For the history on the Chain of Rocks area of the Mississippi....
At one time it *was* navigable, but was very difficult and depending on the current and the season (spring or late fall) there were times it would close temporarily.
The "rocks" today are partially submerged due to a nearby dam providing enough backwater to flow into the canal north of the current bridge. In dry seasons, the rocks become more visible.
If it was never navigable, they wouldn't have built the original COR bridge so high and with the kink in the middle. (boats used to clear on the west side).
The current COR river bridge (1966) couldn't be built until the COR canal was complete. Which was built over time between 1947-1956.
The original canal bridge with I-270 was actually finished in 1963, but was not used for many years due to the delays in the new COR River bridge a few miles west.
Since the lawsuits by the City of Madison were holding up the new river bridge, IDOT couldn't open the canal bridge either.
That is why I-270 traffic was diverted at MP3.8 on the former US-40 for so many years. (May 1965 is when I-270 opened to that MP from I-55)
September 1, 1966 is when the current COR bridge finally opened. But not after many death threats and mysterious construction mishaps. No root cause for the mishaps was ever found, but they stopped after Missouri threatened to close the Missouri side on the older toll bridge. Madison was collecting around $50k a month in tolls until just a month before the new bridge opened. It was in their interest to delay the new bridge opening as long as possible.