Streams: Flowing Water Shapes the Landscape

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Streams Flowing Water Shapes the Landscape

• Smith Pun, Chapter 16

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Why study streams?

• Streams provide water and deposit fertile soil
  for agriculture.
• They are pathways for commerce and trade.
• They also flood, erode, and sculpt the landscape.
• Streams are everywhere and many cities are
  adjacent to them.
• We Americans use water from streams and lakesone
  trillion liters per day.

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Why study streams?

• Objectives In this chapter you will
• learn how water and sediment move in channels and
  across floodplains.
• learn about the diverse patterns and dimensions
  of stream channels and how they relate to dynamic
  processes.
• explore the origins of landscape features
  produced by river erosion and deposition.
• Analyze the causes of floods and the origins of
  floodplains.
• Learn how streams change through time in response
  to natural processes and human activities.
• Describe how through-flowing streams become
  naturally obstructed to produce lakes.

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Why study streams?
Fig 16.1
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Why study streams?
Fig 16.1
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16.1 Where does the water come from?
The hydrologic cycle diagrams the movement of
water through Earth systems. Evapotranspiration
moves water into the atmosphere precipitation
brings it back to Earth. Once on the ground, it
may flow across the surface as runoff (into stream
channels) or infiltrate down into the subsurface
and become part of the ground water.
Fig 16.2
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16.1 Where does the water come from?
Major drainage basins of the North American
continent. While the Mississippi basin is the
largest river basin, the basins in Canada that
feed Hudson Bay and the Arctic Ocean are indeed
sizable.
The Great Basin, west of the continental divide,
does not drain into a sea or ocean, but instead
exits into lakes and playas.
Fig 16.4
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16.1 Where does the water come from?
Discharge is the amount of water that flows
through a channel. Discharge is calculated by
first finding the cross-sectional area of a
stream and then multiplying this times the
velocity. The result will always be in a measure
of volume per unit time. (See formula below in
Fig. 16.5.) Rather than trying to measure streams
everywhere all the time, stream gages (measure
points) are used to estimate flow.
Fig 16.5
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16.1 Where does the water come from?

• Stream water is surface runoff from rainfall and
  snowmelt plus infiltrated ground water that
  reemerges at the surface where the water table
  intersects stream channels.
• The drainage basin is the area from which water
  flows to a stream. A divide separates adjacent
  drainage basins.
• Discharge is the volume of water that passes
  through a cross section of a stream channel
  during an interval of time.

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16.2 Where does the sediment come from?
One way streams procure sediment is by erosion
itself. Particles loosened by weathering are
picked up in surface runoff and transported to
the stream channel.
Fig 16.6a
Fig 11.12
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16.2 Where does the sediment come from?
Or, mass movement events may move loose material
downslope into the stream channel.
Fig 16.6b
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16.2 Where does the sediment come from?
The stream itself may also erode the banks of its
channel and that
sediment too becomes part of the streams load.
Fig 16.6c
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16.2 Where does the sediment come from?

• How much sediment does a stream carry?
• The particles a stream carries are termed its
  sediment load.
• Delivery of sediment to a stream is a function of
  drainage basin slope and climate
• Vegetation plays a role, as it is climate-varied
  and reduces erosion in general.
• Dissolved load the hidden load
• Ions in solution from chemical weathering are
  also carried by water in streams. Dissolved load
  is a smaller mass, but still significant.

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16.2 Where does the sediment come from?
Sediment load comparison of rivers of the world
Fig 16.7
Fig 16.7
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16.3 How do streams pick up sediment?

• Water in a stream must pick up particles to move
  them.
• Both picking up and moving particles requires
  work.
• As water moves downslope, potential energy is
  converted into motion.
• Motion is sufficient energy to move particles.
• Eroding the stream bed also requires energy does
  the stream have this much energy as well?

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16.3 How do streams pick up sediment?
An alluvial stream one where the channel is in
water-transported sediment as in (a) below has a
different energy structure than a stream that is
cut into solid rock as (b) below.
Fig 16.8
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16.3 How do streams pick up sediment?
Bedrock streams (right) erode their channels by
abrasion. Impact force of tumbling rock particles
serves to act as an erosive force by fracturing
small bits of rock, which in turn tumble and
break themselves and/or break off other small
bits.
Fig 16.11
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16.4 How do streams transport sediment?

• Sediment moves as bedload and suspended load
• Bedload large grains that cannot be picked up,
  but still are able to be moved. They roll,
  bounce, and slide along the bottom.
• Bedload may form structures on the stream bed
  such as dunes and cross-bedding.
• When a stream is at bankfull, all sizes of
  particles typically move due to increased stress.
• Bars are bedload particles not in motion under
  the present stress in a stream.

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16.4 How do streams transport sediment?
Here we can visualize the various motions of
bedload particles and see how small eddy currents
keep suspended load entrained.
Fig 16.13
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16.4 How do streams transport sediment?

• Suspended load smaller grains that can stay
  entrained in the water are the suspended load
  where turbulence keeps them until they reach a
  condition that deposits them.
• Stream power the ability of a stream to do
  work. Commonly measured by multiplying shear
  stress times the average velocity.
• Stream power is consumed moving the load. If
  there is excess power in a stream, then it may
  erode more materials from the stream bed or bank.
  If stream power is insufficient, then deposition
  occurs until load matches power.

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16.4 How do streams transport sediment?

• Sediment that rolls, slides, or bounces along the
  stream bed is bedload, whereas finer sediment
  intimately mixed with the flowing water is
  suspended load.
• Bars are mounds of sediment that are stationary
  and exposed at low discharge, but are submerged
  and transported at high discharge.
• Stream power, the product of shear stress and
  flow velocity, describes a streams ability to do
  work.

