Flooding explained: crests, spillways and levees
Townhomes sit in floodwater from the Mississippi River on Monday in Memphis, Tennessee.
May 10th, 2011
09:43 PM ET

Flooding explained: crests, spillways and levees

The swollen Mississippi River on Tuesday was in the process of cresting many feet above flood stage in Memphis, Tennessee, and residents of states to the south are bracing for serious flooding in their communities.

A slow-moving wave of water has been working its way down the river since torrential rains sparked flooding in the Midwest's Ohio and Mississippi river valleys in late April. Hundreds of households were ordered to evacuate in the Memphis area (see pictures), where the river was expected to crest Tuesday around 48 feet - 14 feet above flood stage and less than a foot under the city's record level set in 1937.

The high water is headed for Mississippi and Louisiana, prompting authorities to open one spillway and consider opening another - moves meant to divert some of the water into less populated or unpopulated areas. Up to 5,000 homes will be evacuated in Mississippi, officials there say. In Louisiana, where the river is expected to crest next week, 21 parishes already have issued emergency declarations.

You will hear a lot about crests, spillways and levees over the next couple of weeks. Below you'll find what these mean and what the Army Corps of Engineers and others are doing to mitigate the flooding threat along the Mississippi.

When a river crests

The National Weather Service says a crest is the highest point in a wave. In the case of river flooding, it is the highest stage or level of a flood wave as it passes a particular point. Gauges along the river record the level of the water, and the highest level recorded at each gauge will be the crest for that gauge.

Observers generally know that cresting is occurring when the water level stops rising and becomes stable. However, it's difficult to identify the official crest at the moment that it happens, because during the cresting, small fluctuations happen - water levels can rise a little after they fall a little. The fluctuations are caused by factors such as wind and water currents.

The crest will have passed when the water level continuously decreases. Once this has happened, observers can note what the highest recorded level was. But the cresting of a river at a certain point doesn't mean immediate relief for that area - the water level will lower gradually, meaning flooding can still be a problem days after a crest. On Tuesday, communities in southwestern Illinois and southeastern Missouri still were flooded, days after crests there. In Memphis, although the river was cresting there Tuesday, residents could be dealing with high water levels into June.

Levees meant to keep river in its place

A 2,200-mile system of earthen levees, floodwalls and other controls were built along the Mississippi, Arkansas and Red rivers after a massive flood overwhelmed the southernmost stretch of the Mississippi River in 1927, according to the U.S. Army Corps of Engineers.

The levees guard riverside communities to some extent, but they are rarely tested with the kind of volume and pressure passing through the river now. On Monday in Memphis, the water was moving at 2 million cubic feet per second. At that speed, water would fill a football field at a depth of 44 feet, according to the Army Corps of Engineers.

The system is designed to manage speeds of up to 2,360,000 cubic feet per second at Cairo, Illinois, and up to 3 million cubic feet per second where the Red River meets the Mississippi River in Louisiana. But below the Red River, floodways and spillways (see section below) would be used to divert half that water to other areas in Louisiana so that only up to 1.5 million cubic feet per second flow down the main river channel, according to the Army Corps of Engineers.

Most levees are piles of dirt lined along edges of a river. In theory, vegetation and its roots will keep the dirt stable as water flows by. But levees can fail, in part because moving water can erode, saturate, undermine and destroy them. (Learn more about how levees can fail).

Floodways and spillways

Floodways and spillways are places where the Army Corps of Engineers diverts water from a swollen river. In some cases, this involves intentionally flooding one populated area to prevent or lessen flooding in a more populated area.

On May 2, the Corps intentionally breached a levee near the confluence of the swollen Mississippi and Ohio rivers at the Missouri/Illinois border, sending some of the Mississippi’s water into the New Madrid Floodway - which is 130,000 acres of Missouri farmland.

Missouri officials, wanting to protect the farmers' land, tried unsuccessfully to block the move. The Corps says the move was designed to prevent devastating flooding in Cairo, Illinois, and elsewhere downstream. (See pictures of New Madrid Floodway taken before, after the levee breach.)

The Corps estimates it will take up to two months for the water to recede from the floodway and another month for the land to dry out.

In Norco, Louisiana, the Corps this week opened 72 gates to the Bonnet Carre Spillway, north of New Orleans, diverting millions of gallons of Mississippi River water into Lake Pontchartrain and, eventually, into the Gulf of Mexico.

The agency is considering whether to open the Morganza Spillway, which is on the Mississippi north of Baton Rouge, Louisiana. When the spillway is opened, water would be diverted to the Atchafalaya Delta to the west and south of Baton Rouge, and could flood populated areas such as the town of Morgan City, which has 13,000 residents.

soundoff (141 Responses)
  1. Cesar

    Has anyone seen my little troll? Oh hi Ellis, so nice to see you. Hahahahahahahahaha!

