Streams: Transport to the Oceans

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Streams Transport to the Oceans Chapter
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Streams-- all bodies of flowing water,
bayous(?) Rivers -- those bigger bodies of
flowing water,e.g. Mississippi River
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STREAM FLOW
Why is it dangerous to row across the Mississippi?
shearing
100 feet
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• Turbulent Flow
• Particles have paths that are no longer straight
  or gently curved (laminar flow)
• Particle paths cross each other
• Turbulence occurs best when the fluid is of low
  viscosity and fast moving

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In water, turbulence begins when the velocity
becomes too high
Shallow, slow, laminar flow
slow
FASTER
Away from the edges, where is the water more
turbulent along the surface of river? Where is
the velocity highest?
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What are the consequences of Turbulent vs.
Laminar Flow in Earth Science??

• Laminar flow can lift and carry only the smallest
  and lightest of clay particles, that are not
  stuck together.
• Turbulent flow
• can move clay, pebbles and cobble stones
• can pick up a sand particle for a little way and
  then drop it (saltation)

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• A stream can transport 2 types of sedimentary
  load depending on water speed
• Bed Load-- Sand, Gravel, Boulders, sliding and
  rolling along the bottom
• Suspended Load--Clay, mainly within the water

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Speed of water and erodability
1cm/s .01 m/s x 3600 s per hour
36 m/hour Boulders are moved at speeds in excess
of 500 cm/s, 500cm/s 5 m/s x 3600 s per hour
18,000 m/hr or 18 km/hr In the
Mississippi River in front of Baton Rouge At
8km/hr we have a speed of 800,000 cm per 3,600 s,
that is 222 cm/s
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Rebellious Attraction by Clay!!

• Q. Clay particles disobey our intuition. Why?
• A. Because clay particles become sticky as a
  result of
• electrical attraction between clay particles and
• water tension between clay particles

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STREAM EROSION
How can streams erode a hard rocky bottom?
By ABRASION sandblasting or by the impact of
particles against the rock bed e.g., potholes
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STREAM VALLEY ANATOMY
Channels and Floodplains Braided Channels In
periods of low stream flow, the stream consists
of many small channels. Water easily erodes the
banks to form the small channels. Meandering
Channels (Between floods), the water flows along
snake-like curves
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RIVER MEANDERS
Some of the best studied meanders in the world
are from Louisiana
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• Competence of the water (ability to carry grains
  of a certain size) depends on the speed of the
  water
• As fast water leaves the river and enters the
  flood plain the same volume of water must spread
  out over a larger area. At that point the speed
  of the water drops.
• As the speed of the water drops the competence of
  the water is reduced and the relatively larger
  particles that may be in turbulent suspension
  drop out.

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• Types of river forms
• levees form from repeated floods
• point bars form on the slow, inside bend of the
  meander
• oxbow lakes form when a meander neck is cut-off

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• How can we lessen the effects flooding?
• Lets discuss the feasibility of
• Not building on the floodplain
• Creating artificial levees
• Shortening the river (e.g. USACE has shortened
  Mississippi river by 243 km)

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Longitudinal Profile

• In general, river valleys tend to develop smooth
  cross-sections which are concave-upward.
• The profile (longitudinal profile) is steepest in
  the uplands portion where erosion is strong. At
  its lower reaches the slope is lesser and there
  is more sedimentation.
• If sea-level drops the lower reaches of the river
  are steepened and in order to regain a normal
  longitudinal profile the river begins to erode
  from the lower reaches to the uplands (headward
  erosion)
• When an equilibirum or steady-state,
  non-changing profile is regained, we also say
  that the stream is a graded stream.

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In Louisiana, many of our former flood plains
have been incised or cut. That is the former
floodplains are now higher with respect to the
present-day river. We call these areas terraces.
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