Submitted by Eryn Bain, 7th grade,

Cherokee Middle School

A. A river's path is primarily determined by the shape of the land, says Jim Knox, a UW-Madison geography professor who studies rivers.

Flowing water follows the land's contours, finding the path of least resistance as it runs downhill in response to gravity. While the Mississippi flows south, other major world rivers flow east (the Amazon), west (the Congo) or north (the Nile).

"The Mississippi flows south because of plate tectonics. The movements of the continents and the uplifts and depressions accompanying tectonic forces set up the topography of the land," Knox says. "These forces are responsible for the high elevation to the east in the Appalachian Mountains and to the west in the Rockies."

The forming Mississippi was further constrained in the north by higher elevations and later by massive glaciers that intermittently covered much of the area. To the south, though, is a depression in the Earth 's crust called the Mississippi Embayment, which spans large parts of Louisiana, Mississippi, Arkansas, and Tennessee. The river flowed through this shallow bowl and out to the Gulf of Mexico.

Sediment and fossil evidence show the Mississippi has flowed into the Gulf of Mexico for at least 65 million years, Knox says.

-- Produced in cooperation with University Communications

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