Roughly where the city of St Louis is now situated, or not too far short, there was a major native American settlement with monumental architecture and the famous mounds. Is is now known as Cahokia – and was at its apogee between 1000 and 1200AD the biggest community in North America, with trade connections over a wide area. It came to prominence during a benign period of climate and it now looks like its demise was associated with the onset a cooler world – and a return to Mississippi River flooding events – see http://westerndigs.org/megafloods-spurred-collapse-of-ancient-city-of-ca…
Cores taken from sediments in the Mississippi flood plain go back 1800 years. At different points the researchers came across sterile sections of the core, with little to no pollen, indicative of a flooding event. Until 1400 years ago, 600AD, the area was prone to frequent and severe floods – or rather, evidence of flooding was found, and presumably this was during the cooling event of the 5th and 6th centuries. The river was able to rise 33 feet above its base level.
After 600AD the floods became less frequent – and people began to farm intensively on the flood plain. The community expanded into a metropolis but after 1200 the climate became cooler and wetter once again – and flooding returned (with increasing frequency as the Little Ice Age set in).
The same story is at http://phys.org/print349970653.html … with a map of Cahokia and present day St Louis. The paper was published in PNAS (May 4th) and it tells us that Cahokia was eventually abandoned – but not until AD1400. Therefore, it struggled along with a reduced population until the floods became too common. Nowadays the river is managed and only floods in extreme conditions – such as the famous flood of 1993. Prior to the 20th century the river was less predictable and there were major flooding events on a regular basis. Hence, there was no incentive to revitalise Cahokia.