Old River Control Structure

The Old River Control Structure (ORCS) is a floodgate system in a branch of the Mississippi River in central Louisiana. It regulates the flow of water leaving the Mississippi into the Atchafalaya River, thereby preventing the Mississippi river from changing course. Completed in 1963, the complex was built by the U.S. Army Corps of Engineers in a side channel of the Mississippi known as "Old River," between the Mississippi's current channel and the Atchafalaya Basin, a former channel of the Mississippi. The Old River Control Structure is actually a complex containing the original low-sill and overbank structures, as well as the auxiliary structure that was constructed after the low-sill structure was damaged during the Mississippi River Flood of 1973. The complex also contains a navigation lock and the Sidney A. Murray, Jr. Hydroelectric Station.

Before the 15th century, the Red River and Mississippi River were entirely separate and more or less parallel to one another. Beginning in the 15th century, the Mississippi River created a small westward loop, later called Turnbull's Bend, near present-day Angola, Louisiana. This loop eventually intersected the Red River, making the downstream part of the Red River a distributary of the Mississippi; this distributary came to be called the Atchafalaya River.

In the heyday of steamboats along the Mississippi River, it would take a boat several hours to travel the 20 miles of Turnbull's Bend, after which it would have progressed only a mile or so from the entrance to the bend. To reduce travel time, Captain Henry M. Shreve, a river engineer and namesake of Shreveport, Louisiana, dug a canal in 1831 through the neck of Turnbull's Bend; this canal became known as Shreve's Cut. At the next high water, the Mississippi roared through this channel. The upper portion of Turnbull's Bend became the smaller Upper Old River while the lower portion became the larger Lower Old River. At first, the Lower Old River would flow eastward, to the Mississippi, until 1839, when locals began removing a log jam that was obstructing the Atchafalaya River. The project was finished in 1840. After that, the Lower Old River would flow eastward to the Mississippi when the Red River was high and the Mississippi was low, and westward to the Atchafalaya when the Mississippi was high and the Red River was low. Over time, the number of days when the river flowed east to the Mississippi decreased and the number of days when the river flowed west increased, until eventually the Lower Old River flowed west over half the time. By 1880, it rarely flowed eastward and was now rapidly capturing more and more of the flow of the Mississippi. With this extra intake of water, the channel of the Atchafalaya River was worn deeper and wider throughout the 1800s and early 1900s. Between 1850 and 1950, the flow increased from <10% of the Mississippi to 30%, and the U.S. Army Corps of Engineers began work on the Old River Control Structure (ORCS) to prevent the Atchafalaya becoming the main channel of the Mississippi. In 1963, the The ORCS was completed, and it completely sealed off the Old river. However, a lock, a diversion channel, and flood spillway were installed to allow barges to pass between the Atchafalaya and Mississippi.

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