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Rivers

Rivers. The Hydrologic Cycle. The hydrologic cycle is driven by solar energy. transfer water from the oceans to the continents (and back). The Hydrologic Cycle. Over continents, evaporation off large lakes can add water vapor to atmosphere.

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Rivers

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  1. Rivers

  2. The Hydrologic Cycle • The hydrologic cycle is driven by solar energy. • transfer water from the oceans to the continents (and back)

  3. The Hydrologic Cycle • Over continents, evaporation off large lakes can add water vapor to atmosphere. • Precipitation can flow across the surface (runoff). Fig. 16.2

  4. The Hydrologic Cycle Precipitation can infiltrate below the surface, becoming groundwater. • Transpiration is water returned to the atmosphere by plants. Fig. 16.2

  5. The Hydrologic Cycle • Groundwater flows through pores spaces in rocks/sediments. • Groundwater eventually returns to the surface. • keeps rivers flowing when it’s not raining. Fig. 16.2

  6. Surface Drainage • A network of streams carries surface water back to the oceans. • A drainage basin is the area from which a stream gathers its water. Fig. 16.3

  7. Drainage of North America • Miss. R. drainage basin: largest in N.A., drains all or part of 33 states • Great Basin: rivers do not drain to ocean (internal drainage). Fig. 16.4 Why isn't the Great Basin filled with water?

  8. Stream Sediment • Sediments in streams are: • eroded from surrounding areas. • eroded from within the stream channel. Fig. 16.6

  9. Sediment Sediment • Streams can erode solid bedrock by: • plucking (removal of large fragments). • abrasion (small fragments chipped by sediment. collisions). Fig. 16.12

  10. Stream Sediment • The bedload is the sediments rolling, bouncing or sliding on the streambed. • The suspendedload is sediments that remain suspended in water. Fig. 16.13

  11. Stream Sediment • The dissolvedload is ions dissolved in the stream water. • the "invisible" sediment load Fig. 16.13

  12. Sediment Transport

  13. Sediment Transport • Sediments can be transported when: shear stress > friction + cohesion + gravity • Shear stress increases with slope and water depth. Fig. 16.9

  14. Sediment Deposition • Deposition occurs due to: • decreased slope and velocity. • decreased discharge (amount of water).

  15. Sediment Deposition Alluvial fans are common features in deserts. • Alluvial fan deposition: • Confined mountain stream, high shear and vel., sed. transported • Valley floor, low shear and vel., sed. deposited Fig. 16.14

  16. Sediment Deposition • Upstream part of a river system: • high slope and shear stress • coarse-grained bedload • mostly erosion, little deposition Fig. 16.16

  17. Sediment Deposition • Downstream part of a river system: • low slope and shear stress • fine-grained bedload • mostly deposition Fig. 16.16

  18. Is Illinois is an area of net erosion or deposition? Fig. 16.4

  19. Stream Channel Patterns • Meandering – sinuous, well defined channel • Low slope, deposit finer grain sizes, suspended load dominated • Channel position migrates slowly Fig. 16.22

  20. Stream Channel Patterns • Braided – multiple channels that divide and reunite. • High slope, coarse grained sediment, bedload dominated • Channels constantly shifting Fig. 16.1

  21. Stream Channel Patterns • There is a complete gradation between channel types, and type changes downstream. Fig. 16.19

  22. Stream Channel Migration • Cutbank – the outside of a meander • Water velocity is high • Shear stress is high • Erosion occurs Fig. 16.18

  23. Stream Channel Migration • Point bar – the inside of a meander • Water velocity is low • Shear stress is low • Deposition occurs Fig. 16.18

  24. Stream Channel Migration • Erosion and deposition on meanders causes the channel to migrate. • An oxbow lake forms where meander is cut off by erosion. migration direction erosion

  25. Stream Channel Migration soon to be formed oxbow recently formed oxbows

  26. Stream Channel Migration

  27. straightened and channelized. lined with concrete. Urban Streams • Most urban streams do not meander and are:

  28. Urban Streams • After straightening, streams gradually return to meandering channel. floodplain Point bar Cut bank

  29. Urban Streams • Urban streams require continual maintenance (tax $$) to minimize meandering and erosion. erosion downstream of a bridge Rip-rap preventing erosion downstream of a bridge

  30. What happens when a meandering stream is artificially straightened?

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