The Mississippi River along with its main tributary the Missouri River is the fourth largest river in the world. The Mississippi-Missouri river system is the largest watershed in North America. It drains more than 40 percent of of the contiguous United States.
The Mississippi-Missouri stretches 3,710 miles (5,970 km). Only the Nile, Amazon, and Yangtze rivers are longer. The Mississippi River with its tributaries forms a vast ancient river system. The drainage basin of the Mississippi stretches from the Rocky Mountains to the Appalachian mountains. Its western tributaries drain the Great Plains, and the eastern tributaries drain the Appalachian Plateau.
The Mississippi River begins at Lake Itasca in northern Minnesota. It is 10 feet (3 m) wide at the source, It winds eastward through marshes and is fed by other lakes and streams. It skirts the western end of Lake Superior and then turns southward, As it moves southward in Minnesota if is fed by tributaries. Near St. Louis, Missouri, the Missouri River joins the Mississippi, and further south the Ohio River joins the Mississippi. The Mississippi flows through a gap between the Cumberland and Ozark plateaus and cuts a wide course towards the Gulf of Mexico. Along the way, it is joined by the Arkansas and Tennessee rivers.
The Mississippi River drains 1.2 million square miles (3.2 million square km). When the Missouri River–known as the Big Muddy–joins the Mississippi, the clear waters of the upper Mississippi become less clear. When the Ohio River–the Mississippi’s largest tributary by volume–joins the Mississippi near Cairo, Illinois, the river takes on a breath and fullness that earned the Algonquian native American name Misi-ziibi–“Great River.” At points, the Mississippi River is more than 1 mile (1,600 m) wide.
Southeast of New Orleans the Mississippi becomes the Mississippi Delta. The Mississippi Delta is an alluvial floodplain that is 200 miles (320 km) long and 87 miles (140 km) across at its widest point and encompasses about 4,415,000 acres (17,870 km2), or, almost 7,000 square miles. The Mississippi Delta drains into the Gulf of Mexico.
The Mississippi River basin was largely shaped by a large ice sheet or glacier that stretched across the middle of the current North American continent between 11,000 and 150,000 years ago. As the ice sheet began to recede, the vast Mississippi River Basin was formed. Giant glacial rivers drained into the Mississippi watershed creating the flat, fertile landscape seen today. Today, the Mississippi River is 3 feet (1 m) deep at its source and 60 feet (18 m) deep at its deepest.