Now that the Morganza Spillway will be opened up over the next few days (intention is diversion of a portion of the raging Mississippi River from the Main STEM channel) eyes will be turning from that structure about 140 river miles north from NOLA/Baton Rouge to the Old River Control Structure less than 40 miles North of NOLA/Baton Rouge. Also the New Yorker has resurrected from its archives the wonderful article by John McPhee on the ORCS!
Old River Control Structure
The Old River Control Structure complex. View is to the east-southeast, looking downriver on the Mississippi, with the three dams across channels of the Atchafalaya River to the right of the Mississippi. Concordia Parish, Louisiana is in the foreground, on the right, and Wilkinson County, Mississippi, is in the background, across the Mississippi on the left.
The Old River Control Structure (ORCS) is a floodgate system located in a branch of the Mississippi River in northern Louisiana. 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.”
The Old River Control Structure is necessary to prevent the main flow of the Mississippi River from finding an easier way to the Gulf of Mexico through the natural geologic process of “avulsion,” also known as “delta switching.” Downstream of the facility, the Old River channel eventually merges with the Atchafalaya River which in turn enters the Gulf of Mexico at Morgan City, Louisiana. This alternative path to the gulf is much shorter and steeper. The increased drop allows for greater erosion of the river bottom, while the decreased flow in the main channel allows for more sediments to build up. Left unchecked, more and more water would take the steeper and shorter path to the Gulf of Mexico until eventually this would become the new main channel of the Mississippi. Historically, this natural process has occurred about every 1000 years, and is overdue. Accordingly, between 1900 and 1975 more and more water from the main channel was taking the Atchafalaya River path to the sea. In 1975, artificial control took effect. Some researchers believe that the likelihood of this event increases each year, despite artificial control efforts.
If the Mississippi diverts its main channel to the Atchafalaya Basin and the Atchafalaya River, it would develop a new delta south of Morgan City in southern Louisiana, greatly reducing water flow to its present channel through Baton Rouge and New Orleans.
The original 1963 floodgates of the Old River Control complex are the Low Sill Structure, which regulates flow in the waterway, and the Overbank Control Structure, for when the Mississippi exceeds its banks. The Auxiliary Structure was added to reduce pressure on the other floodgates after extensive damage caused by the flood of 1973. The Sidney A. Murray, Jr. Hydroelectric Station, completed in 1990, and a navigation lock are also at the site.