Post on 08-Jan-2018
description
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.
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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.
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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
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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
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2 1
5
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Stream Load
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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
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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
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Meandering Rivers
Describe how bank erosion can cause a river to meander.
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Braided Rivers
What causes a river to become braided?
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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
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Types of Deposition
Delta: When a stream
empties into a larger
body of water. A trianglular-
shaped deposit forms.
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Types of Deposition
Alluvial Fan: A fan shaped deposit of sediment.
Streams descend a steep slope
Form on land.
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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.
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Tucson Floods
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Human Impacts on Flooding
Clearcutting Logging Agriculture Housing
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Flood Control: Indirect Methods Soil and forest conservation that prevent runoff.
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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.
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Levees of New Orleans
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Breaking Levees
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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?