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 covered with ice over and over again. Ice came as far south as the land around the Ohio River. Then Earth started warming up. The glaciers slowly started melting. They left behind lots of rocks and dirt they had brought with them. This material dammed up old river valleys. It created the Great Lakes. It also forced rivers to find new ways to the sea. The Mississippi found a new course southward. Today, more than 12 million people live along this river.
