River Systems. Objective Students will describe factors that affect the erosive ability of a river and the evolution of a river system

River Systems

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Objective Students will describe factors that

affect the erosive ability of a river and the evolution of a river system.



Arizona State Standards Strand 6: Concept 1: PO2

Demonstrate how dynamic processes such as erosion and deposition lead to the redistribution of materials with the Earth system.



Formation of Rivers When precipitation exceeds evapo-

transpiration The excess water moves down slope as runoff Runoff erodes the rock and land forming

gullies The process continues until the gullies grow

bigger and bigger



Parts of a River Main stream Tributaries are feeder streams Watershed is the land from which water

runs off into the streams Divides are ridges or elevated regions

that separate watersheds Channels are narrow depressions that a

stream follows The edge that is above water level is

the bank The part below the water level is the

stream bed

Draw a mountain diagram and label the parts to a river system



2 1

5

3

7

6

4

Stream Load



Minerals in water

Particles of fine sand & silt

Large coarsematerial

Stream Discharge The volume of water moved by a stream Fast streams have more discharge and carry a

greater load Fast streams erode channels quicker


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Stream Gradient The steepness of a stream’s slope Near the beginning of the stream

(headwaters)the gradient is steep Near the mouth, where the stream enters a

larger body of water, the gradient flattens



Evolution of River Channels

When streams become wider and deeper they are called rivers

Meandering channels have low gradients Results in winding patterns of wide curves Braided streams are composed of multiple channels

that divide and rejoin around sediment bars because of the large sediment load



Meandering Rivers

Describe how bank erosion can cause a river to meander.



Braided Rivers

What causes a river to become braided?



River Formation

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Partner Work Summarize how a river develops. Describe the parts of a river system. Describe the three types of stream load. Explain how stream discharge and gradient affect

the erosive ability of a river. Describe the factors that control whether a river is

braided or meandering. If geologic forces were to cause an uplift of the

land surface, what would be the effect on stream channel erosion?

Stream Deposition When water velocity

decreases, the streams ability to carry its load

decreases. The stream load is deposited as

sediment



Types of Deposition

Delta: When a stream

empties into a larger

body of water. A trianglular-

shaped deposit forms.



Types of Deposition

Alluvial Fan: A fan shaped deposit of sediment.

Streams descend a steep slope

Form on land.



Types of Deposition Floodplain: The

part of the valley floor that may be covered with water.

Natural levees are created when coarse sediment is deposited along the banks of a river.



Tucson Floods



Human Impacts on Flooding

Clearcutting Logging Agriculture Housing



Flood Control: Indirect Methods Soil and forest conservation that prevent runoff.



Flood Control: Direct Methods

Dams: The lake behind the dam serves as a reservoir for runoff.

Artificial levees: prevent overflow, but must be protected against erosion.





Levees of New Orleans



Breaking Levees



Partner Work Describe two ways that floods can be

controlled. Explain the differences between the

deposition of a sediment in deltas and alluvial fans with the deposition of sediment on a floodplain.

Summarize how human activities can affect the number of floods.

Identify methods of flood control. What do you think would happen to

cities in Southwestern Arizona, if rivers in our area could not be dammed?

Documents

River Systems. Objective Students will describe factors that affect the erosive ability of a river and the evolution of a river system