The Mississippi River formed largely during the Great Ice Age when large sheets of ice began to melt, and the resulting water pooled in glacial lakes in what is now Wisconsin and Minnesota. This water slowly drained towards the Gulf of Mexico, carving the Mississippi River as it flowed.
The Mississippi River and its tributaries comprise the largest drainage system in North America. Beginning in northern Minnesota, it travels south, all the way to the Gulf of Mexico, carrying water that drains down from the Rocky and Appalachian Mountains to the ocean. The Mississippi River has the fourth-largest drainage basin in the world, encapsulating more than 1.2 million square miles and approximately 40 percent of the landmass of the United States.
The Mississippi River Delta, where it flows into the Gulf of Mexico, is thought to have initially formed about 7,000 years ago. However, it continues to change as the river carries sediment from the upper portions of the continent and deposits them in the gulf. As sediment levels rise, plants begin to grow, essentially extending land further into what was once ocean. Thus, the Mississippi River's path to the Gulf of Mexico is continually becoming longer. The U.S. Army Corps of Engineers has constructed several structures to prevent the river from changing course as a result of the changes in its delta.