Sediment Regime Sediment Supply to Rivers Sediment BudgetErosional Processes Erosional Processes Soil CreepErosional Processes Overland FlowErosional Processes Bedrock landslidesBedrock landslidesSoil landslidesLandslidingErosional Processes Glacial ErosionErosional Processes River IncisionErosional Processes What controls erosion?Effect of Precipitation and Vegetation on Sediment YieldsMountains: ErosionErosionSediment 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 RiversSize and composition of sediment delivered to rivers reflects:• soil properties• rock properties•the process that delivered it.Graduate student for scaleSediment BudgetSoil CreepLandslidingBank ErosionUpstreamInputStream ReachDownstreamOutputI - O = ΔSSediment inputs from upstream and across channel banks are balanced by either downstream sediment transport or changes in sediment storage.Erosional Processes• Soil “Creep”• Overland Flow• Landslides• Glaciers• River incision into bedrock•Bank ErosionErosional Processes• Soil “Creep”• Overland Flow• Landslides• Glaciers•River Incision•Bank ErosionSoil creep is the gradual, non-catastrophic 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. 1935National Archives: RG083 G 36711Soil CreepSoil CreepLandslidingBank ErosionUpstreamInputStream ReachDownstreamOutputSlow, steady input of material across channel banks, or delivered to valley bottom.Typical rates of 0.1 to 1 mm yr-1.Erosional Processes• Soil “Creep”• Overland Flow• Landslides• Glaciers•River Incision•Bank ErosionErosion 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.XcXc is the critical distance needed to incise a channel.Badlands environments are an extreme example where Xcmay be just cm’s!Unchanneled valleys occur where the erosion resistance of the ground surface is high relative to the amount of overland flow; Xcis very large.Entrenched channels and gullies can develop in landscapes where overgrazing decreases the erosion resistance of the valley floor.Overland FlowErosion 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 landscapesErosional Processes• Soil “Creep”• Overland Flow• Landslides• Glaciers•River Incision•Bank ErosionLandslides 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 landslidesBedrock 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.ScarpRunoutzoneDepositBedrock landslides•Earth Flows–Lots of internal deformation; typically slow.Earthflow in New ZealandSoil landslides•Debris Flows–Lots of internal deformation; rapid.Failure typically occurs along well-defined shear plane at soil-bedrock interface.Debris flows along Tolt RiverLandslidingSoil CreepLandslidingBank ErosionUpstreamInputStream ReachDownstreamOutputRapid, infrequent inputs of large volumes of sediment.Rates of delivery set by landslide frequency, which is often centuries to millennia at a point.Erosional Processes• Soil “Creep”• Overland Flow• Landslides• Glaciers•River Incision•Bank ErosionGlaciers can both entrain loose surface materials and gouge deeply into bedrock.Glacial ErosionRapid 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 ErosionRivers 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 IncisionErosion = 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-1to 1 mm yr-1, but can exceed 5 mm yr-1in extreme topography.Erosional Processes• Soil “Creep”• Overland Flow• Landslides• Glaciers•River Incision•Bank ErosionBank 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 • VegetationLandscape-scale rates of erosion vary with:• Erodibility• Slope• Climate• VegetationRates of erosion vary with:• Erodibility• Slope• Climate• VegetationErodibilityThere is at least a 5 order of magnitude range in bedrock erodiblityRates of erosion vary with:• Erodibility• Slope• Climate• Vegetation0 500 1000 1500 200000.10.20.30.40.50.60.70.8Mean Local Relief (m)GangesBramaputra-1)Erosion Rate (mm yrErosion rate versus slope00.20.40.60.810 5 10 15 20 25 30 35Erosion Rate (mm yr-1)Mean Slope (degrees)Olympic MountainsRates of erosion vary with:• Erodibility• Slope• Climate• VegetationEffect of Precipitation and Vegetation on Sediment YieldsTectonicProcessesSurface ProcessesClimateMantleContinental CrustClass Concept: Rivers and beaches are part of sediment transfer
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