Geology lecture 16

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Geology lecture 16

  1. 1. Rivers Chapter 17
  2. 2. Outline• Streamflow -streams/rivers, runoff, hydrologic cycle component -forming streams/rivers• Drainage networks -Patterns (dendritic to trellis) -Drainage basins, drainage divides• Rivers/Streams -Permanent vs. ephemeral -Discharge, channel velocity, erosion/transport/deposition -Longitudinal (downstream) changes• Further details -Base level, valleys & canyons, terraces, rapids/waterfalls -Depositional environments (e.g. alluvial fans, braiding, deltas..) -Drainage evolution and flooding Chapter 17 Chapter 17
  3. 3. Streamflow• Stream/River – water flow down channels• Runoff – water flow over land surface• Stream runoff is crucial for humans: • Drinking water • Transportation • Waste disposal • Recreation • Commerce • Irrigation • Energy Chapter 17
  4. 4. Streamflow• Stream flow/runoff also causes many problems • Flooding destroys lives and property Chapter 17
  5. 5. Streamflow• Stream flow/ runoff is an important geologic age • Flowing water… • Erodes, transports, deposits sediments • Sculps landscapes • Transfers mass from continents to oceans • Earth: only planet in solar system with liquid water Chapter 17
  6. 6. The Hydrologic Cycle• Stream flow – important component of hydrologic cycle Chapter 17
  7. 7. Forming Streams• Streamflow begins as water is added to the surface Chapter 17
  8. 8. Forming StreamsStreamflow begins as moving sheetwash thin surface water layer moves down steepest slope erodes substrate• Sheetwash erosion createstiny channels (rills)• Rills coalesce & deepen into channels. Chapter 17
  9. 9. Forming Streams• Scouring can mark entry into the channel• Rapid erosion lengthens channel upslope • Process is called headward erosion Chapter 17
  10. 10. Forming Streams• Over time, channels merge. • Smaller tributaries join larger trunk stream• A drainage network – array of linked channels • They change over time Chapter 17
  11. 11. Outline• Streamflow -streams/rivers, runoff, hydrologic cycle component -forming streams/rivers• Drainage networks -Patterns (dendritic to trellis) -Drainage basins, drainage divides• Rivers/Streams -Permanent vs. ephemeral -Discharge, channel velocity, erosion/transport/deposition -Longitudinal (downstream) changes• Further details -Base level, valleys & canyons, terraces, rapids/waterfalls -Depositional environments (e.g. alluvial fans, braiding, deltas..) -Drainage evolution and flooding Chapter 17 Chapter 17
  12. 12. Drainage Networks• Drainage networks form geometric patterns• Patterns reflect geology and landscape form• Several common drainage patterns: 1. Dendritic – branching, “treelike”- due to uniform material Chapter 17
  13. 13. Drainage Networks• Common drainage patterns: 2. Radial – form a point uplift (e.g. volcano) Chapter 17
  14. 14. Drainage Networks• Common drainage patterns: 3. Rectangular – controlled by jointed rocks Chapter 17
  15. 15. Drainage Networks• Common drainage patterns: 4. Trellis (garden) – due to alternating resistant/weak rocks common in fold-trust belts Chapter 17
  16. 16. A Drainage Basin• Land area that drains into a specific trunk stream  Also called catchment or watershed• Divides are boundaries that separate drainage basins Chapter 17
  17. 17. Drainage Divides• Watersheds exist across scales. • Tiny tributaries • Continental rivers• Large watersheds… • Feed large rivers • Section continents• Continental divides separate flow to different oceans Chapter 17
  18. 18. Outline• Streamflow -streams/rivers, runoff, hydrologic cycle component -forming streams/rivers• Drainage networks -Patterns (dendritic to trellis) -Drainage basins, drainage divides• Rivers/Streams -Permanent vs. ephemeral -Discharge, channel velocity, erosion/transport/deposition -Longitudinal (downstream) changes• Further details -Base level, valleys & canyons, terraces, rapids/waterfalls -Depositional environments (e.g. alluvial fans, braiding, deltas..) -Drainage evolution and flooding Chapter 17 Chapter 17
  19. 19. Permanent vs. Ephemeral• Permanent streams • Ephemeral streams • Water flows all year. • Do not flow all year. • At or below the water table. • Above the water table. • Humid or temperate. • Dry climates. • Sufficient rainfall. • Low rainfall. • Lower evaporation. • High evaporation. • Discharge varies seasonally. • Flow mostly during rare flash floods. Chapter 17
  20. 20. Discharge• Amount of water flowing in a channel • Water volume passing a point per unit time • Cubic meters per second (m3/s)• Given bycross-sectional area (Ac) x flow velocity• Varies seasonally due to precipitation and runoff Chapter 17
  21. 21. Channel Velocity• Velocity is not uniform in the channel • Friction slows water along edges • Greater in wider, shallower streams • Lesser in wider, shallower streams • Magnitude determined by wetter perimeter • Greater wetted perimeter, slower the velocity • In straight channels, highest velocity in center Chapter 17
