Sediment Budgets for Mountain RiversMountain RiversBase LevelBase LevelHolocene Sea Level ChangesYouth Maturity Maturity (Late) Old Age Sediment YieldSediment YieldDamsNatural DamsArtificial DamsMountain RiversGutta cavat lapidem (Dripping water hollows out a stone)-Ovid, Epistulae Ex Ponto, Book 3, no. 10, 1. 5Mountain Rivers• Fixed channel boundaries (bedrock banks and bed) • High transport capacity• Low Storage• Input ≈ OutputSediment Budgetsfor Mountain RiversSoil CreepLandslidingBank ErosionUpstreamInputStream ReachDownstreamOutputInput = Output ΔS = 0Little sediment storage implies that sediment inputs balanced by downstream sediment transport.Mountain RiversStrong hillslope-channel coupling in mountain streams means that sediment inputs can move downstream as a pulse.TaiwanTaiwanTaiwanTaiwanIn steep terrain, where landslides are common, the rate of river incision sets the pace for landscape lowering because if the river can’t carry away material stripped from the slopes AND carve the valley deeper, then the valleys will fill with sediment and the hills will lower.BerkeleyPacificaBedrock ChannelsChannels floored by bedrock and lacking an alluvial bed cover. Indicative of transport capacity well in excess of sediment supply.WaterfallsOccur where barriers to down-cutting exist.Usually only last as long as the barrier exists.WaterfallsFundamentally transient features!Comet Falls, Mt Rainier, Aug. 2001Waterfalls: “hanging valleys”Yosemite FallsFine scale lithologicalcontrasts such as from layers of hard and soft rock.WaterfallsBase Level• The limiting level below which a stream cannot erode the land is called the base level of the stream.• The base level for most streams is global sea level.Base Level• Exceptions are streams that drain into closed interior basins having no outlet to the sea.• Where the floor of a tectonically formed basin lies below sea level (for example, Death Valley, California), the base level coincides with the basin floor.• When a stream flows into a lake, the surface of the lake acts as a local base level.HoloceneSea Level ChangesPrimarily Cause ---Glacial MeltingIn TheNorthern HemisphereStreams respond to changes in sea level.Erode rapidly downward when sea level falls.Deposit rapidly when sea level rises.• Streams erode rock and sediment over which if flows and are very effective in developing landscapes• Streams erode rock and sediment in three main ways• Hydraulic action•Solution•AbrasionBedrock channel erosionHydraulic Action – the ability of flowing water to pick up and move rock and/or sediment• Running water can flow into a fracture or joint, forcing a fragment loose and pushing it along the streambed• Pressure of flowing water and swirling eddies lift the fragment or grain• Hydraulic action is very effective at the base of rapids and waterfallsBedrock channel erosion• Solution – chemical weathering (dissolution) of limestone over which a stream flows• Flowing water increases the dissolution rate and is effective in deepening a stream channel• Dissolution of a calcite cement from a sandstone may allow the grains to be set free by hydraulic action•Abrasion– the grinding away of the stream channel by friction and impact with grains and fragments carried by the streamBedrock channel erosionGraded rivers are those which maintain a balance between erosion and uplift and/or deposition.Graded RiversSteady state river profiles are concave up because of the trade off between greater discharge and lower slope as drainage area increases downtream.Graded RiversGraded RiversSteady-state channelsWhen erosion rates balance uplift rates topography can achieve a steady state, despite active erosion.When uplift rates exceed erosion rates topography rises, rivers incise into the rising topography and eventually sculpt mountains.Mountain range upliftMountain range decayWhen erosion rates exceed uplift rates rivers wear down mountainous topography and eventually re-create low-gradient depositional plains.Physiographic CycleYouth • V-Shaped Valley •Rapids • Waterfalls • No Flood Plain • Drainage Divides Broad and Flat, Undissected by Erosion • Valley Being Deepened • General Agreement on this stage, lots of examples• V-Shaped Valley • Beginnings of Flood Plain • Sand and Gravel Bars •Sharp Divides •Relief Reaches Maximum • Valleys stop deepening • General Agreement on this stage, lots of examplesMaturityMaturity(Late) • Valley has flat bottom • Narrow Flood Plain • Divides begin to round off • Relief diminishes • Sediment builds up, flood plain widens • River begins to meander • Many geologists believe slopes stay steep but simply retreat.Old Age • Land worn to nearly flat surface (peneplain)•Resistant rocks remain as erosionalremnants (monadnocks)• Rivers meander across extremely wide, flat flood plainsGlobal Sediment YieldRange of 1 m per million years to 1 m per yearSediment Yield– In southern Alaska and the southern Andes, large active glaciers contribute to high sediment yields.– The clearing of forests, cultivation of lands, damming of streams, construction of cities, and numerous other human activities also affect erosion rates and sediment yields.Sediment Yield• In arid regions, reduced precipitation limits vegetation, makingthe land vulnerable to erosion.• Areas receiving abundant precipitation may actually experience less erosion than some relatively dry regions,• Fields measurements suggest that some of the greatest local sediment yields are from desert landscapes.• Monsoon regions of southeastern Asia receive abundant precipitation that generates high runoff.Modern sediment yield is >10 long-term (geological) rate.Dams• Both natural and artificial dams built across a stream create a reservoir that traps nearly all the sediment that the stream formerly carried to the ocean.• Globally, dams have reduced the sediment load that reaches the oceans by half.Natural Dams• The courses of many streams are interrupted by lakes that have formed behind natural dams consisting of:– Landslide sediments.– Glacial deposits. – Glacier ice.–Lava flows.–• Such a dam acts as a local base level and creates an irregularity in a stream’s long profile.Tsangpo River BasinNatural glacier dams on the Tsangpo River, eastern TibetTsangpo River above Namche Barwa gorgeSand-bedded river over 500 m wideEntrance to Po-Tsangpo GorgeBedrock river <100 m
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