In fact, it my be the longest pedestrian and bicycle bridge in the world.
Movie fans might recognize this bridge. It was featured in the 1981 movie Escape From New York.