    May 10, 2011 at 5:08 pm | Report abuse |
  2. Ellis From Panama

    The Mississippi River is like a sore crack. How awful.

    May 10, 2011 at 5:14 pm | Report abuse |
  3. iamthefreman

    You mean to tell me that the builders and the city really thought a gigantic river like the Mississippi would never reach over the banks? Really? The home buyers didn't think so either? Really? I feel like that bad SNL skit... Really??

    May 10, 2011 at 5:14 pm | Report abuse |
    • Laurie

      How uninformed are you? Memphis has all kinds of precautions as do the cities on the river south of Memphis. If it didn't, the flood would be affecting many more in Memphis then it is. After a major flood in 1927, levees were built even higher and better to protect cities along the Mississippi. If you are suggesting that cities shouldn't be built near a river and that every city should be completely inland, then you don't know much about history or commerce or chances of floods. If we go by people like you, maybe nobody should live in Kansas – don't they know it's tornado alley and nobody should live on either coast – isn't there a chance of earthquakes and even a tsunami. It's so refreshing to hear people like you, who aren't involved in the floods and know nothing about them, to not help but blame hundreds of small towns or cities and their people for an act of God! What a great person!

      May 10, 2011 at 5:29 pm | Report abuse |
  4. Gene

    So, CNN, you're saying that f'(x) =0 and f"(X) <0 at x = Memphis?

    May 10, 2011 at 5:19 pm | Report abuse |
  5. Ellis From Panama

    I love cracks, especially, well, where is Ricky Martin. Get it?? *snicker/blush*

    May 10, 2011 at 5:27 pm | Report abuse |
  6. bezerkur

    whether u are religious or not one cannot ignore the world is in crisis from not just eviromental catastrophys but political and social problems as well. how far can the rubber band stretch? no one can deny the last 10 years hasnt been epic on every level. i try and stay positive and not dwell. i believe this world is in a turning point. to what end its hard to say. i could fill this entire blog with examples of what is wrong. whats sad is everyone who is participating in this blog can do the same.

    May 10, 2011 at 5:27 pm | Report abuse |
    • Gene

      Well, the Mongols overran most of and slaughtered much of the civilized world in the thirteenth century. In the fourteenth, a third of Europe died from fleas. After WWI, we thought that, as least, we've finally probed the depths of human depravity. As the subsequent twenty-five years demonstrated, we weren't even close. Four billion years ago, the earth was a molten mess uninhabitable.

      Epic? That's a conceit. Our troubles are small potatoes

      May 10, 2011 at 5:46 pm | Report abuse |
  7. Outsider

    Cesar, you are getting on my nerves, I told you to stop posting but you don't listen. I have your IP address and will proceed against you. Have a nice life. Jorma

    May 10, 2011 at 5:30 pm | Report abuse |
  8. Gene

    Of course, I meant f'(x) = 0 and f"(x) < 0 at x = Memphis.

    Also, 5 = atonement, 10 = completeness, and 17 = Heaven. Saturday a week comes the Big Bwahahaha.

    May 10, 2011 at 5:32 pm | Report abuse |
  9. Cesar

    @Outsider: I will post what and when I feel like it. You get in my nerves. Get out!

    May 10, 2011 at 5:38 pm | Report abuse |
  10. Joey

    I just tried to post and failed.
    Have I been banned along with my troll?

    May 10, 2011 at 5:43 pm | Report abuse |
  11. Joey

    Thank you.
    I see no posts from the fake Joey, only from an honestly "fat" Joey troll.

    May 10, 2011 at 5:46 pm | Report abuse |
  12. Jeff Frank - (R-Ohio)

    Seems CNN has their hands full, with a lot of people, that have a lot of time on their hands.

    May 10, 2011 at 6:12 pm | Report abuse |
    • Gene

      So, you're saying CNN has their hands full of people who have time on their hands?

      Is this like Russian dolls?

      Is CNN in Ted Turner's hands?

      Also, could I borrow some of the time you evidently have? Mine's about to run ou

      May 10, 2011 at 6:23 pm | Report abuse |
  13. Jazzzzzzzz

    can I post

    May 10, 2011 at 6:15 pm | Report abuse |
  14. Ellis From Panama

    Outsider is a BIG ASS......

    May 10, 2011 at 6:19 pm | Report abuse |
  15. Sean

    Jeff Frank.... You do have more time don't you? Start wiping your butt, you are full of it.

    May 10, 2011 at 6:26 pm | Report abuse |
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