Streams g110

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Streamsg110

• Lecture prepared by Bob Hall
• Revised 8/2/2000

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Topics To Be Covered

• Why study streams?
• Floods
• Drainage basins
• Streams as component of hydrologic cycle
• Masters of adjustment
• Stream variables
• Stream profile

• Masters of adjustment
• Channel patterns
• Meandering
• Braided
• Flood plain formation
• Lateral accretion
• Vertical accretion
• Stream terraces

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StreamsWhy we need to know about themTheir
shifty behavior

• Flowing water the dominant agent in shaping the
  landscape.
• Rivers are the source of most marine sediments.
• Absolutely essential use as water source.
• Almost as important in transportation, generating
  electricity, recreation
• In flood, they are our greatest life-taking
  geologic hazard.

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White River, Indianapolis1999
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Flood!!
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The Drainage Basin (Watershed)
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United States Drainage Basins
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Streams in the Context ofthe Hydrologic Cycle
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Concept of Discharge
Q A x V
Jm slide15
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Concept of the Hydrograph
Jm slide 16
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Quiz

• What would the hydrographs look like for a
  stream draining the same watershed under the
  following conditions
• Under a forest cover.
• Planted to crops
• Urbanized

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Streams as Masters of Adjustment

• Stream Variables
• Stream Profiles and Base Level
• Stream-Channel Patterns

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Stream Variables

• Independent
• Discharge (Q)
• Sediment load

• Dependent
• Width (W)
• Depth (D)
• Velocity (V)

The interrelationships between variables is
expressed as the discharge equation Q W x D x
V As Q changes, changes must also occur in W,
D, and V. Examples flood, withdrawal of
irrigation water.
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Stream Variables (cont.)

• Solution By the continuity equation, Q W x D
  x V,
• An increase in W will force a decrease in depth
  and/or velocity.
• If change is in depth, the stream will be only ½
  as wide

• Problem If discharge remains the same through 2
  stream cross sections, but the second is twice as
  wide as the first, what changes in variables will
  occur?

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Stream Profiles

• Transverse trapezoids to troughs to Vs
• Longitudinal concave upward
• Smooth an equilibrium profile stream said
  to be graded.

knickpoint
A profile with knickpoints is not in equilibrium.
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Stream Profiles (cont.)

• Problem A landslide adds much material to the
  load carried by the stream. How will the stream
  respond?

• Solution To handle the extra load, the stream
  must increase velocity.
• Steepening the gradient is one way.
• How could the stream adjust using the continuity
  equation?

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Base Level

• Lowest level to which a stream can erode.

• Ultimate b.l. sea level
• Temporary b.l.
• lakes
• tributary junctions
• resistant rocks in channel

What happens to a stream profile if base level
falls? If it rises?
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Longitudinal Profile and Base Level
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Channel Patterns

• Braided multiple channels, indiv. ones may
  meander.

• Meandering series of bends, usually as single
  channel

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Meandering Stream
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Stream Meanders
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Meander Cutoff
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Entrenched Meanders
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Braided Channels

• Jm slide 27

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Meandering vs. Braided Patterns

• Meandering
• Less total load
• Finer-grained load
• Cohesive banks

• Braided
• More total load
• Coarser-grained
• Non-cohesive banks

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Meandering vs Braided Streams

• Problem A stream flows with a meandering pattern
  through a stretch, then flows with a braided
  pattern for another stretch, then returns to
  meandering once again. Why?

• Braided The stream bed materials change from fine
  to coarse to fine.

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Stream Behavior
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Adjustments of Longitudinal Profile
Jm slide 21
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Flood Plain Formation by Lateral Accretion
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Flood Plain Formation by Vertical Accretion
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Stream TerracesHow do they form?

• Abandoned flood
• plains
• Another shift in stream behavior.

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Terms Used in Studying Streams

• Base level
• Discharge
• Drainage basin
• Entrenched meander
• Graded stream
• Hydrograph
• Knickpoint
• Longitudinal profile

• Meander cutoff
• Stream-channel pattern
• Stream variable
• Temporary base level
• Transverse profile
• Ultimate base level
• Watershed

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Student Responsibilities on Streams

• What is a drainage basin? How are streams part
  of the hydrologic cycle? Explain how streams are
  masters of adjustmentwith at least 4 examples
  include the concepts of stream variables, stream
  profileless and base level, channel patterns, and
  stream behavior. How do flood plains form? How
  do stream terraces form?