• The Mississippi river, with its sand and silt, had created most of Louisiana, and it could not have done so by remaining in one channel.

• The Mississippi had jumped here and there, within an arc about 200 miles wide.

• Always it is the river's purpose to get to the Gulf by the shortest and steepest gradient.

• Major shifts have occurred once in a millennium.

  Mississippi river geomorphology

• The Red river had been a tributary of the Mississippi, but in the 1940's, the Atchafalaya distributary captured it and drew it away.

• It formed the letter H: the Atchafalaya and Red on the left side; the Mississippi on the right side.

• The cross piece was named Old River.

• The structure was named Old River Control Structure (ORCS).

• In times of flood, the Atchafalaya was useful as a safety valve.

• The Corps of Engineers could not plug the Atachafalaya.

• The Corps would have to built something that would give the Atchafalaya a portion of the Mississippi, but prevent it from taking it all.

• The Corps established that 30% of the flow would go to the Atchafalaya.

• Today the Atchafalaya's waters are 10 ft lower than the Mississippi.

• Just how the Army Corps got involved in the Lower Mississipi has no easy answer.

• It started in the War of 1812.

• After that war, Congress instructed the Corps of Engineers to survey the Mississippi and its tributaries with an eye to improving inland navigation.

• In terms of hydrology, what the Corps has done at ORCS is to stop time.

• In southern Louisiana, the bed of the Mississippi river is so far below sea level that a flow of 120,000 cfs is required to keep the ocean salt water from flowing inland.

• "Mother Nature has more time than we do. She has nothing but time."

• Prior to 1972, the Mississippi's main channel, in its relative lethargy, had given up a lot of volume to accumulations of sediment.

• High water would flow that much higher.

• Under absence of floods, channels deposit their sediment load in the bed: aggradation.

• Under floods, channel erode their beds: degradation.

• In 1973, 20 inches of precipitation fell in 36 hours.

• During the flood of 1973, a hole developed under ORCS that was deeper than a football stadium.

• Structure was founded on piles, driven though clayey peats and organic muck.

• The shallowest rock was at a depth of 7000 ft!

• The water is from the state of New York, the state of Montana, the province of Alberta.

• The floodplain is more than 100 miles wide.

• "This river used to meander all over its floodplain. People would move their tepees. You cannot move Vicksburg."

• Rivers overflow their banks, and create natural levees.

• In response to major floods, the levees were extended and raised.

• Shreve made a bold move on the Mississippi: He cut a meander loop, which increased the flow of the Mississippi into its distributary, advancing the date of ultimate capture.

• In 1850, the levees averaged 6 ft.

• River stages, in their wide variations, became generally higher through time, as the water was presented with fewer outlets.

• Even at normal stages, the Mississippi was beginning to stand up like a large vein on the back of a hand.

• Hold by levees was the way to do the job.

• In 1882, the levees failed in 284 places.

• According to the Corps, it was important that no water be allowed to escape the river, because its full power would be most effective in scouring the bed.

• This was the hydraulic and hydrological philosophy of the Army Corps of Engineers.

• By 1927, it was apparent that the levees were helping to aggravate the problem they were meant to solve.

• The Corps built levee fuses, which would break on purpose, releasing the pressure on the system.

• The Design Flood was called the Project Flood.

• The Atchafalaya, continuing to grow, had become, by volume of discharge, one of there or four largest rivers in the U.S.

• Compared with the Mississippi, it had a 3:1 advantage in slope.

• Every shopping center, every drainage improvement, every square foot of new pavement in nearly half the United States was accelerating runoff toward Louisiana.

• As contributing factors grew, the river delivered more flood for less rain.

• After 1973, the flow lines were recalculated and the levees had to be raised.

• "When the river would pool against the stratosphere was only a question of time."

• The original ORCS cost 86 million dollars.

• The work of repair would increase this amount to 500 million.

• "Nobody knows where the 100-yr flood is. The data have lost their pristine character.

• "Flood across the century are getting higher, low stages lower."

• The Atchafalaya is now one of the 20 largest rivers in the world.

Levee on the Lower Mississippi River near Romeville, LA.

Atchafalaya slide show

• Mississippi river meanders across its flood plain.

• Holding by levees concentrates all sediment flow within the channel.

• Aggradation is caused by baseflow; degradation by floods.

• The balance of baseflow vs floods determines the net effect, either net aggradation or net degradation.

• In the Mississippi, it looks more like net aggradation.

• The Corps hoped it would be net degradation.

• The levee solution solves some problems, but it creates many other problems.
  
  
  • False sense of security as floods increase in magnitude and frequency with upstream development.

  • Starvation of flood plains due to nutrient shortage.

  • Loss of the sight of the river.

• Paving increases the magnitude and frequency of floods.

• Based on the geomorphology, it is concluded that it is a matter of time before the Atchafalaya becomes the master stream.

• The Army Corps of Engineers is charged by Congress not to let this happen.