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Erosion and Landscape Evolution

Erosion and Landscape Evolution. 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.

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Erosion and Landscape Evolution

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  1. Erosion and Landscape Evolution

  2. 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

  3. 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

  4. Anatomy of a Drainage System

  5. Stream Order

  6. The Ideal Stream Cycle (W.M. Davis, 1880) Not a Literal Time Sequence • Youth • Maturity • Old Age • Rejuvenation  

  7. 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

  8. 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

  9. 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.

  10. Old Age • Land worn to nearly flat surface (peneplain) • Resistant rocks remain as erosional remnants (monadnocks) • Rivers meander across extremely wide, flat flood plains

  11. 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.

  12. Rejuvenation of an old-age landscape

  13. Rejuvenation of an early mature landscape

  14. 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.

  15. Sea Level and River Profile

  16. Superposed (Antecedent) DrainageStreams Cut Right Through High Topography

  17.  Rejuvenated Peneplain: the Northeastern US

  18. Rejuvenated Peneplain

  19. Rivers and Crustal Movement, California

  20. The Ultimate Antecedent Drainage, India-Nepal-Tibet

  21. Arid and Humid Weathering Compared

  22. Arid Erosion Cycle: Youth • V-shaped Valleys • Divides Flat, Undissected • Much Like Youth in Humid Climates

  23. Arid Erosion Cycle: Maturity • Slopes Stay Steep, Retreat • Alluvial Fans • Playa Lakes • Pediments

  24. Arid Erosion Cycle: Old Age • Inselbergs • Playa Lakes • Pediplain

  25. Closed Basins are Typical of Arid Regions

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