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GEOG 100--Lecture 16--Streams

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GEOG 100--Lecture 16--Streams

  1. 1. Chapter 11Fluvial Geomorphology
  2. 2. StreamsThe work that streams do:• Erosion/Denudation – Movement away of rock, rock fragments and/or soil from its original position• Transportation – Continuous movement of material from one place to another• Deposition – Setting down (depositing) of eroded and transported material
  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 basinor Watershed• A single network system; includes both the channeled valley and any other land surface contributing overland flow or groundwater to the stream• Scale varies
  5. 5. Erosion• Overland flow— unchannelized flow of water• Splash erosion—Raindrops hitting exposed ground surface, shifting individual particles, which can then be transported away
  6. 6. Overland Flow• Sheet erosion—if there is very little ground cover (vegetation), not much water may infiltrate and sheet flow can occur, moving particles downslope
  7. 7. 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
  8. 8. How Quickly Erosion OccursDepends On…• Flow Speed• Turbulence• Resistance of the bedrock
  9. 9. 1. Flow Speed
  10. 10. 1. Flow Speed• Flow Speed –The faster the water, the more force it has –Faster water = more erosion
  11. 11. What makes the water go faster?
  12. 12. What makes the water go faster?1. Steeper gradient
  13. 13. What makes the water go faster?1. Steeper gradient – Gradient—The rate of fall in elevation of the stream surface in the downstream direction (as in 20 ft./mi.)
  14. 14. What makes the water go faster?1. Steeper gradient – Gradient—The rate of fall in elevation of the stream surface in the downstream direction (as in 20 ft./mi.) • Steeper gradient = faster flow = more erosion
  15. 15. What makes the water go faster?1. Steeper gradient – Gradient—The rate of fall in elevation of the stream surface in the downstream direction (as in 20 ft./mi.) • Steeper gradient = faster flow = more erosion2. Volume of flow (discharge)
  16. 16. What makes the water go faster?1. Steeper gradient – Gradient—The rate of fall in elevation of the stream surface in the downstream direction (as in 20 ft./mi.) • Steeper gradient = faster flow = more erosion2. Volume of flow (discharge) – More water = higher speed = more erosion
  17. 17. What makes the water go faster?1. Steeper gradient – Gradient—The rate of fall in elevation of the stream surface in the downstream direction (as in 20 ft./mi.) • Steeper gradient = faster flow = more erosion2. Volume of flow (discharge) – More water = higher speed = more erosion • Flood events move bigger material—and more of it!
  18. 18. What makes the water go faster?1. Steeper gradient – Gradient—The rate of fall in elevation of the stream surface in the downstream direction (as in 20 ft./mi.) • Steeper gradient = faster flow = more erosion2. Volume of flow (discharge) – More water = higher speed = more erosion • Flood events move bigger material—and more of it!3. Channel width—the narrower the channel, the swifter the flow, for the same volume of water (Remember the Venturi effect?)
  19. 19. How much erosive force?
  20. 20. How much erosive force?
  21. 21. How much erosive force?
  22. 22. How much erosive force?
  23. 23. 2. Turbulence
  24. 24. 2. Turbulence• Water swirling and tossing, not smooth flow
  25. 25. 2. Turbulence• Water swirling and tossing, not smooth flow• Turbulence is determined by: –Flow speed
  26. 26. 2. Turbulence• Water swirling and tossing, not smooth flow• Turbulence is determined by: –Flow speed •Faster flow = increased turbulence
  27. 27. 2. Turbulence• Water swirling and tossing, not smooth flow• Turbulence is determined by: –Flow speed •Faster flow = increased turbulence –Roughness of the stream channel
  28. 28. 2. Turbulence• Water swirling and tossing, not smooth flow• Turbulence is determined by: –Flow speed •Faster flow = increased turbulence –Roughness of the stream channel •A rough, irregular channel = more turbulence
  29. 29. 2. Turbulence• Water swirling and tossing, not smooth flow• Turbulence is determined by: –Flow speed •Faster flow = increased turbulence –Roughness of the stream channel •A rough, irregular channel = more turbulence
  30. 30. 3. Resistance of bedrock• More resistance (harder rocks) = less erosion
  31. 31. Transportation: Stream Load• Stream load (rock material transported by streams) is carried in three forms:
  32. 32. Transportation: Stream Load• Stream load (rock material transported by streams) is carried in three forms:
  33. 33. Transportation: Stream Load• Stream load (rock material transported by streams) is carried in three forms: 1. Dissolved load—dissolved minerals carried in solution
  34. 34. Transportation: Stream Load• Stream load (rock material transported by streams) is carried in three forms: 1. Dissolved load—dissolved minerals carried in solution 2. Suspended load—small particles that never touch the stream bed
  35. 35. Transportation: Stream Load• Stream load (rock material transported by streams) is carried in three forms: 1. Dissolved load—dissolved minerals carried in solution 2. Suspended load—small particles that never touch the stream bed 3. Bedload—larger rock fragments that drag, roll, skip or bounce along the stream bed
  36. 36. Transportation: Stream Load• Stream load (rock material transported by streams) is carried in three forms: 1. Dissolved load—dissolved minerals carried in solution 2. Suspended load—small particles that never touch the stream bed 3. Bedload—larger rock fragments that drag, roll, skip or bounce along the stream bed • Material is picked up, dropped and picked up again
  37. 37. Transportation: Stream Load• Stream load (rock material transported by streams) is carried in three forms: 1. Dissolved load—dissolved minerals carried in solution 2. Suspended load—small particles that never touch the stream bed 3. Bedload—larger rock fragments that drag, roll, skip or bounce along the stream bed • Material is picked up, dropped and picked up again
