Erosion and Landscape Evolution

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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
• Transport
• Lowlands, Coastal Plain, Lakes and Seas
• Deposition Dominates
• Constructive Processes
• History Preserved
• Good Geological Record

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Anatomy of a Drainage System
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Stream Order
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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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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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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 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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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 of an old-age landscape
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Rejuvenation of an early mature landscape
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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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Rivers and Crustal Movement, California
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The Ultimate Antecedent Drainage,
India-Nepal-Tibet
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Arid and Humid Weathering Compared
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Arid Erosion Cycle Youth

• V-shaped Valleys
• Divides Flat, Undissected
• Much Like Youth in Humid Climates

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Arid Erosion Cycle Maturity

• Slopes Stay Steep, Retreat
• Alluvial Fans
• Playa Lakes
• Pediments

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Arid Erosion Cycle Old Age

• Inselbergs
• Playa Lakes
• Pediplain

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Closed Basins are Typical of Arid Regions