Old River Control Structure

Old River Control Structure
Michael Maples, U.S. Army Corps of Engineers · Public domain · source
Name	Old River Control Structure
Location	Point Coupee Parish, Louisiana, United States
Purpose	Flood control, Mississippi River flow distribution
Owner	United States Army Corps of Engineers
Dam type	Control structure complex
Reservoir	Old River

Old River Control Structure. It is a critical flood control and water distribution system in central Louisiana, operated by the United States Army Corps of Engineers. The complex's primary function is to regulate the flow of water from the Mississippi River into the Atchafalaya River, thereby preventing the larger Mississippi from changing its main course. This engineering feat is vital for protecting regional communities, preserving navigation routes, and maintaining the economic and ecological balance of the Mississippi River Delta.

History and background

The geological tendency of the Mississippi River to seek a steeper, shorter path to the Gulf of Mexico via the Atchafalaya River had been recognized for centuries. Following the Great Mississippi Flood of 1927, which devastated vast areas of the Southern United States, the need for comprehensive federal flood control became urgent. This led to the passage of the Flood Control Act of 1928, authorizing the Mississippi River and Tributaries Project. By the mid-20th century, measurements indicated the Atchafalaya was capturing an increasing percentage of the Mississippi's flow, threatening the stability of ports like Baton Rouge and New Orleans. In response, Congress authorized the construction of the Old River Control Structure under the Flood Control Act of 1954, with major work commencing that decade.

Design and engineering

The complex is not a single dam but a system of structures built across the Old River connection between the Mississippi and Atchafalaya rivers. Its core components include the original Low Sill Structure, the Auxiliary Structure, and the Overbank Structure. The design incorporates massive concrete weirs, navigable locks, and a network of floodgates operated by powerful hydraulic cylinders. Engineering challenges were immense, requiring foundations sunk deep into the river's alluvial deposits to resist scour and the immense hydraulic forces of the Mississippi River. The structures were designed to maintain a precise 70/30 flow distribution, sending 70% of the combined flow down the Mississippi past Baton Rouge and 30% into the Atchafalaya Basin.

Operational role and function

Daily operations are managed by the United States Army Corps of Engineers from the New Orleans District. Using real-time data from gauges throughout the Lower Mississippi River, engineers adjust the gates to maintain the mandated flow split, which is crucial for flood control during high-water events. The structure also includes a navigation lock, the Old River Lock, which allows commercial barge traffic to pass between the Mississippi River and the Atchafalaya River, supporting the vital Inland waterways of the United States. This operational control preserves freshwater supplies for industries and municipalities along the Mississippi, including the city of New Orleans, and prevents saltwater intrusion from the Gulf of Mexico.

Flood events and structural challenges

The system faced its most severe test during the Mississippi River Flood of 1973. Extremely high water levels and turbulent flow caused severe scour at the base of the Low Sill Structure, undermining its foundation and nearly causing a catastrophic failure. Emergency repairs involved dropping thousands of riprap slabs from United States Air Force helicopters to stabilize the area. This event led to the design and construction of the reinforced Auxiliary Structure, completed in 1986, and the Overbank Structure, completed in 1991. Subsequent high-water events, including floods in 1997, 2011, and 2019, have further tested the complex, but the additional structures have thus far succeeded in maintaining control.

Environmental and economic impact

The control of the Mississippi River has profound consequences. Economically, it maintains the viability of the Port of South Louisiana and the Port of New Orleans, two of the nation's busiest ports, and protects billions of dollars in agricultural, petrochemical, and shipping infrastructure. However, it also significantly alters natural sediment distribution, contributing to land loss in the Mississippi River Delta and the erosion of coastal wetlands in Louisiana. This has heightened conflicts between navigation interests, flood protection for cities like Morgan City, and restoration efforts led by agencies like the Coastal Protection and Restoration Authority of Louisiana. The structure remains a focal point in debates over managing the Mississippi River for both human and ecological needs.