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. saints

    Jsizzle you are a looser

    May 11, 2011 at 4:58 am | Report abuse |
  2. saints

    Jsizzle you are sick. I woldnt want to know these worthless idiots

    May 11, 2011 at 5:06 am | Report abuse |
  3. Cesar

    @Saints: Shut up and Kiss me.

    May 11, 2011 at 6:07 am | Report abuse |
  4. joAnn

    I,do feel for all the people,that has lost everything.I,wonder how,the people in the flood,of 1927 had to deal in this situation?I,do believe God's in control with this situation. JoAnn

    May 11, 2011 at 6:22 am | Report abuse |
  5. searchingForAnAthiestExtremist

    If _this_ is "god in control" I think your "god" has a real crappy sense of humor.

    May 11, 2011 at 6:41 am | Report abuse |
    • ds

      Obviousely, because you don't quite have a grasp on Go's plan (which is all written). How could you know since, you choose to open your mouth without engaging brain.

      Typical atheists tout "critical thinking"...... I guess you missed that memo.

      May 11, 2011 at 7:49 pm | Report abuse |
  6. i cant belive is not butter

    Good one cnn! People starveing in africa and you write about cresting....sweet

    May 11, 2011 at 6:43 am | Report abuse |
    • mel

      what does it matter anyway, you will never be satisfied.

      May 14, 2011 at 6:14 pm | Report abuse |
    • smc2009

      Do you really think all of this is simply about a river cresting? Learn a little about your own country and the potential impact of this before making a pat reference to people starving in Africa. The Mississippi is the largest water transportation artery in the country. It drains over 40% of the continent , its delta provides the most productive farmland in the country, and over 60% of America's grain exports are transported on the river. Everyone would be starving without it.

      May 16, 2011 at 11:50 am | Report abuse |
  7. AJ

    What a terrible situation for those along the Mississippi river. I love how people are saying it's the End times, or questioning God, when the fact of the matter truly is, these types of events, while rare, are known to happen, and after 60+ years should honestly be EXPECTED at some point. One Natural Disasters class I took gave specific statistics, and now I realize how spot on it truly was. We've been lulled into the idea of safety thinking we can protect ourselves, and prepare, but nothing can prepare us for a once every 50-60 year event or even every 100 year event. The Earth has proven that to us all this year with tragic tragic consequences. With that said, I still pray for all those affected, it is terrible, yes, but no random act of God or End times situation. Just the Earth doing what she always does with no regard to human life. I pray that God comforts those most affected, and that the community would rally together and show how strong they truly are in during these horrible disasters.

    May 11, 2011 at 8:50 am | Report abuse |
    • AgentStorm

      Very true. And I would like to add to your statement "Earth doing what she always does with no regard to human life" as we humans have little to no regard for her if you think about it. It is a sad situation but you are correct about these disasters.

      May 11, 2011 at 3:19 pm | Report abuse |
    • Lars

      You are absoulutly correct about nature. Two factors you cannot really anticipate all the time is wind and water. Both are very powerfull. My father was 15 for the flood of 37 and is living in the same town now as he did then. Listening to his memories are a marvel. But people that have never seen floods and tornados don't really know what to expect and become complacent. I'll bet that a lot of these people who are now and soon will be flooded out do not have flood insurance because" well it hasn't happened in my lifetime." I live in San Antonio, Texas for the most of the last 3 years we have been in a drought. In 98 the area I live in was hit with a 750 year flood 17 inches on rain in 12 hours or so.. You just never know.

      May 15, 2011 at 1:53 pm | Report abuse |
  8. John

    Very sad, worried about the young kids and old.

    May 11, 2011 at 12:57 pm | Report abuse |
  9. humberto

    Maby Obama wants a retake on that .

    May 11, 2011 at 2:22 pm | Report abuse |
  10. Augie

    So just why is the sky wizard punishing the bible belt who has their hands out for federal assistance dollars they don't want to pay to receive? Curious minds like to know . . . and all that.

    May 11, 2011 at 3:10 pm | Report abuse |
  11. searchingForAnAthiestExtremist

    Lol @ Augie! Spot on.

    May 11, 2011 at 3:30 pm | Report abuse |
  12. searchingForAnAtheistExtremist

    I say we only give them fed money if they agree to stop whining about the feds for a whole term

    May 11, 2011 at 3:32 pm | Report abuse |
    • John

      Fidel Casto, Lenin, Stalin

      May 14, 2011 at 6:08 pm | Report abuse |
  13. skeptic

    They got our water. Texas is bone dry.

    May 11, 2011 at 8:32 pm | Report abuse |
    • Bob Jones

      That's not the only thing Texas is missing.

      May 14, 2011 at 7:53 pm | Report abuse |
  14. guido

    This land was not meant to be settled on.it's a flood plain.Good luck!

    May 12, 2011 at 7:00 pm | Report abuse |
  15. Mark Felton

    Flooding explained? Could it be that it is "loads of water"?

    May 12, 2011 at 7:37 pm | Report abuse |
    • krozareq

      What's worse is all the stuff in the water. The best is the entire fire ant population floating on top of the water and if a colony gets on you then you're pretty much fooked.

      May 14, 2011 at 3:56 pm | Report abuse |
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