  22. 22. Channel Velocity• Velocity is not uniform within a channel • Max. velocity near outside in bending channels • Outside is preferentially scoured and deepened (cut bank) • Inside is locus of desposition (point bar) due to reduced velocity • Deepest part is called the thalweg Chapter 17
  23. 23. Channel Velocity• Velocity is not uniform in all areas of a channel • Stream flow is turbulent • Chaotic and erratic• Turbulence caused by… • Flow obstructions • Shear in water• Eddies scour channel bed. Chapter 17
  24. 24. Erosion Processes• River flow does work • Energy imparted is derived from gravity • Do work by converting potential to kinetic energy• Erosion is maximized during floods • Large water volumes, high velocities, abundant sediment Chapter 17
  25. 25. Erosion Processes• Stream erosion: scour, break abrade, dissovle material 1. Scouring – running water picks up sediment and moves it 2. Breaking & lifting – the force of moving water can… break chunks off the channel bottom/walls can lift rocks off the channel bottom Chapter 17
  26. 26. Erosion Processes3. Abrasion – sediment grains in flow “sandblast” rocks • Exposed bedrock in channels gets polished smooth • Gravel swirled by turbulent eddies drills holes • Bowl-shaped depressions are called potholes • Potholes are unusual, intricately sculpted4. Dissolution – mineral matter dissolves in water Chapter 17
  27. 27. Sediment Transport• Sediment load – material moved by rivers• 3 types: 1. Dissolved load – Ions from mineral weathering 2. Suspended load – fine particals (silt and clay) in the flow 3. Bed load – large articles roll, slide, bounce along bottom Chapter 17
  28. 28. Sediment Transport• Competence – maximum size transported• Capacity – maximum load transported • Change with discharge: • High discharge – large cobbles and boulders may move • Low discharge – large clasts are stranded Chapter 17
  29. 29. Sediment Deposition• When flow velocity decreases… • Competence is reduced and sediment drops out • Grain sizes are sorted by water. • Sands are removed from gravels; muds from both. • Gravels settle in channels. • Sands drop out in near channel environments. • Silts & clays drape floodplains away from channels. Chapter 17
  30. 30. Sediment Deposition• Sediment size tracks with river slope • Coarsest particles typify steep slopes in headwaters • Fine particles typify gentler slopes near the mouth Chapter 17
  31. 31. Sediment DepositionFluvial (river) sediments are called alluvium • Channels may have mid-channel bars • Sands build up point bars inside channel bends • A stream builds a delta upon entering a lake/ocean Chapter 17
  32. 32. Longitudinal Changes• Stream character changes with flow distances• In profile, the gradient is a concave-up curve Chapter 17
  33. 33. Longitudinal Changes• Near stream headwaters… • Gradient is steep, discharge is low • Sediment sizes are course (large) • Channels are straight, rocky Chapter 17
  34. 34. Longitudinal Changes• Toward the mouth (downstream end)… • Gradient is low, higher discharges • Smaller grain sizes typical • Channels are larger, bend more Chapter 17
  35. 35. Outline• Streamflow -streams/rivers, runoff, hydrologic cycle component -forming streams/rivers• Drainage networks -Patterns (dendritic to trellis) -Drainage basins, drainage divides• Rivers/Streams -Permanent vs. ephemeral -Discharge, channel velocity, erosion/transport/deposition -Longitudinal (downstream) changes• Further details -Base level, valleys & canyons, terraces, rapids/waterfalls -Depositional environments (e.g. alluvial fans, braiding, deltas..) -Drainage evolution and flooding Chapter 17 Chapter 17
  36. 36. Base Level ConceptLowest point to which a stream • Ultimate base level is sea level • Streams cannot erode below sea level • A lake serves as a local (or temporary) base level • Base level changes cause stream to adjust • Raising base level results in an increase in desposition • Lowering base level accelerates erosion Chapter 17
  37. 37. Valleys and Canyons• Land far above base level is subject to down cutting• Rapid down cutting creates eroded trough • Valley – gently sloping trough sidewalls define a V-shape • Canyon – steep trough sidewalls form cliffs• Determined by rate of erosion vs. strength of rocks Chapter 17
  38. 38. Stream Terraces• Valleys store sediment when base level is stable/raised• Stability, then renewed incision creates stream terraces • Terraces are former, now abandoned, floodplains Chapter 17
  39. 39. Rapids & Waterfalls• Rapids are turbulent water with a rough surface• Waterfalls are free-falling water columns• Reflect geologic control: • Flow over bedrock steps or large clasts • Flow constriction (channel narrowing) • Sudden increase in gradient Chapter 17
  40. 40. Alluvial Fans• Build at mountain front by river (or debris) flow• Sediments rapidly dropped near stream source• Sediments create a conical, fan-shaped structure Chapter 17
  41. 41. Braided Streams• Form where channels are choked by sediment• Flow is forced around sediment obstructions • Diverging - converging flow creates sand and gravel bars • Bars are unstable, rapidly formed and eroded• Flow occupies multiple channels across a valley Chapter 17