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16.5 Why do streams deposit sediment?

• A stream must lose power to enter a state of
  deposition. As power is the result of shear
  stress and velocity
• Loss of shear stress will cause deposition.
• Changes in slope and in water depth, so
  deposition occurs where slope or water depth
  decreases
• Loss of velocity will cause deposition
• Loss of discharge will cause deposition, because
  without water there is no shear stress, nor any
  velocity!

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16.5 Why do streams deposit sediment?
Deposition occurs where water depth decreases. In
the case of a stream in arid landscapes, depth
goes to zero very rapidly. The result is a fan of
alluvium that forms
where the stream goes from confined to
unconfined. Stream power drops off where the
channel stops and water depth (and thus stress)
go to zero.
Fig 16.14
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16.5 Why do streams deposit sediment?
Here we can see the channel ending and flow
broadening out. This means depth goes to zero and
power along with it. Sediment
falls out quickly, forming the fan-shaped deposit
shown here.
Fig 16.14
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16.5 Why do streams deposit sediment?

• Deposition occurs where velocity decreases, since
  power is a function of velocity.
• When a river enters a quieter body of water
  (ocean, sea, lake), the drop in velocity causes
  such a drop in power.

Fig 16.15
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16.5 Why do streams deposit sediment?
Fig 16.15 bottom
This deposition into a still body creates a
delta. Here we see the terminus of the
Mississippi delta and the distributary channels
taking sediment farther out.
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16.5 Why do streams deposit sediment?

• Streams deposit sediment where the stream power
  decreases because of decreases in one or more of
  water depth, slope, discharge, or velocity.
• Alluvial fans form where sediment deposition
  results from abrupt change from a narrow, deep,
  confined channel to a wide, shallow, unconfined
  sheet.
• Deltas result from deposition of sediment because
  of a sharp decrease in flow velocity where a
  stream enters still water.

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16.6 Why does a stream change along its course?
Slope decrease to base level stream power per
area of channel is fairly constant. As velocity
increases downstream, shear stress
must decrease. The stream adjusts to this
relation. Here we see two streams, both trying to
achieve their base levelthe state of power
equality along the course. In the lower example,
geology disturbs this adjustment.
Fig 16.17
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16.9 Why do streams flood?

• Discharge that exceeds a stream channels
  capacity is due to greater than normal
  precipitation.
• Floods occur when a drainage basin cannot absorb
  the water from precipitation or snowmelt and it
  must then run off onto the surface.
• High-precipitation events can lead either to
  brief flash floods, if of short duration, or to
  longer duration floods that may last days or
  weeks.

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16.9 Why do streams flood?

• Due to high intensity rainfall, usually seasonal.
• Intense rain cannot be absorbed quickly enough,
  so it runs off into streams.
• Flash floods associated with canyons are
  generally the deepest and often most damaging
  (below right).
• These events are rapid (flash) and difficult to
  predict.

Fig 16.24
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16.9 Why do streams flood?
Prolonged floods due to long and unusual
precipitation patterns The infamous 1993 flood of
the Mississippi River that submerged 26,000 km2
(roughly the size of MD). Floods of this
magnitude have very large economic impact due to
the sheer area of coverageinfrastructure,
commerce, and
other segments of society are affected.
Fig 16.25
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16.9 Why do streams flood?
Grand Forks, ND, 1997 The Red River submerged
nearly the entire city.
Fig 16.26
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16.9 Why do streams flood?

• Floods occur because of unusually heavy rainfall
  or snowmelt that generates water far in excess of
  the grounds capacity to absorb it. The excess
  runoff and ground water flow fills stream
  channels beyond their capacity to contain it.
• Flash floods start and end abruptly and usually
  result from heavy thunderstorm rainfall in steep,
  rocky drainage basins where infiltration is low
  and surface runoff is rapid.
• Prolonged floods with slowly rising and falling
  discharges persist for days or weeks and result
  from unusually rainy conditions over extended
  periods.

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16.11 How do human activities affect streams?
Dams increase deposition upstream due to
decreased velocity and also increase erosional
capacity downstream due to increased slope below
the dam,
and produce sediment-free water which will strive
to reach its load capacity.
Fig 16.29
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16.11 How do human activities affect streams?
Changes in discharge due to urbanization. Natural
landscapes and vegetation slow the delivery of
water to streams. However, urban settings divert
water from themselves by design (pavement, storm
sewers), which changes the delivery dynamic.
Fig 16.30
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16.12 How do stream-formed landscapes change
through geologic time?
Terraces evidence of downcutting and filling of
valleys. Bedload deposits well above a current
floodplain indicates the stream was at a higher
elevation in the geologic past. Climate change,
tectonic processes, and fluctuations of sea level
drive the imbalances that rivers respond to by
creating terraces.
Fig 16.33
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16.12 How do stream-formed landscapes change
through geologic time?
Stream Terraces
Fig 16.32
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16.12 How do stream-formed landscapes change
through geologic time?
Effects of climate on rivers
Fig 16.34
Precipitation and temperature vary over
time. This affects discharge and sediment load.
(Recall, climate affects vegetation, which alters
erodibility.) Pictured are two climate alteration
scenarios. As effective power changes, so does
base level over time.
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16.12 How do stream-formed landscapes change
through geologic time?
Alteration of stream dynamics by tectonic change.
Uplift alters power in an obvious manner. As the
headwater end of a stream is raised, it begins to
incise in an effort to achieve a base level
condition again.
Fig 16.35