River Systems

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.  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  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  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

Stream Load Minerals in water Particles of fine sand & silt Large coarse material

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  The volume of water moved by a stream  Fast streams have more discharge and carry a greater load  Fast streams erode channels quicker

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  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  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

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?  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  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.  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.  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.  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  Clearcutting Logging Agriculture Housing

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