Why the Mississippi River Wants to Shift to the Atchafalaya: The Old River Control Structure

## The Law of Fluvial Avulsion

Rivers are lazy physical systems: water follows the steepest, most direct gravitational route to the sea. Over centuries, a meandering delta river deposits sediment at its mouth, building out a long, shallow pathway that gradually loses its gradient.

In the lower Mississippi River valley, the river has created such a long, sluggish loop past Baton Rouge and New Orleans (over 300 miles to the Gulf) that another river—the **Atchafalaya River**—offers a path to the Gulf of Mexico that is **over 150 miles shorter and more than twice as steep**.

### The Catastrophic Avulsion Threat
Left to natural physical laws, the Mississippi River would have naturally jumped its banks and permanently captured the Atchafalaya channel by the early 1970s. Had this occurred:
* The ports of New Orleans and Baton Rouge would become stagnant saltwater tidal estuaries, crippling US agricultural grain exports.
* Highway and railway bridges across Louisiana would be swept away.

### The Old River Control Structure
To prevent this economic catastrophe, the US Congress authorized the US Army Corps of Engineers to construct the colossal **Old River Control Structure** in 1963. By federal law, exactly 70 percent of the combined flow must continue down the Mississippi mainstem, while 30 percent is diverted through gates into the Atchafalaya.