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.

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