GEOG 100--Streams (Fluvial Geomorphology) edit

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GEOG 100--Streams (Fluvial Geomorphology) edit

  1. 1. Chapter 11 Fluvial Geomorphology
  2. 2. Streams The work that streams do: • Erosion/Denudation • Transportation • Deposition
  3. 3. Drainage Systems • Drainage system—A branched, hierarchical network of streams and tributaries • Valley—Where a drainage system is clearly established • Interfluve—(“inter”=between, “fluvia”= rivers) High ground that separates valleys • Drainage divide—The invisible line separating two drainage basins
  4. 4. Drainage basin / Watershed • A single network system; includes both the channeled valley and any other land surface contributing overland flow or groundwater to the stream • Overland flow—unchannelized flow of water
  5. 5. Three Processes of Stream Erosion • Hydraulic action—The physical force of water pounding on rocks and land materials, breaking them apart • Abrasion—Rock materials hitting the bed (bottom) of the river and its banks (sides) • Corrosion—The chemical action of water dissolving minerals and rock material
  6. 6. How Quickly Erosion Occurs Depends On… • Flow Speed • Turbulence • Resistance of the bedrock
  7. 7. 1. Flow Speed • Flow Speed – The faster the water, the more force it has – Faster water = more erosion
  8. 8. What makes the water go faster? 1. Steeper gradient (slope) 2. Volume of flow (discharge) 3. Channel width – The narrower the channel, the swifter the flow, for the same volume of water (remember the Venturi effect!)
  9. 9. Which has more erosive force?
  10. 10. 2. Turbulence • Turbulence is determined by: – Flow speed • Faster flow = increased turbulence – Roughness of the stream channel • A rough, irregular channel = more turbulence
  11. 11. 3. Resistance of bedrock • Harder rocks = less erosion
  12. 12. Transportation: Stream Load • Stream load (rock material transported by streams) is carried in three forms: 2. Suspended load—small particles that never touch the stream bed (Most of the stream load is suspended.) 3. Bedload—larger rock fragments that drag, roll, skip or bounce along the stream bed • Material is picked up, dropped and picked up again
  13. 13. Deposition • Deposition occurs when either flow speed or volume decrease • Conditions that cause deposition: – Change in gradient – Channel widening – Flowing into less active water – Change of direction
  14. 14. Deposition: Alluvium • Alluvium—Any stream-deposited debris – Smaller particles are carried farther than large ones – Larger particles drop out of suspension first – The longer alluvial material is transported in the stream, the more rounded and smooth it becomes
  15. 15. Stream Flow What if the stream isn’t straight? What if the stream isn’t straight? • Patterns – Straight – Sinuous – Meandering – Braided
  16. 16. Equilibrium • All factors are balanced (gravity, stream load, deposition, down-cutting) • More theoretical than actual, because equilibrium is so difficult to achieve—and maintain
  17. 17. What happens as a stream tries to reach “equilibrium”? • Valley deepening • Valley widening and flattening • Valley lengthening
  18. 18. Valley Deepening: Knickpoint Migration • Downcutting progresses upstream, until the valley is all at one level
  19. 19. Valley Deepening: Knickpoint Migration
  20. 20. Valley Widening and Flattening The Role of Meanders
  21. 21. Valley Widening and Flattening
  22. 22. Meanders and Oxbow Lakes
  23. 23. Can you find an oxbow lake? Where are cutoffs about to form?
  24. 24. 25
  25. 25. Structures of a Floodplain
  26. 26. Structures of a Floodplain
  27. 27. Valley Lengthening: Headward Erosion
  28. 28. Valley Lengthening: Delta Formation and Distributaries
  29. 29. 30
  30. 30. 31
  31. 31. Stream Rejuvination: Rapid uplift
  32. 32. Entrenched meanders: Slow uplift 33
  33. 33. Entrenched meanders 34

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