Sunday, May 04, 2008
A Short Geologic History of New Orleans
High ground, you say? Everyone knows that a portion of New Orleans is below sea level. This is true but much has changed since the City was founded in 1718. The fact is the mean elevation of Louisiana is only 100' above sea level. To put this into perspective, Morgan City is 7' above sea level, Lafayette 39' above sea level, Baton Rouge 60' above sea level and the far northwestern city of Shreveport only 177' above sea level. Why then did Bienville situate the City of New Orleans at the second lowest spot in the United States, higher only than Death Valley that has an elevation of 282' below sea level? The answer is, he didn’t.
There were no topographic maps or GPS devices in 1718. Still, seasoned explorers Bienville and his brother D’Iberville understood the concept of high ground. They had located and chosen the site for New Orleans on an expedition more than a decade before the City’s founding. Although no records exist to confirm this assumption, a look at present-day Louisiana geography and geology indicates New Orleans in 1718 may have been at or near the highest elevation at the mouth of the Mississippi River.
New Orleans is part of the Mississippi River Delta, a geographic region that encompasses 13,000 square miles, fully 25% of Louisiana. Deltas are comprised mainly of silt. A look at the mechanics of the Mississippi River explains why. The Mississippi River drops 1,475' from its source in Minnesota to its mouth at the Gulf of Mexico. Water flows down river because of gravity. Along the way the Mississippi is intersected by many smaller rivers.
The Mississippi and the rivers that feed it transport many tons of alluvium picked up along the way because of erosion. The energy of the flowing river carries this alluvium in suspension. As the elevation nears sea level and this energy is dissipated, the river can no longer maintain its load and it is deposited in the form of silt. Often, extra silt is deposited at a meander in the river where energy is locally dissipated. This is a likely scenario for the location of New Orleans in 1718.
Just north of the small town of Donaldsonville the Mississippi turns abruptly eastward. Interestingly, Donaldsonville is near the point the modern Mississippi River threatens to abandon its present course and flow into the Atchafalaya River Basin. The Corp of Engineers has prevented this occurrence for many years by constructing special levees along the course of the Mississippi River. Near Donaldsonville, the Mississippi River flows eastward until it reaches a point just east of New Orleans where it again turns, this time abruptly southward.
Old New Orleans is located in a crescent-shaped bend in the river, a meander. The crescent that formed the Crescent City is really a meander. What happened in 1718 at this meander was a dissipation of energy that resulted in higher ground because of an unloading of sediment. Likely, New Orleans was the highest point near the mouth of the Mississippi River in 1718.
Why is much of New Orleans presently below sea level. The answer is subsidence. Geologically speaking, silt is very unstable. When loaded, it readily compresses and subsides. During the early days of New Orleans, there were no man-made levees separating the City from the Mississippi River. Because of this, the City was flooded with silt and knee-deep water every Spring. City fathers soon began building up the natural levees to prevent this from happening. The result is that much of New Orleans, without the yearly addition of silt from the river, has subsided in the centuries following 1718. Even with this subsidence, the French Quarter and the Central Business District, part of the original settlement, remains at or near sea level and was surely even higher in 1718.
Another reason Bienville chose the present site of New Orleans was because of access. Native Americans had shown the French a short cut from the Gulf of Mexico to New Orleans - a strategic advantage over any foreign power that might attempt to wrest the region from France. This short cut came through a pass from the Gulf of Mexico to Lake Pontchartrain, and then from St. Johns Bayou to present-day New Orleans.
Everyone is aware of the tremendous damage done in 2005 by Hurricanes Katrina and Rita. How can we alleviate a future disaster without moving the venerable old City? Here is my suggestion. Cut the levees near Donaldsonville and let the mighty Mississippi follow its preferred course: into the Atchafalaya Basin to the Gulf of Mexico. Will it change history? Only time will tell.