Extensive flooding along the Mississippi River is being touted as evidence of global warming by the alarmist press including the Arizona Daily Star (see article). It really is a result of the La Niña phase of the El Niño–Southern Oscillation (ENSO) and the Pacific Decadal Oscillation. The extensive tornadoes have a similar cause (see The storm over tornadoes.) Accuweather said: “The combination of a weakening La Nina and the anticipated sharp temperature anomaly gradient between the northern U.S. and the southern U.S. told us that the jet stream running across the U.S. would be abnormally strong this spring. A strong jet stream leads to more powerful storms and thunderstorms, which increases the chances of large tornadoes and widespread flooding.”

The Mississippi River system drains 41% of the continental U.S. Floods are common and cyclical. The Star article claims, “Flooding on the Mississippi has become more frequent and more extensive since about 1950…” The record for the lower Mississippi from NOAA would disagree. That record, beginning in the year 1543, shows that the most floods occurred in the 1920s and 1930s, and the most extensive flood occurred in 1927 on the lower Mississippi. Perhaps the author of the Star article was looking at this record for Wisconsin and Minnesota.

The Mississippi River has an extensive floodplain. It’s called that for a reason. We have tried to modify the floodplain, and make land available, with an extensive system of levees which work much of the time, but this is one of the times the system did not work as hoped. In fact, much land was flooded due to purposely breeching the levee to sacrifice part of the floodplain in the hope of saving cities downstream. We’ve tried to tame the mighty Mississippi with our engineering works, and many people relied on these government efforts. But there is always a risk. The risk that nature is stronger. Ultimately, if you inhabit the floodplain, you should expect to get wet once in a while.

Some geology:

The Mississippi River, left to itself, wanders across the alluvial flood plain. The graphic below, a LANDSAT image shows a portion of the river. Notice the arcuate structures on the sides. These represent former reaches of the river. Below the LANDSAT image is a map of a portion of the river showing the deposits made by the various river courses (Map by Harold Fisk, 1944 US. Corps of Engineers).

As the river flows around a bend, erosion takes place on the outside of the curve and deposition occurs in the inside. Eventually, the river cuts off the curve leaving an arcuate oxbow lake. The graphic below from the Iowa DOT shows how this occurs.

Finally, we have a pair of photos (USGS/NASA) showing a portion of the river in May 2006 and during the flooding in May 2011.

This entry was posted on Tuesday, May 31st, 2011 at 7:23 am and is filed under Climate change, Geology. Tags for this post: Climate change, floods, Mississippi. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.