Mississippi River Flooding- Charts

Beginning in 1973, the Mississippi River started to rise and to flood more from Greenville to Baton Rouge.  The floods are more frequent, longer and higher.  Why?  It isn’t more rain.  Average rainfall has increased very little since 1940.  What’s causing it?  And what can be done about it?

We think work by the US Army Corps of Engineers on the river and urban development upriver are the primary causes.  The Corps has shortened, straightened, and channelized the river.  This has increased the flow to the lower river.  Faster runoff from development upriver has also increased the flow.  But the Corps has not increased the discharge to the Gulf to handle the greater flow.  So the river rises, backs up, and floods more.

In fact, the discharge to the Gulf has actually decreased.  This was confirmed recently by Dr. Hi-Jun Xu, hydrologist at LSU.  He said a section of the river bed just below the Old River Control Complex has risen more than 30 feet in the last 20 years.  He said it acts like a “stopper” causing the water to back up and rise

We have correlated actual river stage measurements and increased flooding since 1940 when the Corps first big changes (cutoffs) to shorten the river were nearing completion.  The lower river was above flood stage only 5 times in 32 years from 1940 to 1973 or once every 6.5 years on average.

But It has been above flood stage once every other year on average since 1973, and it’s getting worse.  It has been above flood stage 8 of the last 10 years at Natchez and Greenville, 7 years out of 10 at Vicksburg, and 6 of 10 years at Baton Rouge.  And the mean high stage is 6 feet higher at Baton Rouge, 10 feet higher at Natchez, and 7 feet higher at Greenville.

We think the higher river and more frequent and more severe floods are due to the effects of the cutoffs over time and to other changes by the Corps.   These changes include the Old River Control Complex (ORCC) above Baton Rouge and training weirs or dikes in the river between Greenville and New Orleans that direct its flow to the narrower main channel.

The Corps built the ORCC to keep the Mississippi from changing course down the shorter, steeper Atchafalaya River to the Gulf — and turning the existing main channel below Baton Rouge into a salt water estuary.   Prior to the completion of ORCC in 1964 the Mississippi at flood stage naturally flowed down an old river bed (Old River) into the Atchafalaya.  The higher the flood, the greater the flow.  It was sometimes over a third of the Mississippi’s flow.  Old River was nature’s relief valve.  It increased the discharge to the Gulf and lowered the level of the Mississippi.  And mitigated flooding.

But engineers feared the flow could increase to the point that the Mississippi would change course permanently.  So the Corps dammed up Old River, and built the ORCC to control the Mississippi’s flow to the Gulf via the Atchafalaya.   The Corps operates the ORCC to restrict the discharge to 23% of the Mississippi’s flow although it has much more capacity.  The Corps refuses to use extra capacity without specific authorizing legislation by Congress.

The unintended consequence of the reduced discharge is a rising river and higher and longer floods contained by levees not designed to be dams.  The faster flow downriver and slower discharge to the Gulf increase the risk of levee failure and of the course change the ORCC was supposed to prevent.

Dr. Yi-Jun Xu says that course change could happen when the next mega flood occurs.  He didn’t define “mega flood”.  But the ORCC almost failed in the 1973 flood.  And a two-foot higher crest below ORCC in 2011 or 2016 could have been catastrophic.

We think the Corps should operate the ORCC to increase the discharge as the river rises — and that Congress should authorize this.  Now.  This would lower flood crests, make floods shorter, and reduce the risk of levee failure — and a course change.  It would also reduce batture and backwater flooding and the resulting economic and environmental damage on some 1.5 million acres in Mississippi and Louisiana.

It’s time for change.  It’s time for the Corps and Congress to stop playing river roulette.

The following charts show historical flood records for Greenville, Vicksburg, Natchez, and Baton Rouge from 1940 to present.