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    To understand how floods happen, you need to know the shape and depth of the river. Rivers have a main channel where the water flows, but it also has an area on each side of the bank called a flood plain. The plain holds any water that flows over the river's bank. The height of a river depends on how deep it is in the center of the channel. If the river is deep it will hold more water. Large rivers that have a lot of tributaries flowing into them tend to run deeper because the water runs faster.    
                 
    NOAA Photograph

Hydrologists are scientists that study rivers and flooding. Before they can predict whether a river will over flow its banks and flood the land, they need information from weather forecasters.

Hydrologist need to know how much water is already in the river, how much more water will come from rainfall or snowmelt, how to track the flow of water in the river, and how to track water draining into the river from its tributaries.They get this information from local river forecast centers that are located around the United States.


    
   

Hydrologists use rain gauges, weather radar, and weather satellites to tell them where rain is falling and how much rain is falling. That information will tell them if there are any areas in danger of flooding. Hydrologists keep a close eye on how much rain is falling in the areas that flood most often. When snow and ice at the top of mountains begins to thaw and run into the river systems, the hydrologists keep track of river water levels. If they know how much rain has fallen and how much more is expected, then they can issue a flood watch or warning.

  NOAA Photograph      
       
       
       
       
    Some hydrologists measure snow levels on mountains and glaciers. Their research will allow river hydrologists to make predictions about when melting snow and ice might cause problems along flood plains. River Forecast Center in Your Area      
    Not only do floods occur during heavy rains, but they also happen down stream away from the source of extra water. As flood water drains off of the land and into rivers, the water begins to move swiftly towards its drainage outlet. Small rivers drain into large rivers, and large rivers always flow towards an ocean or the sea. The front edge of the moving flood water is called the crest. The crest passes quickly at first, but as it moves towards its outlet the crest moves more slowly and causes flooding to last longer.    
                 
    Learn more about hydrology     Ask a Hydrologist questions      
             
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Photographic Citations:
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Text Citations:

Encyclopedia.com:
http://www.encyclopedia.com/html/section/flood_floodcharacteristicsandcontrol.asp

NOAA:
http://www.nws.noaa.gov/oh/tt2/wwater2/TOUR/tour2a-go.shtml