Erosion and Landscape Evolution. How Do We Know Rivers Cut Their Valleys? John Playfair, 1800...
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![Page 1: Erosion and Landscape Evolution. How Do We Know Rivers Cut Their Valleys? John Playfair, 1800 Tributary valleys almost always join the main valley at.](https://reader036.fdocuments.us/reader036/viewer/2022062320/56649cb75503460f9497d693/html5/thumbnails/1.jpg)
Erosion and Landscape Evolution
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How Do We Know Rivers Cut Their Valleys?
John Playfair, 1800• Tributary valleys almost always join the main
valley at exactly the same elevation, even though the valleys may begin many miles apart. This is very unlikely unless the rivers have cut the valleys.
• How Rivers Widen Valleys
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Constructive and Destructive Processes
Highlands• Erosion Dominates • Destructive Processes • History not Preserved • Little Geological Record TransportLowlands, Coastal Plain, Lakes and Seas• Deposition Dominates • Constructive Processes • History Preserved • Good Geological Record
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Stream Abrasion, Marathon County
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Stream Potholes, Marathon County
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Mega-Potholes, St. Croix Valley
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Anatomy of a
Drainage System
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The Continental
Divide, Colorado
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Stream Order
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The River That Did This….
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Looks Like This Near Its Source
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The Ideal Stream Cycle (W.M. Davis, 1880)
Not a Literal Time Sequence• Youth •Maturity • Old Age • Rejuvenation
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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
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Youthful Landscape, Arizona
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Maturity (Early) • 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
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Young-Mature Landscape, California
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Mature Landscape, Kentucky
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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.
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Old Mature Landscape, Tennessee
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Old Age • Land worn to nearly flat surface (peneplain)
• Resistant rocks remain as erosional remnants (monadnocks)
• Rivers meander across extremely wide, flat flood plains
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Monadnock, Colorado
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Monadnocks, Maine
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Old Age Landscape, South America
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The Onset of Old Age? Indiana
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Old Age? Or Maybe Not: Nebraska
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Old Age? No! (Wisconsin)
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Rejuvenation • Some change causes stream to speed up
and cut deeper. –Uplift of Land – Lowering of Sea Level–Greater stream flow
• Stream valley takes on youthful characteristics but retains features of older stages as well.
• Can happen at any point in the cycle.
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Rejuvenation, Utah
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Rejuvenation of an old-age landscape
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Rejuvenation, San Juan River, Utah
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Rejuvenation of an early mature landscape
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Machu Pichu, Peru
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Machu Pichu, Peru
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Why the Stream Cycle Doesn't Explain Everything
• Rises and falls in sea level during the ice ages rejuvenated most landscapes to some extent.
• Climate changes mean that mass-wasting processes in temperate regions may have undergone radical changes repeatedly in the last few million years.
• In places where conditions have remained uniform for long times, like the stable interiors of Africa, Australia and South America, the ideal stream cycle seems to work best.
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Sea Level and River Profile
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Superposed (Antecedent) DrainageStreams Cut Right Through High Topography
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Rejuvenated Peneplain:
the Northeastern
US
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Rejuvenated Peneplain
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Superposed Drainage, Delaware
Water Gap
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Water Gap, Pennsylvania
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Cumberland Mountains, Virginia
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Cumberland Gap
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Devil’s Gap, Wyoming
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Approach to Devil’s Gap
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Rivers and Crustal
Movement, California
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Tectonic Uplift, Colorado
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Tectonic Uplift, Grand Canyon
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The Ultimate
Antecedent Drainage,
India-Nepal-Tibet
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Drainage Diversion
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The Huang He: “China’s
Sorrow” • 1887: 2,000,000 dead • 1931: 3,700,000 dead • 1938: The Chinese
dynamite levees to slow the Japanese; half a million Chinese died.
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River Diversions in the Caspian Region
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Stream Piracy:
Northeast England
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Why is the Danube Blue?
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Piracy on the Danube
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Flood, Ecuador
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Flood, Green Bay, June 1990
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Flood, Green Bay, June 1990
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Building Smart in a Flood Plain
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Channeled Scablands, Washington
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Fluid Flow is Scale-Invariant
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Erosion of Bedrock River Beds
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Scabland Terrain, Oregon
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Erosion of Soft River
Beds
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Mega-Gravel Bar, Washington
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Mega-Flood Deposits,
Washington