Though rivers have flowed across what is now the Mississippi River basin for millions of years, the modern river emerged from the drainage of melting glaciers around 12,000 years ago. The Mississippi has shifted course continuously since then, and its course remains erratic and unpredictable.
The last glacial period lasted over 60,000 years and ended around 10,000 BC. During this time, much of North America sat under vast ice sheets 1 to 2 miles thick. When these glaciers retreated, they left behind a chain of temporary lakes that drained across the Midwest. In time, the many river systems consolidated into what is now the Mississippi system.