Mountain Rivers Gutta cavat lapidem Dripping water hollows out a stone Ovid Epistulae Ex Ponto Book 3 no 10 1 5 Mountain Rivers Fixed channel boundaries bedrock banks and bed High transport capacity Low Storage Input Output Sediment Budgets for Mountain Rivers Little sediment storage implies that sediment inputs balanced by downstream sediment transport Input Output S 0 Landsliding Soil Creep Upstream Input Stream Reach Downstream Output Bank Erosion Mountain Rivers Strong hillslopechannel coupling in mountain streams means that sediment inputs can move downstream as a pulse Taiwan Taiwan Taiwan Taiwan In 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 Berkeley Pacifica Bedrock Channels Channels floored by bedrock and lacking an alluvial bed cover Indicative of transport capacity well in excess of sediment supply Waterfalls Occur where barriers to down cutting exist Usually only last as long as the barrier exists Waterfalls Fundamentally transient features Waterfalls hanging valleys Comet Falls Mt Rainier Aug 2001 Yosemite Falls Waterfalls Fine scale lithological contrasts such as from layers of hard and soft rock Base 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 Holocene Sea Level Changes Primarily Cause Glacial Melting In The Northern Hemisphere Streams respond to changes in sea level Erode rapidly downward when sea level falls Deposit rapidly when sea level rises Bedrock channel erosion 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 Abrasion Bedrock channel erosion Hydraulic 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 waterfalls Bedrock 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 stream Graded Rivers Graded rivers are those which maintain a balance between erosion and uplift and or deposition Graded Rivers Steady state river profiles are concave up because of the trade off between greater discharge and lower slope as drainage area increases downtream Graded Rivers Steady state channels When erosion rates balance uplift rates topography can achieve a steady state despite active erosion Mountain range uplift When uplift rates exceed erosion rates topography rises rivers incise into the rising topography and eventually sculpt mountains Mountain range decay When erosion rates exceed uplift rates rivers wear down mountainous topography and eventually re create low gradient depositional plains Physiographic Cycle Youth 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 Maturity 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 examples Maturity 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 erosional remnants monadnocks Rivers meander across extremely wide flat flood plains Global Sediment Yield Range of 1 m per million years to 1 m per year Sediment 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 making the 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 Natural glacier dams on the Tsangpo River eastern Tibet Tsangpo River Basin Tsangpo River above Namche Barwa gorge Sand bedded river over 500 m wide Entrance to Po Tsangpo Gorge Bedrock river 100 m wide Moraine dam at entrance to Tsangpo gorge Lake 3 elevation Lake 2 elevation Moraine plug at entrance to Tsangpo gorge Aster DEM Truncated moraine Moraine dam Lake 3 Namche Barwa Tsangpo River Moraine plug Lake 2 Nyang River Tsangpo River Lake 2 81 km3 Delta terrace from tributary Repeated episodes of glacially driven damming of the Tsangpo River Evidence for repeated filling and dam failure during ice recession Tsangpo Lakes
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