Millions of years ago, the Upper Mississippi River flowed north and emptied into a big lake. Then came the Ice Age.
From about 3 million to 10,000 years ago, the North American continent was repeatedly covered with ice as far south as the area around the Ohio River. Then the climate began to warm. The glaciers slowly started melting, leaving behind the rock and soil they had carried. This material dammed up old river valleys and created the Great Lakes. It also forced rivers to make new routes to the sea. The Mississippi found a new course southward. Today, lands that border the Mississippi are home to more than 12 million people.
