Mountains Erosion Erosion 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 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 Sediment Budget I O S Sediment inputs from upstream and across channel banks are balanced by either downstream sediment transport or changes in sediment storage Landsliding Soil Creep Upstream Input Stream Reach Downstream Output Bank Erosion Erosional Processes Soil Creep Overland Flow Landslides Glaciers River incision into bedrock Bank Erosion Erosional Processes Soil Creep Overland Flow Landslides Glaciers River Incision Bank Erosion Soil creep is the gradual noncatastrophic downslope movement of weathered material under the influence of gravity i e not by flowing water The burrowing activity of animals results in a net downslope transport of material that in some environments can be the dominant sediment transport process Tree throw can uproot rocks and also usually results in a net downslope transport of soil and broken rock Plowing a hillslope ca 1935 National Archives RG083 G 36711 Soil Creep Slow steady input of material across channel banks or delivered to valley bottom Landsliding Soil Creep Typical rates of 0 1 to 1 mm yr 1 Upstream Input Stream Reach Downstream Output Bank Erosion 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 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 Badlands environments are an extreme example where Xc may be just cm s Unchanneled valleys occur where the erosion resistance of the ground surface is high relative to the amount of overland flow Xc is very large Entrenched channels and gullies can develop in landscapes where overgrazing decreases the erosion resistance of the valley floor 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 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 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 Scarp Runout zone December 14 1991 Deposit Bedrock landslides Earth Flows Lots of internal deformation typically slow Earthflow in New Zealand Soil landslides Debris Flows Lots of internal deformation rapid Failure typically occurs along welldefined shear plane at soilbedrock interface Debris flows along Tolt River Landsliding Rapid infrequent inputs of large volumes of sediment Rates of delivery set by landslide frequency which is often centuries to millennia at a point Landsliding Soil Creep Upstream Input Stream Reach Downstream Output Bank Erosion Erosional Processes Soil Creep Overland Flow Landslides Glaciers River Incision Bank Erosion Glaciers can both entrain loose surface materials and gouge deeply into bedrock 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 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 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 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 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 What controls erosion Potential process drivers Climate Topography Vegetation Landscape scale rates of erosion vary with Erodibility Slope Climate Vegetation Rates of erosion vary with Erodibility Slope Climate Vegetation Erodibility There is at least a 5 order of magnitude range in bedrock erodiblity Rates of erosion vary with Erodibility Slope Climate Vegetation Erosion rate versus slope 1 0 8 Erosion Rate mm yr 1 Erosion Rate mm yr 1 0 8 0 6 0 4 0 2 0 0 0 7 Bramaputra Olympic Mountains 0 6 0 5 0 4 Ganges 0 3 0 2 0 1 5 0 10 0 15 20 25 Mean500 Slope 1000 degrees 1500 Mean Local Relief m 30 35 2000 Rates of erosion vary with Erodibility Slope Climate Vegetation Effect of Precipitation and Vegetation on Sediment Yields Class Concept Rivers and beaches are part of sediment transfer systems Climate Surface Processes Tectonic Processes Continental Crust Mantle
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