Friday Oct 9 Nisqually field trip tomorrow if you did not get an email from Issac yesterday please talk to her today Bring clothing rain gear for cold and wet weather erosion moving away from the source Weathering soil erosion Sediment regime amount size characteristic sediment transport style in a river governed by not surprisingly the amount and size of material delivered from both hillslopes and upstream this in turn depends on erosional processes today s lecture weathering rates style rock properties soil properties Sediment Budget along a stream reach 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 Downstrea m 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 non catastrophic downslope movement of weathered material under the influence of gravity i e not by flowing water ways to move soil The burrowing activity of animals results in a net downslope transport of material that in some environments can be the dominant sediment transport process ways to move soil Tree throw can uproot rocks and also typically results in a net downslope transport of soil and broken rock ways to move soil human modifications 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 Upstream Input Typical rates of 0 1 to 1 mm yr 1 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 channe Badlands environments are an extreme example where Xc may be just centimeters 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 Xc was large became smaller 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 earth flows some internal deformation typically slow relatively little water earth flow in NZ Soil landslides debris flows lots of internal deformation rapid relatively high water content fluid like flow Failure typically occurs along welldefined shear plane at soilbedrock interface debris flows along Tolt R landslides Bedrock landslides can limit the relief of mountain ranges Mt Cook New Zealand top 10 meters of summit fell away in a massive landslide avalanche on December 14 1991 Scarp Runout zone Deposit 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 River Incision 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 bank erosion reworking valleybottom sediments entraining sediments delivered by other erosional processes Rangitata R NZ erosion controls climate topography slope vegetation lithology erodibility land use climate precip veg sediment yield total amount of sediment generated by a particular landscape slope 1 1 0 8 Olympic Mountains 0 6 Erosion Rate mm yr 0 4 0 2 0 0 5 10 15 20 25 Mean Slope degrees 30 35 erodibility bedrock erodibility ranges over at least 5 orders of magnitude land use mountains the big picture weathering soil formation erosion interact ultimately provide sediment for rivers beaches a final thought rivers and beaches are important sediment transfer material transfer systems within larger Earth system remember this context
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