  38. 38. Transportation: Stream Load• Stream load (rock material transported by streams) is carried in three forms: 1. Dissolved load—dissolved minerals carried in solution 2. Suspended load—small particles that never touch the stream bed 3. Bedload—larger rock fragments that drag, roll, skip or bounce along the stream bed • Material is picked up, dropped and picked up again
  39. 39. Transportation: Competence
  40. 40. Transportation: Competence• The faster the stream is flowing, the larger the particles it can transport
  41. 41. Transportation: Competence• The faster the stream is flowing, the larger the particles it can transport• This measurement is called competence
  42. 42. Transportation: Competence• The faster the stream is flowing, the larger the particles it can transport• This measurement is called competence – Competence varies to the sixth power of the water’s speed
  43. 43. Transportation: Competence• The faster the stream is flowing, the larger the particles it can transport• This measurement is called competence – Competence varies to the sixth power of the water’s speed If flow speed doubles:
  44. 44. Transportation: Competence• The faster the stream is flowing, the larger the particles it can transport• This measurement is called competence – Competence varies to the sixth power of the water’s speed If flow speed doubles: 26 = 64 times the size!
  45. 45. Transportation: Competence• The faster the stream is flowing, the larger the particles it can transport• This measurement is called competence – Competence varies to the sixth power of the water’s speed If flow speed doubles: 26 = 64 times the size! – This is why flood events are so significant in stream transportation
  46. 46. Transportation: Capacity
  47. 47. Transportation: Capacity• Capacity—A measure of the amount of solid material potentially transported (volume/time past a given point: gal/sec)• The capacity of a given stream depends on:
  48. 48. Transportation: Capacity• Capacity—A measure of the amount of solid material potentially transported (volume/time past a given point: gal/sec)• The capacity of a given stream depends on: 1. Water volume (volume↑, capacity↑)
  49. 49. Transportation: Capacity• Capacity—A measure of the amount of solid material potentially transported (volume/time past a given point: gal/sec)• The capacity of a given stream depends on: 1. Water volume (volume↑, capacity↑) 2. Flow speed (flow speed↑, capacity↑)
  50. 50. Transportation: Capacity• Capacity—A measure of the amount of solid material potentially transported (volume/time past a given point: gal/sec)• The capacity of a given stream depends on: 1. Water volume (volume↑, capacity↑) 2. Flow speed (flow speed↑, capacity↑) 3. The type of load material
  51. 51. Transportation: Capacity• Capacity—A measure of the amount of solid material potentially transported (volume/time past a given point: gal/sec)• The capacity of a given stream depends on: 1. Water volume (volume↑, capacity↑) 2. Flow speed (flow speed↑, capacity↑) 3. The type of load material (lighter material, capacity↑)
  52. 52. Deposition• Deposition occurs when either flow speed or volume decrease• Conditions that cause deposition: –Change in gradient –Channel widening –Change of direction –Flowing into less active water
  53. 53. Deposition: Alluvium• Alluvium—Any stream-deposited debris –Sorting—Based on size (smaller particles carried farther than large ones; as stream flow decreases, larger particles will drop out of suspension first)
  54. 54. Sorting after a flood event erosion
  55. 55. Sorting after a flood event
  56. 56. Sorting after a flood event deposition
  57. 57. Sorting after a flood event deposition (particles settle biggest first, smallest last)
  58. 58. Deposition: Alluvium• Alluvium –Particle shape—Alluvial material is generally smooth and round due to rocks hitting each other and the stream bed; the longer it’s transported in the stream, the more rounded and smooth a rock will become
  59. 59. (some well-rounded alluvial deposits)
  60. 60. Stream Flow• How does friction from the channel sides and bottom affect stream flow?
  61. 61. Stream Flow• Patterns – Straight – Sinuous – Meandering – Braided
  62. 62. Stream Flow• Patterns – Straight – Sinuous – Meandering – Braided
  63. 63. Stream Flow• Patterns – Straight – Sinuous – Meandering – Braided
  64. 64. Stream Flow• Patterns – Straight – Sinuous – Meandering – Braided
  65. 65. Just like a lake, water balance mustbe maintained, or the stream willbecome an ephemeral stream
  66. 66. Equilibrium: A Graded Stream• All factors are balanced (gravity, stream load, deposition, down-cutting)• A graded condition is more theoretical than actual, because equilibrium is so difficult to achieve—and maintain
  67. 67. Which processes happen as a streamtries to attain a graded condition?• Valley deepening• Valley widening and flattening• Valley lengthening
  68. 68. Valley Deepening:Knickpoint Migration• Downcutting progresses upstream, until the valley is all at one level
  69. 69. Valley Deepening:Knickpoint Migration• The Case of Niagara Falls
  70. 70. Valley Widening and Flattening
  71. 71. Valley Widening and Flattening The Role of Meanders
  72. 72. Meanders and Oxbow Lakes
  73. 73. Meanders and Oxbow Lakes
  74. 74. Can you find the oxbow lake?Where is a cutoff about to form?
  75. 75. 34
  76. 76. Structures of a Floodplain
  77. 77. Structures of a Floodplain
  78. 78. Valley Lengthening:Headward Erosion
  79. 79. Valley Lengthening:Delta Formation and Distributaries
  80. 80. Valley Lengthening:Delta Formation and Distributaries
  81. 81. Valley Lengthening:Delta Formation and Distributaries
  82. 82. Valley Lengthening:Delta Formation and Distributaries
  83. 83. Stream Rejuvinationand Entrenched Meanders
  84. 84. Stream Rejuvinationand Entrenched Meanders
  85. 85. Stream Rejuvinationand Entrenched Meanders
  86. 86. Next: Oceans

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