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Geologylecture16 130408195848-phpapp02
- 2. Chapter 17 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
- 3. Chapter 17 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
- 4. Chapter 17 Streamflow • Stream flow/runoff also causes many problems • Flooding destroys lives and property
- 5. Chapter 17 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
- 6. Chapter 17 The Hydrologic Cycle • Stream flow – important component of hydrologic cycle
- 7. Chapter 17 Forming Streams • Streamflow begins as water is added to the surface
- 8. Chapter 17 Forming Streams Streamflow begins as moving sheetwash thin surface water layer moves down steepest slope erodes substrate • Sheetwash erosion creates tiny channels (rills) • Rills coalesce & deepen into channels.
- 9. Chapter 17 Forming Streams • Scouring can mark entry into the channel • Rapid erosion lengthens channel upslope • Process is called headward erosion
- 10. Chapter 17 Forming Streams • Over time, channels merge. • Smaller tributaries join larger trunk stream • A drainage network – array of linked channels • They change over time
- 11. Chapter 17 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
- 12. Chapter 17 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
- 13. Chapter 17 Drainage Networks • Common drainage patterns: 2. Radial – form a point uplift (e.g. volcano)
- 14. Chapter 17 Drainage Networks • Common drainage patterns: 3. Rectangular – controlled by jointed rocks
- 15. Chapter 17 Drainage Networks • Common drainage patterns: 4. Trellis (garden) – due to alternating resistant/weak rocks common in fold-trust belts
- 16. Chapter 17 A Drainage Basin • Land area that drains into a specific trunk stream Also called catchment or watershed • Divides are boundaries that separate drainage basins
- 17. Chapter 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
- 18. Chapter 17 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
- 19. Chapter 17 Permanent vs. Ephemeral • Permanent streams • Water flows all year. • At or below the water table. • Humid or temperate. • Sufficient rainfall. • Lower evaporation. • Discharge varies seasonally. • Ephemeral streams • Do not flow all year. • Above the water table. • Dry climates. • Low rainfall. • High evaporation. • Flow mostly during rare flash floods.
- 20. Chapter 17 • 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 Discharge
- 21. Chapter 17 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
- 22. Chapter 17 • 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 Channel Velocity
- 23. Chapter 17 • 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. Channel Velocity
- 24. Chapter 17 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
- 25. Chapter 17 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
- 26. Chapter 17 Erosion Processes 3. 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 sculpted 4. Dissolution – mineral matter dissolves in water
- 27. Chapter 17 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
- 28. Chapter 17 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
- 29. Chapter 17 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.
- 30. Chapter 17 Sediment Deposition • Sediment size tracks with river slope • Coarsest particles typify steep slopes in headwaters • Fine particles typify gentler slopes near the mouth
- 31. Chapter 17 Sediment Deposition Fluvial (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
- 32. Chapter 17 Longitudinal Changes • Stream character changes with flow distances • In profile, the gradient is a concave-up curve
- 33. Chapter 17 Longitudinal Changes • Near stream headwaters… • Gradient is steep, discharge is low • Sediment sizes are course (large) • Channels are straight, rocky
- 34. Chapter 17 Longitudinal Changes • Toward the mouth (downstream end)… • Gradient is low, higher discharges • Smaller grain sizes typical • Channels are larger, bend more
- 35. Chapter 17 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
- 36. Chapter 17 Base Level Concept Lowest 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
- 37. Chapter 17 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
- 38. Chapter 17 Stream Terraces • Valleys store sediment when base level is stable/raised • Stability, then renewed incision creates stream terraces • Terraces are former, now abandoned, floodplains
- 39. Chapter 17 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
- 40. Chapter 17 Alluvial Fans • Build at mountain front by river (or debris) flow • Sediments rapidly dropped near stream source • Sediments create a conical, fan-shaped structure
- 41. Chapter 17 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
- 42. Chapter 17 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
- 43. Chapter 17 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)
- 44. Chapter 17 Meandering Streams • Meanders become more sinuous with time • Cut bank erodes; point bar accretes. • Curves become more pronounced
- 45. Chapter 17 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
- 46. Chapter 17 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
- 47. Chapter 17 Drainage Evolution • Streamflow is cause of most Landscape changes • Example: •Uplift changes base level •Streams cut down •Valleys widen; hills erode •Landscape lowered to new base level
- 48. Chapter 17 Drainage Evolution Stream 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
- 49. Chapter 17 Drainage Evolution Drainage reversal • Tectonic uplift can alter a river course • South America used to drain westward • Western uplift raised the Andes, changed Amazon flow to east
- 50. Chapter 17 Drainage Evolution Antecedent 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
- 51. Chapter 17 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
- 52. Chapter 17 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
- 53. Chapter 17 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
- 54. Chapter 17 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.
- 55. Chapter 17 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.
- 56. Chapter 17 Raging Waters • Kosi River flood before and after. • New channel width ~20 km!
- 57. Chapter 17 Raging Waters • Kosi River flood before and after.
- 58. Chapter 17 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
- 59. Chapter 17 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
- 60. Chapter 17 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
- 61. Chapter 17 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)