Physical Geography 102
Fluvial Processes I


Erosion is the mechanical and chemical destruction of the land and the removal of material by moving water, wind or ice.
Sediment and organic material is removed from one location, then transported to another location where it is deposited

Slope Erosion

Precipitation impacts on the surface - dislodges soil and sediment
- Splash Erosion
Small movements of material add up to large movements over time
As soil and sediment are removed by erosion, new material is created by chemical, mechanical and biologic processes
Amount of loss usually equals amount of replacement - therefore the soil tends to maintain itself
Overall the total amount of material is reduced as the hill or slope is eroded
Vegetation will slow the erosion process by holding soil on the slope and blocking rainfall from causing splash erosion

Accelerated Erosion

Reductions in the amount of vegetation can lead to accelerated erosion
- rapid removal of material
Accelerated erosion occurs naturally and through human influences
Causes - forest fires, flooding, volcanic eruptions, disease, overgrazing, mass wasting, agricultural practices
Deserts and Badlands are natural vegetation free areas
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Gully Erosion

In areas cleared of vegetation, water flowing downhill quickly cuts small channels or rills into the slope
Widening and deepening of these rills results in gullies
Gullies will keep the water flow concentrated, further increasing the rate of erosion


Colluvium - sediment which has moved down slope due to splash erosion, overland flow and creep, and accumulates at the base of the slope
Alluvium - sediment deposited by streams

Stream Erosion

Not only do streams transport sediment but they also are a major force of erosion
Erosion in streams occurs through:
- Hydraulic Action
- Abrasion
- Corrosion

Hydraulic Action

Hydraulic action is erosion due to the force of moving water
Moving water is able to move objects
Loose and consolidated sediment on the stream bed can be swept along
If this process removes material below the waterline on a stream bank, the bank may collapse
- this process is called calving


Sediment carried along in the stream can impact on the channel bed or with other sediment
Abrasion will reduce the size of the sediment or remove material from the stream bed if the channel is cut in bedrock
Potholes form as large sediment is moved around in a depression, widening the depression


Chemical erosion of the rock and sediment due to acids in the streamwater

Stream load

Defined as the amount of sediment transported by a stream
Three main means by which sediment is moved in the stream
- Bed Load
- Suspended Load
- Solution Load

Bed Load is sediment which moves along the bottom of the channel by rolling or sliding - sand and gravel

Suspended Load is sediment which is carried above the channel bed - typically clay and silt sized sediment

Solution Load - sediment which is dissolved in the water and carried as solution - example: salt

Suspended Load accounts for the greatest volume of sediment transported by a stream
Stream load is controlled by stream velocity
The greater the velocity the greater the load carrying capacity
Low flow velocities will result in sediment being deposited
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