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Geology lecture 16

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  • 1. Rivers Chapter 17
  • 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. 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. Streamflow• Stream flow/runoff also causes many problems • Flooding destroys lives and property Chapter 17
  • 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. The Hydrologic Cycle• Stream flow – important component of hydrologic cycle Chapter 17
  • 7. Forming Streams• Streamflow begins as water is added to the surface Chapter 17
  • 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. Forming Streams• Scouring can mark entry into the channel• Rapid erosion lengthens channel upslope • Process is called headward erosion Chapter 17
  • 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. 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. 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. Drainage Networks• Common drainage patterns: 2. Radial – form a point uplift (e.g. volcano) Chapter 17
  • 14. Drainage Networks• Common drainage patterns: 3. Rectangular – controlled by jointed rocks Chapter 17
  • 15. Drainage Networks• Common drainage patterns: 4. Trellis (garden) – due to alternating resistant/weak rocks common in fold-trust belts Chapter 17
  • 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. Longitudinal Changes• Stream character changes with flow distances• In profile, the gradient is a concave-up curve Chapter 17
  • 33. Longitudinal Changes• Near stream headwaters… • Gradient is steep, discharge is low • Sediment sizes are course (large) • Channels are straight, rocky Chapter 17
  • 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. Meandering Streams• Meanders become more sinuous with time • Cut bank erodes; point bar accretes. • Curves become more pronounced Chapter 17
  • 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. Raging Waters• Kosi River flood before and after. • New channel width ~20 km! Chapter 17
  • 57. Raging Waters• Kosi River flood before and after. Chapter 17
  • 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. 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. 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. 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