Streams: Transport to the Ocean

1
Streams Transport to the Ocean
2
Rivers and streams

• Stream Body of water flowing in a channel.
• The floor of the channel is called the bed.
• When rainfall is very heavy or snow melts
  rapidly, bodies of water overflow their banks
  and water covers the adjacent land called the
  floodplain.

3
Rivers and streams

• Carry away runoff to lakes and seas.
• Erode land (degradation).
• Transport and deposit sedimentary debris, forming
  fertile farmlands.

4
How does water flow in a stream?
5
Viscosity

• A materials resistance to flow. Higher
  viscosity slower flowing.
• NOT related to density.
• Viscosity demo.

6
Viscosities (Pa s)
7
Laminar flow

• Fluid moves in straight lines or slightly curved
  paths. Streamlines (lines of motion of water
  molecules) do not cross.

Fig. 14.1
8
Turbulent flow

• Irregular, swirling flow. Streamlines cross.
• Occurs at most rates of stream flow.
• Keeps smaller particles in suspension.

Fig. 14.1
9
Laminar to turbulent transition
Fig. 14.1
10
Reynolds Number

• r fluid density, U fluid velocity, H
  length, and m viscosity.
• If Re gt 500, turbulent.
• If Re lt 500, laminar.

11
Streams move material in three forms

• Dissolved load Material dissolved in stream
  waters.
• Suspended load Material permanently or
  temporarily suspended in the stream flow.
• Bed load Material that travels along the stream
  bed by sliding and rolling.

12
Flow velocity vs particle size
Faster current
Larger particles carried
13
Settling velocity

• How fast a particle of a given size settles out
  of stream waters.
• V 2gr2 (rparticle-rwater)/(9m)
• g acceleration of gravity (a constant)
• r radius of the particle
• rparticle density of the particle
• rwater density of the water
• m viscosity of the water
• The smaller the particle, the slower it settles
  out of suspension (clays stay in suspension for a
  long time).

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Current velocity and particle size
Fig. 14.3
15
Lower Velocities form Ripples
Saltating grain
Ripples migrate downstream and are cross-bedded.
Fig. 14.4
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Higher Velocities form Dunes
Fig. 14.4
17

• Very low velocities No ripples or dunes (not
  enough sediment moving).
• Very high velocities No ripples or dunes
  (eradicated by fast flow, and most particles are
  in suspension rather than saltating).

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Parts of a River System
19
Two important stream types

• 1. Meandering Streams
• Gentle gradients, fine-grained and unconsolidated
  (i.e., easily eroded) alluvium
• Examples point bars, oxbow lake, migrating
  meanders

20
Meandering River
Fig. 14.9
21
Fig. 14.9
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Low velocity Low sediment Low slope
Shifting meanders
Current faster on outside banks (erosion), slower
on inside banks (deposi- tion)
Fig. 14.9
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Formation of an oxbow lake
Major flood event
Oxbow lake
Fig. 14.9
24
Two important stream types

• 2. Braided Streams
• Large sediment supply.
• Large variations in flow volume.
• At any one moment the active channels may account
  for only a small proportion.
• Common in glacial, deserts, and mountain regions.

25
Braided River
Fig. 14.9
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Formation of Natural Levees
Fig. 14.10
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Formation of Natural Levees
Fig. 14.10
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Formation of Natural Levees
Fig. 14.10
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Formation of Natural Levees
Fig. 14.10
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Discharge

• Total amount of water that passes a given point
  in a stream per unit time
• Q w d v
• Q flux (m3/s)
• w width (m)
• d depth (m)
• v velocity (m/s)

31
River with Low Discharge
Fig. 14.11
32
River with High Discharge
Fig. 14.11
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Flooding

• Water in the stream is greater than the volume of
  the channel.
• Interval between floods depends on the climate of
  the region and the size of the channel.

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Recurrence interval

• Average time between the occurrences of a given
  event (e.g., flood, earthquake).
• The recurrence interval of a flood of a given
  size at a given place depends on
• climate of the region
• width of the floodplain
• size of the channel

35
Annual Flood Frequency Curve
Fig. 14.12
36
Longitudinal profile

• Cross-sectional view or profile of a river.
• Height of a riverbed at any point along its
  course.

37
Longitudinal Stream Profile of the Platt and
South Platt Rivers
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Base level

• Elevation at which a stream
• enters a large body of water such
• as a lake or ocean

39
Role of Base Level in Controlling Longitudinal
Profile of Rivers
Fig. 14.14
40
Graded stream

• Stream in which neither erosion nor deposition is
  occurring, due to an equilibrium of slope,
  velocity, and discharge.

41
Geologic evidence of changesin stream equilibrium

• Alluvial fans Cone-shaped or fan-shaped
  accumulations at a steep slope change.
• Terraces Erosional remnants of former
  floodplains.

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Alluvial Fans
Fig. 14.16
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Formation of River Terraces
44
Drainage divides separate adjacent drainage
basins
Fig. 14.18
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Drainage basin

• Area of land surrounded by topographic divides in
  which all the water is directed to a single
  point.

46
Drainage basin
Fig. 14.19
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Delta

• Location of significant sedimentation where a
  river meet the sea.

48
Mississippi Delta
Fig. 14.24
49
Typical Large Marine Delta
Fig. 14.23