15 rivers forstudents
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15 rivers forstudents

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15 rivers forstudents 15 rivers forstudents Presentation Transcript

  • Rivers 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 Chapter 17
  • Streamflow• Stream/River –• Runoff –• Stream runoff is crucial for humans: Chapter 17
  • Streamflow• Stream runoff also Chapter 17
  • Streamflow• Stream runoff is an important • Flowing water… • Earth: only planet Chapter 17
  • The Hydrologic Cycle• Stream flow – Chapter 17
  • Forming Streams• Streamflow begins Chapter 17
  • Forming StreamsStreamflow begins as• Sheetwash erosion• Rills coalesce & deepen into channels. Chapter 17
  • Forming Streams• Scouring can mark entry into the channel• Rapid erosion lengthens channel upslope • Process is called headward erosion Chapter 17
  • Forming Streams• Over time, channels merge.• Smaller tributaries join larger trunk steam• A drainage network –array of linked channels 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 Chapter 17
  • Drainage Networks• Drainage networks form geometric patterns• Patterns reflect geolgoy and landscape form• Several common drainage patterns 1. Dendritic – branching, tree like, due to uniform material Chapter 17
  • Drainage Networks• Common drainage patterns: 2. Radial – form a point uplift Chapter 17
  • Drainage Networks• Common drainage patterns: 3. Rectangular – controlled by jointed rocks Chapter 17
  • Drainage Networks• Common drainage patterns: 4. Trellis (garden) – due to atlernating resistant/weak rocks common in fold-in thrust belts 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 Chapter 17
  • Drainage Divides• Watersheds exist across scales.• Tiny tributaries• Continentals rivers• Large watersheds…• Feed large rivers• Section continents• Continental divides separate flow to different oceans 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 Chapter 17
  • 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
  • Discharge• Amount of water flowing in a channel • Water volume passing a point per unit time• Cubic meters per second• Given by cross-sectional area• Varies seasonally due to precipation and runoff Chapter 17
  • Channel Velocity• Velocity is • Friction • Greater in • Lesser in • Magnitude determined by • In straight channels, Chapter 17
  • Channel Velocity• Velocity is not uniform within a channel • Max. velocity near outside in bending channels • Outside is preferentially scoured and deepened • Inside is locus of deposition due to reduced velocity • Deepest part is called the thalweg Chapter 17
  • Channel Velocity• Velocity is not uniform in all areas of a channel • Stream flow is turbulent • Chaotic• Turbulence caused by…• Flow obstructions• Shear in water• Eddies scour channel bed. 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 floodsLarge water volumes, high velocties Chapter 17
  • Erosion Processes• Stream erosion: 1. Scouring – 2. Breaking & lifting – Chapter 17
  • Erosion Processes3. Abrasion – • Exposed bedrock in channels gets • Gravel swirled by turbulent eddies • Bowl-shaped depressions are called4. Dissolution – Chapter 17
  • Sediment Transport• Sediment load -• 3 types: 1. Dissolved load – 2. Suspended load – 3. Bed load – Chapter 17
  • Sediment Transport• Competence –• Capacity – • Change with discharge: • High discharge – • Low discharge – Chapter 17
  • Sediment Deposition• When flow velocity decreases… • Competence is • 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
  • Sediment Deposition• Sediment size tracks with • Coarsest particles typify • Fine particles typify Chapter 17
  • Sediment DepositionFluvial (river) sediments are called • Channels may have • Sands build up • A stream builds a Chapter 17
  • Longitudinal Changes• Stream character• In profile, the gradient describes Chapter 17
  • Longitudinal Changes• Near stream headwaters… • Gradient is • Sediment sizes are • Channels are Chapter 17
  • Longitudinal Changes• Toward the mouth (downstream end)… • Gradient is • Smaller • Channels are 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 Chapter 17
  • Base Level ConceptLowest point to which a stream • Ultimate base level is • A lake serves as a • Base level changes cause • Raising base level results in an • Lowering base level Chapter 17
  • Valleys and Canyons• Land far above base level• Rapid down cutting creates • Valley – • Canyon –• Determined Chapter 17
  • Stream Terraces• Valleys store sediment when• Stability, then renewed Chapter 17
  • Rapids & Waterfalls• Rapids are• Waterfalls are• Reflect geologic control: • Flow over • Flow • Sudden increase Chapter 17
  • Alluvial Fans• Build at mountain front by• Sediments rapidly• Sediments create Chapter 17
  • Braided Streams• Form where channels• Flow is forced around • Diverging - converging flow • Bars are unstable• Flow occupies Chapter 17
  • Meandering Streams• Channels can form • Along • Where streams travel over • When substrates are• Meanders increase• Meanders Chapter 17
  • Meandering Streams• Max velocity swings back & • Fast water erodes • Point bar• Meanders change due to natural variation in... • Thalweg • Get cutoff when Chapter 17
  • Meandering Streams• Meanders become • Cut bank erodes; point bar accretes. • Curves become Chapter 17
  • Deltas• Deltas form  a river enters • Flow• Channel divides into a Chapter 17
  • Deltas• Mississippi has a• Distinct lobes indicate• River periodically • River breaks • Establishes a shorter, steeper path Chapter 17
  • Drainage Evolution• Streamflow is cause of most• Example: • Uplift changes • Streams cut • Valleys • Landscape lowered Chapter 17
  • Drainage EvolutionStream piracy • One stream • Results from • A stream with • Captured • Below capture point, Chapter 17
  • Drainage EvolutionDrainage reversal • Tectonic uplift can • South America used to • Western uplift Chapter 17
  • Drainage EvolutionAntecedent drainages • Tectonic uplift can raise • If erosion keeps pace with uplift, • Called • If uplift rate exceeds incision, Chapter 17
  • Drainage Evolution• Some antecedent streams • Meanders initially develop • Uplift raises • Meanders incise Chapter 17
  • Raging Waters• During a flood… • Flow exceeds • Velocity (thus, competence & capacity) • Water leaves • Moving water & debris • Water slows away from the Chapter 17
  • Raging Waters• Numerous causes of floods: Chapter 17
  • Raging Waters• Case history: Mississippi and Missouri Rivers, 1993. • Spring 1993: • July 1993: • Covered 40,000 mi2. • Flood lasted 79 days. • 50 people died. • 55,000 homes destroyed. • $12 billion in damage. Chapter 17
  • Raging Waters• Seasonal floods recur on an annual basis. • Monsoons – • Intense period of • 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
  • Raging Waters• Kosi River flood before and after. • New channel width ~20 km! Chapter 17
  • Raging Waters• Kosi River flood before and after. Chapter 17
  • Raging Waters• Ancient floods: Ice-Age megafloods. • 11 Ka, • Water scoured • Created Chapter 17
  • Living with Floods• People living in floodplains• Land use changes may • Establish floodways – • Remove Chapter 17
  • Living with Floods• Flood risk borne by • Use hydrologic data to • Maps allow agencies to • Building in flood-prone settings is Chapter 17
  • Living with Floods• Flood risk is calculated • Discharges are plotted against • On semi-log, this plots as a • Probability (% chance of occurrence) given discharge will happen Chapter 17