River Rejuvenation


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River rejuvenation

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River Rejuvenation

  1. 1. River Rejuvenation All your knowledge of rivers should help you to understand this final process which forms a unique set of landforms. The big picture
  2. 2. Rejuvenate <ul><li>To restore to a former state; to make fresh or new again </li></ul>
  3. 3. Learning Outcomes <ul><li>To understand how changes in base level create different landforms. </li></ul><ul><li>Explain the landforms produced by river rejuvenation (15 marks) </li></ul>
  4. 4. Base Level <ul><li>Listen to the explanation of base level and then produce a diagram to explain what it is and how it changes. </li></ul>
  5. 5. Base Level <ul><li>Base level is the lowest level to which erosion by running water can take place. </li></ul>
  6. 6. The River System <ul><li>River’s are always attempting to reach a state of dynamic equilibrium where there is a balance between the rate of erosion and deposition. </li></ul>
  7. 7. Dynamic Rivers <ul><li>In reality a river is rarely, if ever, in a state of dynamic equilibrium. </li></ul><ul><li>Changes in discharge and sediment load are constantly changing the profile of the river. </li></ul>
  8. 8. For example…. <ul><li>Waterfall </li></ul><ul><ul><ul><li>Erosion near the waterfall </li></ul></ul></ul><ul><ul><ul><li>Deposition lower downstream </li></ul></ul></ul><ul><li>Lengthy period of heavy rain </li></ul>
  9. 9. Base Level changes <ul><li>Eustatic Change </li></ul><ul><ul><li>Changes to sea Level </li></ul></ul><ul><li>Isostatic change </li></ul><ul><ul><li>Glacial rebound </li></ul></ul>
  10. 10. Changes <ul><li>Positive </li></ul><ul><li>Sea levels rise in relation to the land. </li></ul><ul><li>The river loses energy and as such deposition dominates and coastal areas flood. </li></ul><ul><li>Negative </li></ul><ul><li>Sea levels fall in relation to the sea. </li></ul><ul><li>Land emerges from the sea, steepening the river gradient thus increasing the rivers erosional power. </li></ul>rejuvenation
  11. 11. Base Level <ul><li>Produce your diagram </li></ul>
  12. 12. Key Points <ul><li>Rivers begin to regrade from the point nearest the sea. </li></ul><ul><li>It often happens again before a river has reached dynamic equilibrium giving it a partly graded profile. </li></ul><ul><li>Knickpoints </li></ul><ul><li>Incised meanders </li></ul><ul><li>River terraces </li></ul><ul><li>Waterfalls </li></ul>Changes to the cross profile of the river
  13. 13. River in grade Flood plain
  14. 14. Waterfall retreats cutting a lower valley Fall in sea level River bluffs New flood plain forms
  15. 15. New flood plain forms River bluffs Waterfall decreases in size to form rapids (knick point) River Terrace
  16. 16. Terraces
  17. 17. Knickpoints and waterfalls
  18. 20. Incised Meanders
  19. 25. <ul><li>The likely story is that the streams once flowed across nearly flat lowlands. </li></ul><ul><li>Then, uplift of the rocks began, giving the streams a steeper slope to the sea and so speeding their flow and causing them to erode. </li></ul><ul><li>But, the uplift was gradual enough that the streams held their old courses.  </li></ul><ul><li>The streams cut downward without a change in pattern, which is called incision.  </li></ul>
  20. 26. Write three level descriptors for this question <ul><li>Explain the landforms produced by river rejuvenation (15 marks) </li></ul><ul><li>Level 1 : BASIC </li></ul><ul><li>Level 2 : CLEAR </li></ul><ul><li>Level 3 : DETAILED </li></ul>
  21. 27. Review <ul><li>BASE LEVEL </li></ul><ul><li>INCISION </li></ul><ul><li>ISOSTATIC UPLIFT </li></ul><ul><li>KNICK POINT </li></ul>