flooding

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 1.1 and 1.2). Click image to the left or use the URL below. URL: 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.

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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.

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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 Figure 1.2 was created with the recognition that the Mississippi River sometimes simply cannot be contained by levees and must be allowed to flood.

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 waters passage and increase the time between rainfall and the waters 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.

flash floods

Flash floods are sudden and unexpected, taking place when very intense rains fall over a very brief period (Figure streambed. A 2004 flash flood in England devastated two villages when 3-1/2 inches of rain fell in 60 minutes. Pictured here is some of the damage from the flash flood. Click image to the left or use the URL below. URL:

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instructional diagrams

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questions

which of these can buffer flooding?

a) vegetation

b) wetlands

c) levees

-->  d) all of the above

a levee that protects the land near a river from flooding

-->  a) may cause there to be flooding upstream or downstream.

b) may cause there to be flooding downstream only.

c) may protect the entire river from flooding.

d) will keep that land safe from flooding as long as it is kept in working order.

a raised structure designed to hold back the waters of a stream or river in case of a flood.

a) river

b) levee

-->  c) dam

d) none of the above

this river flooded sometimes causes extreme floods in the midwestern u.s.

a) the sacramento river

b) the colorado river

c) the des moines river

-->  d) the mississippi river

flooding is often worse when vegetation is cleared.

-->  a) true

b) false

floods are relatively recent phenomenon, only occurring since humans have altered the landscape.

a) true

-->  b) false

the lands downstream from a dam are safe from flooding.

a) true

-->  b) false

plants reduce flooding by

a) keeping the rainwater from striking the ground.

-->  b) slowing the raindrops down so they dont all hit the ground at once.

c) allowing the water to flow downhill in its own way.

d) all of these.

how did flooding of the nile river help the egyptians?

-->  a) floodwaters were a source of nutrients along the floodplain.

b) flooding protected human settlements from invaders.

c) flooding moved sediment into the river delta and stopped large waves from coming ashore.

d) all of these.

diagram questions

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