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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. Stream Abrasion, Marathon County

  5. Stream Potholes, Marathon County

  6. Mega-Potholes, St. Croix Valley

  7. Anatomy of a Drainage System

  8. The Continental Divide, Colorado

  9. Stream Order

  10. The River That Did This….

  11. Looks Like This Near Its Source

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

  13. 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

  14. Youthful Landscape, Arizona

  15. 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

  16. Young-Mature Landscape, California

  17. Mature Landscape, Kentucky

  18. 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.

  19. Old Mature Landscape, Tennessee

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

  21. Monadnock, Colorado

  22. Monadnocks, Maine

  23. Old Age Landscape, South America

  24. The Onset of Old Age? Indiana

  25. Old Age? Or Maybe Not: Nebraska

  26. Old Age? No! (Wisconsin)

  27. 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.

  28. Rejuvenation, Utah

  29. Rejuvenation of an old-age landscape

  30. Rejuvenation, San Juan River, Utah

  31. Rejuvenation of an early mature landscape

  32. Machu Pichu, Peru

  33. Machu Pichu, Peru

  34. 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.

  35. Sea Level and River Profile

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

  37.  Rejuvenated Peneplain: the Northeastern US

  38. Rejuvenated Peneplain

  39. Superposed Drainage, Delaware Water Gap

  40. Water Gap, Pennsylvania

  41. Cumberland Mountains, Virginia

  42. Cumberland Gap

  43. Devil’s Gap, Wyoming

  44. Approach to Devil’s Gap

  45. Rivers and Crustal Movement, California

  46. Tectonic Uplift, Colorado

  47. Tectonic Uplift, Grand Canyon

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

  49. Drainage Diversion

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