Historic Mississippi River Flood Brings Highest Crest on Record Below St. Louis

A historic and unseasonable flood has brought the highest flood levels ever recorded to the Mississippi River south of St. Louis, thanks to more than 10 inches of rain that fell over a three-day period that began Christmas Day. At least 25 deaths in Illinois and Missouri are being blamed on the flooding. The Mississippi River crested at its third highest water level on record in St.Louis on January 1. On January 2, the southward-propagating crest brought the second highest flood on record to Chester and the highest flood on record to Cape Girardeau and Thebes. On Monday, January 4, the crest is expected to be about 200 miles south of St. Louis at Caruthersville, Missouri, bringing the third highest flood ever observed there. The latest flood forecasts for the Mississippi River issued Sunday evening by NWS River Forecast Center predicted no other locations would see an all-time record crest, with crests between the 2nd and 10th highest on record expected along most of the Lower Mississippi and the lower portions of two main tributaries, the Ohio and Arkansas Rivers.


Figure 1. Precipitation over much of the drainage basin of the Mississippi River in Wisconsin, Iowa, Illinois, Missouri, and portions of surrounding states during December 2015 averaged 200 - 600% of normal (dark blue to purple colors.) Image credit: NOAA/NWS/AHPS.


Figure 2. The Mississippi River downstream of St. Louis at Thebes crested on January 2 at the highest level ever observed. Flood records at this location extend back to 1844. The sudden dips in the water levels may be due to levee breaches. Image credit: NOAA/AHPS.

Huge stresses being put on Mississippi River levees
The record flood is putting record stresses on portions of the Mississippi River levee system. According to the Associated Press (AP), the Len Small Mississippi River levee near Miller City, IL--just north of the river's confluence with the Ohio River at Cairo, Illinois--was breached Saturday, bringing the total number of levee breaches from the flood to eleven. The area affected was mostly agricultural, but about 500 people living behind the levee in the towns of Olive Branch, Hodges Park, and Unity were told to move to higher ground. The Len Small levee is a local flood control district levee that fronts the Mississippi River, and is not a U.S. Army Corps of Engineers levee.


Video 1. Aerial view of the Len Small levee break in southern Illinois on January 2, 2016. Thanks go to wunderground member beell for posting this video in my blog comments.

Andy Revkin's December 30 Dot Earth blog in the New York Times quoted Nicholas Pinter, a professor of applied geology at the University of California, Davis, who has extensively studied flood risks on the Mississippi River. Pinter was an author of a 2011 paper showing areas where surface erosion, compaction of soil or subsidence lowered levee elevations in ways that raise risks in such floods. Here’s his quote:

"The big big issue for the next couple of days is levee performance, as flood levels exceed 1993 in some locations. The current forecasts call for large levee systems like Grand Tower and Degognia and others…to be at overtopping levels. Most of the Middle Mississippi River levees are currently rated Unacceptable (like 44.5% of rated levee miles nationwide); plus there are major geotechnical issue documented by our research--subsidence and compaction and un-permitted levee modifications using road gravel that is unsuited to flood control. There is room for optimism--this flood wave is very sharp and will be days long…rather than weeks. My personal fear is that many of these levees are degraded in ways that may rear up suddenly as near-record flood levels roll through. I am very apprehensive for neighbors and colleagues and friends on the bottoms."


Figure 3. Changes in levee elevation as calculated between 1998 - 2007 along the Mississippi River near its confluence with the Ohio River near Cairo, Illinois. Red indicates places where levees along the Mississippi River have subsided in ways that increase flood risks. The maximum sinking measured was 4.1 feet. Image credit: Nicholas Pinter and Southern Illinois University.

Army Corps may be forced to open two flood structures
On January 12, the flood crest is expected to arrive in New Orleans, bringing the Mississippi River to its 17-foot flood stage in the city--just 3 feet below the tops of the levees. In past years, though, when the river has been forecast to rise to 17 feet in the city, the Army Corps of Engineers has opened up the Bonnet Carré Spillway in St. Charles Parish, which diverts water into Lake Pontchartrain and keeps the river from reaching flood stage in New Orleans. According to a December 30 news story in The Advocate, the Army Corps will make a decision by January 9 on whether or not to open the Bonnet Carré Spillway. The last time the spillway was opened in January was back in 1937, its first year of operation. There is a lower chance that the Corps will be forced to open the Morganza Floodway in Pointe Coupee Parish between New Orleans and Baton Rouge, which would divert water down the Atchafalaya River. Opening this spillway has a considerably higher cost than opening the Bonnet Carre Spillway, due to the large amount of agricultural lands that would be flooded below the Morganza Floodway. The Corps also has the option of increasing the flow of Mississippi River water into the Atchafalaya at the Old River Control Structure in Concordia Parish. Operating the Old River Control Structure in this way, though, puts stress on the structure, as I explained in my 2011 blog post, America's Achilles' heel: the Mississippi River's Old River Control Structure. The Sunday evening forecast from the NWS River Forecast Center predicted that the Mississippi River would crest at Red River Landing, where the Old River Control Structure is located, on January 18. The predicted crest of 62.5' is just 0.9' below the all-time record crest of 63.39' set on May 18, 2011, when the Corps was forced to open the Morganza Floodway in order to relieve pressure on the Old River Control Structure.

Another huge concern is what shape the levees will be in after this record January flood subsides. After the great 2011 flood, about a billion dollars was required to bring the levee system back to the state it was in before is was damaged by the flood. The Army Corps will not have much time to do repairs before the annual spring flood arrives in May on the Mississippi, so the levee system will be much more vulnerable than usual to major flooding. So far, though, the odds of major spring flooding along the Mississippi appear to be below average. The winter snow pack in the Dakotas, Minnesota, and Wisconsin is much below average, thanks to a warm December. The water content of the snow pack on January 3, 2016 was about 20% of what was present on January 3, 2011, in advance of the great 2011 spring flood. In addition, we may be helped out by the fact that El Niño tends to cause lower-than-average precipitation over the key Mississippi drainage area of the Midwest.  See the NWS outlooks (scroll down pages to get to the precip maps) at:
http://www.cpc.ncep.noaa.gov/products/predictions/long_range/seasonal.php?lead=01
http://www.cpc.ncep.noaa.gov/products/predictions/long_range/seasonal.php?lead=02
http://www.cpc.ncep.noaa.gov/products/predictions/long_range/seasonal.php?lead=03

I'll have a new post on Tuesday, when my top-ten list of 2015 weather events is scheduled to go up.

Jeff Masters