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

Stream Deposition The 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.

Delta A stream may deposit sediment on land or in water. Example: the load can be deposited when the stream reaches an ocean or lake The 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 Fan When 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.

Floodplains The 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 Levees When 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 Sediments Not 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.

Flooding Human 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 Control Indirect 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 Control Artificial Levee Man-made Levee Must be protected by water erosion When they break, flooding and property damages results. Examples: Seattle, New Orleans, Galveston

Review This is an example of a ____.

Review This 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. Delta 2. Alluvial fan 3. Floodplain 4. Natural Levee 5. Dam