Journal #5What is a flood?Why are floods harmful?What can humans do to prevent

floods?

Today, we will describe the two types of stream deposition and identify methods of flood control.

Stream DepositionThe total load that a stream can carry

is greatest when a large volume of water is flowing swiftly.

When the speed of the water decreases (gets slower) the ability of the stream to carry its load decreases.

As a result, part of the stream load is deposited as sediment.

DeltaA stream may deposit sediment on land or

in water.Example: the load can be deposited when

the stream reaches an ocean or lakeThe load is usually deposited at the mouth

of the stream in a triangular shape.A triangular shaped deposit that forms

where the mouth of a stream enters a larger body of water is called a delta.

The exact size and shape is determined by waves, tides, off-shore depths and the sediment load

Alluvial FanWhen a stream descends a steep slope

and reaches a flat plain, the speed of the stream suddenly decreases.

As a result, the stream deposits some of its load on the level plain at the base of the slope.

A fan shaped deposit, called an alluvial fan, forms on the land and point upstream.

FloodplainsThe volume of water in nearly all streams

varies depending on the amount of rainfall and snowmelt in the watershed.

A large increase in volume can cause a stream to overflow its banks over the valley floor.

The part of the valley floor that may be covered with water during a flood is called a floodplain.

Natural LeveesWhen a stream overflows, the

stream loses speed and deposits sediment along the banks.

The accumulation of theses deposits eventually make a raised bed called natural levees.

Finer Flood SedimentsNot all of the load deposited by a stream

will form levees.Finer (smaller) sediments are carried

farther out.A series of floods makes the floodplain soil

rich and good for farming. Floodplains provide convenient access the

river for shipping fishing and transportation.

FloodingHuman activity can contribute to the size

and number of floods.Vegetation, such as trees and grass,

protects the ground surface from erosion by taking in much of the water.

Therefore, the likelihood of flooding increases.

Natural events, such as forest fires, can also increase the likelihood of flooding.

Flood ControlIndirect methods of flood control include

forest and soil conservation measures.More-direct methods include the building of

structures that redirect the flow of water.Dams

The lakes that form behind dams act as reservoirs for excess runoff.

The stored water can be sued to generate electricity, supply fresh water and irrigate farmland.

Flood ControlArtificial Levee

Man-made LeveeMust be protected by water erosionWhen they break, flooding and

property damages results.Examples: Seattle, New Orleans,

Galveston

ReviewThis is an example of a ____.

ReviewThis is an example of a ____.

What is a flood plain?

How is a natural levee formed?

What are the advantages and disadvantages of living in a floodplain?

How does human activity contribute to the size and number of floods?

What are some methods of flood control?

Your assignment:1. Delta2. Alluvial fan3. Floodplain4. Natural Levee5. Dam