Rivers are restless things. The Mississippi has changed course roughly every thousand years throughout its geological history, and by the mid-twentieth century, it was overdue. The trouble began centuries earlier, when the Mississippi's meandering created an oxbow loop called Turnbull's Bend near present-day Angola, Louisiana. In the fifteenth century, this loop intersected the Red River, birthing a new distributary: the Atchafalaya. At first the connection was modest, but in 1839, locals removed a massive natural log jam called the Great Raft from the Atchafalaya, and the balance of power between the two rivers began to shift. By 1880, the Atchafalaya was capturing an ever-growing share of the Mississippi's flow. The shorter, steeper route to the Gulf was winning. Left unchecked, the Mississippi would have fully redirected itself into the Atchafalaya Basin, creating a new delta south of Morgan City and leaving New Orleans stranded on a sluggish backwater.

Congress directed the Army Corps of Engineers to intervene, and in 1963 the Old River Control Structure became operational. Its mandate was deceptively simple: maintain a 70-30 split, sending seventy percent of the combined flow of the Mississippi and Red rivers down the Mississippi's existing channel and thirty percent down the Atchafalaya. The original complex included a low-sill structure for routine flow regulation and an overbank structure activated only when the Mississippi exceeded its banks. A navigation lock allowed barge traffic to pass between the rivers. The system worked -- until the Mississippi flood of 1973 nearly destroyed it. Floodwaters scoured the foundations of the low-sill structure, and for a harrowing period, engineers feared the entire complex would fail. It held, barely, but the close call prompted construction of the Old River Control Auxiliary Structure in 1986 to share the load during future floods.

What makes the Old River Control Structure so consequential is not the engineering itself but what lies downstream. Baton Rouge and New Orleans depend on the Mississippi for drinking water, industrial cooling, and the navigation channel that makes them among the busiest port complexes in the world. A diversion would strand petrochemical plants, grain elevators, and shipping terminals along a hundred miles of river. The Atchafalaya route would flood communities around Morgan City. The economic disruption would ripple across the country. And yet the river pushes relentlessly against its concrete restraints. Each spring flood tests the structure anew. The Sidney A. Murray Jr. Hydroelectric Station, completed in 1990 as the newest component, generates power while providing an additional measure of flow control. The nearby Morganza Spillway serves as a safety valve, though it has been opened only twice since the control structure was built.

From the air, the Old River Control Structure reveals its true scale. Three separate dam structures span channels between the Mississippi's main flow and the Atchafalaya, with Concordia Parish, Louisiana, on one side and Wilkinson County, Mississippi, on the other. The landscape is a maze of levees, spillways, and diversion channels -- the visible architecture of humanity's attempt to dictate terms to the continent's greatest river. Some researchers believe the likelihood of a catastrophic course change increases each year despite these artificial controls. The Mississippi, after all, has been rearranging the geography of the Gulf Coast for millennia. The Old River Control Structure is a bet -- an enormous, expensive, constantly maintained bet -- that human engineering can hold a river in place indefinitely. So far, the bet has paid off. But the river is patient.