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Mountains: Erosion

Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

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Page 1: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Mountains: Erosion

Page 2: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Erosion

Page 3: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Sediment Regime

• Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and upstream.

• The amount or rate of sediment supply depends on the processes that govern sediment delivery to rivers.

Page 4: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Sediment Supply to Rivers

Size and composition of sediment delivered to rivers reflects:

• soil properties

• rock properties

• the process that delivered it.

Graduate student for scale

Page 5: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Sediment Budget

Soil Creep

Landsliding

Bank Erosion

UpstreamInput

Stream Reach

DownstreamOutput

I - O = S

Sediment inputs from upstream and across channel banks are balanced by either downstream sediment transport or changes in sediment storage.

Page 6: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Erosional Processes

• Soil “Creep”• Overland Flow• Landslides• Glaciers• River incision into bedrock• Bank Erosion

Page 7: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Erosional Processes

• Soil “Creep”• Overland Flow• Landslides• Glaciers• River Incision• Bank Erosion

Soil creep is the gradual, non-catastrophic downslope movement of weathered material under the influence of gravity (i.e., not by flowing water).

Page 8: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

The burrowing activity of animals results in a net downslope transport of material that in some environments can be the dominant sediment transport process.

Page 9: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Tree-throw can uproot rocks and also usually results in a net downslope transport of soil and broken rock.

Page 10: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Plowing a hillslope, ca. 1935

National Archives: RG083 G 36711

Page 11: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Soil Creep

Soil Creep

Landsliding

Bank Erosion

UpstreamInput

Stream Reach

DownstreamOutput

Slow, steady input of material across channel banks, or delivered to valley bottom.

Typical rates of 0.1 to 1 mm yr-1.

Page 12: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Erosional Processes

• Soil “Creep”• Overland Flow• Landslides• Glaciers• River Incision• Bank Erosion

Erosion by overland flow occurs once enough flow accumulates to overcome the erosion resistance of the ground surface.

Page 13: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Precipitation that runs off as overland flow can cause substantial erosion once enough flow accumulates to incise the

ground surface. Xc

Xc is the critical distance needed to incise a channel.

Page 14: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Badlands environments are an extreme example where Xc may be just cm’s!

Page 15: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Unchanneled valleys occur where the erosion resistance of the ground surface is high relative to the amount of overland flow; Xc is very large.

Page 16: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Entrenched channels and gullies can develop in landscapes where overgrazing decreases the erosion resistance of the valley floor.

Page 17: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Overland Flow

Erosion by overland flow is rare in forested mountain landscapes because:

• rainfall tends to infiltrate into the ground;

•the ground has substantial erosion resistance due to vegetation.

Erosion by overland flow is most common in disturbed or semi-arid landscapes

Page 18: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Erosional Processes

• Soil “Creep”• Overland Flow• Landslides• Glaciers• River Incision• Bank Erosion

Landslides involve the downslope movement of soil and/or rock under the influence of gravity and may be either slow and gradual or rapid and catastrophic.

Page 19: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Bedrock landslides

Bedrock landslides can limit the relief of mountain ranges, such as happened at Mt. Cook, New Zealand when the top 10 meters of summit fell away in a massive landslide/avalanche on December 14,

1991.

Scarp

Runoutzone

Deposit

Page 20: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Bedrock landslides

• Earth Flows– Lots of internal

deformation; typically slow.

Earthflow in New Zealand

Page 21: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Soil landslides

• Debris Flows– Lots of internal

deformation; rapid.

Failure typically occurs along well-defined shear plane at soil-bedrock interface.

Debris flows along Tolt River

Page 22: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Landsliding

Soil Creep

Landsliding

Bank Erosion

UpstreamInput

Stream Reach

DownstreamOutput

Rapid, infrequent inputs of large volumes of sediment.

Rates of delivery set by landslide frequency, which is often centuries to millennia at a point.

Page 23: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Erosional Processes

• Soil “Creep”• Overland Flow• Landslides• Glaciers• River Incision• Bank Erosion

Glaciers can both entrain loose surface materials and gouge deeply into bedrock.

Page 24: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Glacial Erosion

Rapid erosion of material from above perennial snow line.

Rates can exceed 10 mm yr-1.

Processes of erosion and rates depend on temperature, glacier size, precipitation rate, etc...

Page 25: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Erosional Processes

• Soil “Creep”• Overland Flow• Landslides• Glaciers• River Incision• Bank Erosion

Rivers can carve deeply into bedrock and such incision provides another source of sediment.

Page 26: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

In the world there is nothing more submissive and weak than water. Yet for attacking that which is hard and strong nothing can surpass it.

- Lao-Tzu, 6th century B.C.

Page 27: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

River Incision

Erosion = f (discharge, channel width, slope)

More water in a narrower channel down a steeper slope means faster river incision

Rates of bedrock river incision typically range from <0.01 mm yr-1 to 1 mm yr-1, but can exceed 5 mm yr-1 in extreme topography.

Page 28: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Erosional Processes

• Soil “Creep”• Overland Flow• Landslides• Glaciers• River Incision• Bank Erosion

Bank erosion recycles material stored on the valley bottom, typically in the floodplain.

The rate of bank erosion defines a turnover time for valley bottom landforms.

Page 29: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

What controls erosion?

Potential process drivers:

• Climate • Topography • Vegetation

Page 30: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Landscape-scale rates of erosion vary with:

• Erodibility• Slope• Climate• Vegetation

Page 31: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Rates of erosion vary with:

• Erodibility• Slope• Climate• Vegetation

Page 32: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Erodibility

There is at least a 5 order of magnitude range in bedrock

erodiblity

Page 33: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Rates of erosion vary with:

• Erodibility• Slope• Climate• Vegetation

Page 34: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

050010001500200000.10.20.30.40.50.60.70.8

Mean Local Relief (m)GangesBramaputra

Erosion rate versus slope

0

0.2

0.4

0.6

0.8

1

0 5 10 15 20 25 30 35

Erosion Rate (mm yr

-1)

Mean Slope (degrees)

Olympic Mountains

Page 35: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Rates of erosion vary with:

• Erodibility• Slope• Climate• Vegetation

Page 36: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

Effect of Precipitation and Vegetation on Sediment Yields

Page 37: Mountains: Erosion. Erosion Sediment Regime Sediment “regime” of a river is set by the amount and size of material delivered from both hillslopes and

TectonicProcessesSurface ProcessesClimate

MantleContinental Crust

Class Concept: Rivers and beaches are part of sediment transfer systems