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8.6: Flooding

Difficulty Level: At Grade Created by: CK-12
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Why are there so many floods?

Floods are a natural part of the water cycle, but that doesn't make them any less terrifying. Put most simply, a flood is an overflow of water in one place. How can you prepare for a flood? What do you do if you're caught in one?

Causes of Floods

Floods usually occur when precipitation falls more quickly than water can be absorbed into the ground or carried away by rivers or streams. Waters may build up gradually over a period of weeks, when a long period of rainfall or snowmelt fills the ground with water and raises stream levels.

Extremely heavy rains across the Midwestern U.S. in April 2011 led to flooding of the rivers in the Mississippi River basin in May 2011 (Figures below and below).

This map shows the accumulated rainfall across the U.S. in the days from April 22 to April 29, 2011.

Record flow in the Ohio and Mississippi Rivers has to go somewhere. Normal spring river levels are shown in 2010. The flooded region in the image from May 3, 2011 is the New Madrid Floodway, where overflow water is meant to go. 2011 is the first time since 1927 that this floodway was used.

Flash Floods

Flash floods are sudden and unexpected, taking place when very intense rains fall over a very brief period (Figure below). A flash flood may do its damage miles from where the rain actually falls if the water travels far down a dry streambed.

A 2004 flash flood in England devastated two villages when 3-1/2 inches of rain fell in 60 minutes.

Buffers to Flooding

Heavily vegetated lands are less likely to experience flooding. Plants slow down water as it runs over the land, giving it time to enter the ground. Even if the ground is too wet to absorb more water, plants still slow the water’s passage and increase the time between rainfall and the water’s arrival in a stream; this could keep all the water falling over a region from hitting the stream at once. Wetlands act as a buffer between land and high water levels and play a key role in minimizing the impacts of floods. Flooding is often more severe in areas that have been recently logged.

Flood Protection

People try to protect areas that might flood with dams, and dams are usually very effective. But high water levels sometimes cause a dam to break and then flooding can be catastrophic. People may also line a river bank with levees, high walls that keep the stream within its banks during floods. A levee in one location may just force the high water up or downstream and cause flooding there. The New Madrid Overflow in the image above was created with the recognition that the Mississippi River sometimes simply cannot be contained by levees and must be allowed to flood.

Effects of Floods

Within the floodplain of the Nile, soils are fertile enough for productive agriculture. Beyond this, infertile desert soils prevent viable farming.

Not all the consequences of flooding are negative. Rivers deposit new nutrient-rich sediments when they flood, so floodplains have traditionally been good for farming. Flooding as a source of nutrients was important to Egyptians along the Nile River until the Aswan Dam was built in the 1960s. Although the dam protects crops and settlements from the annual floods, farmers must now use fertilizers to feed their cops.

Floods are also responsible for moving large amounts of sediments about within streams. These sediments provide habitats for animals, and the periodic movement of sediment is crucial to the lives of several types of organisms. Plants and fish along the Colorado River, for example, depend on seasonal flooding to rearrange sand bars.

“Floods 101” is a National Geographic video found in Environment Video, Natural Disasters, Landslides, and more: http://video.nationalgeographic.com/video/player/environment/.


  • When the amount of water in a drainage exceeds the capacity of the drainage, there is a flood.
  • Floods are made worse when vegetation is cleared, when the land is already soaked, or when hillsides have been logged.
  • People build dams and levees to protect from flooding.
  • Floods are a source of nutrients on a floodplain.


Use this resource to answer the questions that follow.


1. Where are floods more likely to occur?

2. Why have farmers relied on floods?

3. What causes floods?

4. At what depth can a flood move a car? Why is this dangerous?

5. What cause the Mississippi Flood of 1993?

6. Why did Hurricane Katrina cause so much damage to New Orleans?

7. What could cause massive flooding today?


1. How does a flash flood differ from another type of flood?

2. What was the role of flooding on the Nile River and what was the consequence of damming the river?

3. Why do floods still occur, even though people build dams and levees?

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flash flood A rapid flood in a low-lying area that may allow no preparation.
levee A raised structure designed to hold back the waters of a stream or river in the case of a flood.

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At Grade

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Date Created:
Feb 24, 2012
Last Modified:
Aug 07, 2016
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