Mississippi River sets all-time flood records; 2nd major spillway opens

The Mighty Mississippi continues to wreak havoc as the river's highest flood crest in history pushes southwards near Memphis, Tennessee today. The river crested at its highest height on record over the past four days along a 70-mile stretch from New Madrid, Missouri to Tiptonville, Tennessee, to Caruthersville, Missouri, smashing records that had stood since the great flood of 1937. The flood height of 47.6' at Caruthersville, Missouri, on Saturday was a full 1.6 feet above the previous record flood height, set in 1937. However, thanks in part to decision by the Army Corps of Engineers last Monday to intentionally destroy a levee at Birds Point on the west bank of the Mississippi, pressure on the levees along this stretch of river was substantially reduced, potentially preventing multi-billion-dollar levee breaches. Currently, the Mississippi is expected to reach its 2nd highest level on record at Memphis on May 10, cresting at 48.0'. The all-time record at Memphis occurred during the great flood of 1937, when the river hit 48.7'. Downstream from Memphis, flood waters pouring in from the Arkansas River, Yazoo River, and other tributaries are expected to swell the Mississippi high enough to beat the all-time record at Vicksburg, Mississippi by 1.3' on May 19, and smash the all-time record at Natchez, Mississippi by six feet on May 21, and by 3.2 feet at Red River Landing on May 22. Red River Landing is the site of the Old River Control Structure, the Army Corps' massive engineering structure that keeps the Mississippi River from carving a new path to the Gulf of Mexico. I'll have a detailed post talking about the Old River Control Structure later this week. Its failure would be a serious blow to the U.S. economy, and the great Mississippi flood of 2011 will give the Old River Control Structure its most severe test ever. Also of concern is the forecast for the Mississippi to crest at 19.5 feet in New Orleans on May 23. The levees in New Orleans protect the city for a flood of 20.0 feet--that is not much breathing room. Fortunately, rainfall of at most 0.5 inches is expected over the Lower Mississippi River watershed over the next five days, which should prevent flood heights from rising above the current forecast.


Figure 1. Opening of the Bonnet Carre' Spillway on March 17, 1997. The spillway was operational from March 17 to April 18, 1997, operating at a maximum flow of 243,000 cu ft/s (6,900 m3/s). Image credit: Army Corps of Engineers.

Bonnet Carre' Spillway opens
Today, the Army Corps of Engineers is setting in motion another key part of their plan to control the great flood of 2011. The Corps is opening the Bonnet Carre' Spillway to divert 250,000 cubic feet per second of water from the Mississippi River into Lake Pontchartrain, some 28 miles upstream from New Orleans. A large crane will traverse a 1.5 mile-long stretch of the Mississippi River and remove large wooden slats that will allow the river to spill northwards into a 6-mile long channel lined by guide levees, to Lake Pontchartrain. The land that will be inundated is uninhabited and is not farmed, unlike the land of the Birds Point-New Madrid Spillway that was inundated by last week's intentional levee breach. The main concern with opening the Bonnet Carre' Spillway is the impact of the Mississippi River's fresh water on the salt water ecosystems of Lake Pontchartrain. This is the 10th time since 1937 that the Bonnet Carre' Spillway has been opened. The Army Corps is considering opening the final spillway they have in reserve, the great Morganza Spillway in Louisiana, late this week. The Army Corps has never opened all three Lower Mississippi River spillways at the same time. The Morganza Spillway has been opened only once, back in 1973.


Figure 2. The last time the Bonnet Carre Spillway was opened was in 2008. The International Space Station captured this image of the muddy brown waters of the Mississippi flooding into Louisiana's Lake Pontchartrain, just upstream from New Orleans. Image credit: NASA Earth Observatory.

Good links to follow the flood:
Summary forecast of all crests on Lower Mississippi and Ohio Rivers.
Wundermap for Cairo, IL with USGS River overlay turned on.
National Weather Service "May 2011 Mississippi River Flood" web page

Jeff Masters