Old River Control: The Floodgates Holding the Mississippi in Place, Louisiana

Rivers don't stay in one place. Over thousands of years, the Mississippi has shifted its course across Louisiana again and again, abandoning old channels for shorter, steeper routes to the Gulf of Mexico. Geologists call it avulsion, and it's how the river built most of southern Louisiana in the first place. In the 1800s, the Mississippi began setting up its next move. In 1831, a cutoff was dug across a big loop in the river called Turnbull's Bend to shorten the trip for steamboats. What was left of the loop, a channel that became known as Old River, ended up connecting the Mississippi to the Red River and the Atchafalaya River. The Atchafalaya offered the Mississippi a path to the Gulf much shorter than its own channel past Baton Rouge and New Orleans. And the river started taking it. Between 1850 and 1950, the share of flow going down the Atchafalaya climbed from under 10 percent to about 30 percent. Every year the Atchafalaya got bigger, and every year it pulled harder. By the early 1950s, the Army Corps of Engineers concluded that if nothing was done, the Mississippi could switch its main channel to the Atchafalaya by about 1990. That would leave Baton Rouge, New Orleans and the chain of ports and industry between them on a dwindling channel, while the full force of the river poured down the Atchafalaya Basin. So Congress stepped in. On September 3, 1954, it authorized the Old River Control Project. Construction began in 1955 and was completed in 1963, at a cost of about $67 million. The structures were designed to hold the river in place by freezing its natural 1950 flow split: about 70 percent of the combined flow of the Mississippi and Red Rivers stays in the Mississippi, and about 30 percent goes down the Atchafalaya. The split is measured as an annual average, and it's still set by law. In the spring of 1973, the system nearly failed. Floodwaters scoured the riverbed around the main structure, the Low Sill, and undermined its foundation. The Corps opened the Morganza Spillway downstream to take pressure off, and the structure held. But the damage was permanent. The Corps spent years repairing it, and in December 1986 it completed a second, backup structure, the Auxiliary Control Structure, at a cost of $206 million. In 1990, a privately built hydroelectric plant joined the complex. In 2011, a historic flood pushed 706,000 cubic feet of water per second through the complex, and the Morganza Spillway was opened for only the second time in its history. The control system held. Every day, these gates decide where America's largest river goes. As long as they hold, the Mississippi stays on the course it's followed for generations.