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
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3. Streamflow
• Stream/River –
• Runoff –
• Stream runoff is crucial for humans:
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4. Streamflow
• Stream runoff also
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5. Streamflow
• Stream runoff is an important
• Flowing water…
• Earth: only planet
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6. The Hydrologic Cycle
• Stream flow –
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7. Forming Streams
• Streamflow begins
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8. Forming Streams
Streamflow begins as
• Sheetwash erosion
• Rills coalesce & deepen
into channels.
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9. Forming Streams
• Scouring can mark entry into the channel
• Rapid erosion lengthens channel upslope
• Process is called headward erosion
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10. Forming Streams
• Over time, channels merge.
• Smaller tributaries join larger trunk steam
• A drainage network –array of linked channels
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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
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12. 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
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13. Drainage Networks
• Common drainage patterns:
2. Radial – form a point uplift
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14. Drainage Networks
• Common drainage patterns:
3. Rectangular – controlled by jointed rocks
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15. Drainage Networks
• Common drainage patterns:
4. Trellis (garden) – due to atlernating resistant/weak rocks
common in fold-in thrust belts
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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
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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
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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
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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.
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20. 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
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21. Channel Velocity
• Velocity is
• Friction
• Greater in
• Lesser in
• Magnitude determined by
• In straight channels,
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22. 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
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23. 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.
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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 velocties
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25. Erosion Processes
• Stream erosion:
1. Scouring –
2. Breaking & lifting –
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26. Erosion Processes
3. Abrasion –
• Exposed bedrock in channels gets
• Gravel swirled by turbulent eddies
• Bowl-shaped depressions are called
4. Dissolution –
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27. Sediment Transport
• Sediment load -
• 3 types:
1. Dissolved load –
2. Suspended load –
3. Bed load –
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28. Sediment Transport
• Competence –
• Capacity –
• Change with discharge:
• High discharge –
• Low discharge –
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29. 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.
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30. Sediment Deposition
• Sediment size tracks with
• Coarsest particles typify
• Fine particles typify
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31. Sediment Deposition
Fluvial (river) sediments are called
• Channels may have
• Sands build up
• A stream builds a
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32. Longitudinal Changes
• Stream character
• In profile, the gradient describes
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33. Longitudinal Changes
• Near stream headwaters…
• Gradient is
• Sediment sizes are
• Channels are
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34. Longitudinal Changes
• Toward the mouth (downstream end)…
• Gradient is
• Smaller
• Channels are
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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
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36. Base Level Concept
Lowest 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
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37. Valleys and Canyons
• Land far above base level
• Rapid down cutting creates
• Valley –
• Canyon –
• Determined
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38. Stream Terraces
• Valleys store sediment when
• Stability, then renewed
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39. Rapids & Waterfalls
• Rapids are
• Waterfalls are
• Reflect geologic control:
• Flow over
• Flow
• Sudden increase
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40. Alluvial Fans
• Build at mountain front by
• Sediments rapidly
• Sediments create
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41. Braided Streams
• Form where channels
• Flow is forced around
• Diverging - converging flow
• Bars are unstable
• Flow occupies
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42. Meandering Streams
• Channels can form
• Along
• Where streams travel over
• When substrates are
• Meanders increase
• Meanders
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43. Meandering Streams
• Max velocity swings back &
• Fast water erodes
• Point bar
• Meanders change due to natural variation in...
• Thalweg
• Get cutoff when
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44. Meandering Streams
• Meanders become
• Cut bank erodes; point bar accretes.
• Curves become
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45. Deltas
• Deltas form  a river enters
• Flow
• Channel divides into a
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46. Deltas
• Mississippi has a
• Distinct lobes indicate
• River periodically
• River breaks
• Establishes a shorter, steeper path
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47. Drainage Evolution
• Streamflow is cause of most
• Example:
• Uplift changes
• Streams cut
• Valleys
• Landscape lowered
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48. Drainage Evolution
Stream piracy
• One stream
• Results from
• A stream with
• Captured
• Below capture point,
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49. Drainage Evolution
Drainage reversal
• Tectonic uplift can
• South America used to
• Western uplift
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50. Drainage Evolution
Antecedent drainages
• Tectonic uplift can raise
• If erosion keeps pace with uplift,
• Called
• If uplift rate exceeds incision,
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51. Drainage Evolution
• Some antecedent streams
• Meanders initially develop
• Uplift raises
• Meanders incise
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52. Raging Waters
• During a flood…
• Flow exceeds
• Velocity (thus, competence & capacity)
• Water leaves
• Moving water & debris
• Water slows away from the
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53. Raging Waters
• Numerous causes of floods:
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54. 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.
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55. 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.
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56. Raging Waters
• Kosi River flood before and after.
• New channel width ~20 km!
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57. Raging Waters
• Kosi River flood before and after.
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58. Raging Waters
• Ancient floods: Ice-Age megafloods.
• 11 Ka,
• Water scoured
• Created
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59. Living with Floods
• People living in floodplains
• Land use changes may
• Establish floodways –
• Remove
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60. Living with Floods
• Flood risk borne by
• Use hydrologic data to
• Maps allow agencies to
• Building in flood-prone settings is
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61. 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
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