  42. 42. Meandering Streams• Channels can form looping curves • Along lower river portion with low gradient • Where streams travel over a broad floodplain • When substrates are soft and easily eroded• Meanders increase volume of water in the stream• Meanders evolve Chapter 17
  43. 43. Meandering Streams• Max velocity swings back & forth across channels • Fast water erodes cut back (outside of bend) • Point bar (inside of bend) collects sediment• Meanders change due to natural variation in... • Thalweg (maximum depth) position and friction • Get cutoff when sinuosity gets too severe (cut banks converge) Chapter 17
  44. 44. Meandering Streams• Meanders become more sinuous with time • Cut bank erodes; point bar accretes. • Curves become more pronounced Chapter 17
  45. 45. Deltas• Deltas form  a river enters standing water (base level) • Flow slws, loses competence; sediments drop out• Channel divides into a fan of small distributaries Chapter 17
  46. 46. Deltas• Mississippi has a river-dominated bird’s foot delta• Distinct lobes indicate past desposition centers• River periodically switches course via avulsion • River breaks through a levee upstream • Establishes a shorter, steeper path to the Gulf of Mexico Chapter 17
  47. 47. Drainage Evolution• Streamflow is cause of most Landscape changes• Example: •Upliftchanges base level •Streams cut down •Valleys widen; hills erode •Landscape lowered to new base level Chapter 17
  48. 48. Drainage EvolutionStream piracy • One stream captures flow from another • Results from headward erosion • A stream with more vigorous erosion (steeper gradient), intercepts another stream • Captured stream flows into the new stream • Below capture point, old stream dries up Chapter 17
  49. 49. Drainage EvolutionDrainage reversal • Tectonic uplift can alter a river course • South America used to drain westward • Western uplift raised the Andes, changed Amazon flow to east Chapter 17
  50. 50. Drainage EvolutionAntecedent drainages • Tectonic uplift can raise ground beneath established streams • If erosion keeps pace with uplift, stream will incise into uplift • Called antecedent drainage • If uplift rate exceeds incision, stream is diverted around uplft Chapter 17
  51. 51. Drainage Evolution• Some antecedent streams have incised meanders • Meanders initially develop on a low gradient • Uplift raises landscape (drops base level_) • Meanders incise into the uplifted Chapter 17
  52. 52. Raging Waters• During a flood… • Flow exceeds water volume storage of a channel • Velocity (thus, competence & capacity) increase • Water leaves channel, drowns adjacent land • Moving water & debris scour floodplains • Water slows away from the thalweg, dropping sediment Chapter 17
  53. 53. Raging Waters• Numerous causes of floods: • Torrential rainfall • After soil pores have been filled by prior rainfalls • Abrupt warm weather rapidly melts winter snow • Failure of a natural/artifical dam Chapter 17
  54. 54. Raging Waters• Case history: Mississippi and Missouri Rivers, 1993. • Spring 1993: long rainy season • July 1993: flood waters invaded huge areas • Covered 40,000 mi2. • Flood lasted 79 days. • 50 people died. • 55,000 homes destroyed. • $12 billion in damage. Chapter 17
  55. 55. Raging Waters• Seasonal floods recur on an annual basis. • Monsoons – heavy tropical rains (ie on Indian subcontinent) • Intense period of heavy summer rain • Many people live in floodplain & delta plain settings. • 1990 - monsoon killed 100,000 people in Bangladesh. • 2008 – monsoon caused the Kosi river to avulse, displacing ~2.3 million people in Nepal/India. Chapter 17
  56. 56. Raging Waters• Kosi River flood before and after. • New channel width ~20 km! Chapter 17
  57. 57. Raging Waters• Kosi River flood before and after. Chapter 17
  58. 58. Raging Waters• Ancient floods: Ice-Age megafloods. • 11 Ka, ice dams failed, releasing Glacial Lake Missoula • Water scoured eastern Washington landscape • Created “channeled scablands” • Once of largest floods in geologic histroy Chapter 17
  59. 59. Living with Floods• People living in floodplains face hard choice • Move or expect eventual catastrophic loss• Land use changes may mitigate flood damage • Establish floodways – places designed to transmit floods • Remove people and structures from these places Chapter 17
  60. 60. Living with Floods• Flood risk borne by homeowners, insurance companies, lenders, government agencies • Use hydrologic data to produce flood ricks maps • Maps allow agencies to manage risks • Building in flood-prone settings is tightly regualted Chapter 17
  61. 61. Living with Floods• Flood risk is calculated as a probability • Discharges are plotted against recurrence intervals • On semi-log, this plots as a straight long • Probability (% chance of occurrence) given discharge will happen (determined by graph inspection) Chapter 17

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