Stream Transport

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Stream Transport
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Table of Contents 1. Types of Load 2. Stream
Regime 3. Floods and Erosion 4. Stream
Equilibrium 5. Stages of a Stream 6. Old age or
Lower Stage Stream Features
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1. Types of load
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A rivers ability to do work (that is, erode, or
pick and move material) depends upon its
energy. Approximately 95 of a streams energy is
converted to heat by inner turbulence and by
friction on the bed Material carried by a river
is called its load. A rivers load can be
carried in three ways 1) suspension, 2)
bounced along the bed (bed load) 3) dissolved
into solution
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Physical Geography, Strahler and Strahler
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1) Dissolved load Not usually visible 3.9
million tonnes annually are removed from the land
in dissolved load Amount depends on rock type and
climate
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Solid-debris load Material that is not dissolved
and carried by the river includes bed load and
suspended load 2) Bed load Material that rolls,
bounces along the bottom and is not supported by
the water Moves slowly and is hard to measure The
largest grain a stream can move by bed load is
called its competence A stream can move larger
particles at times of flood Capacity is the
maximum amount of debris a given stream can carry
as bed load Depends on the stream gradient,
discharge, and size of the load
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Critical erosion velocity The lowest velocity at
which grains of a given size will move The
smaller the particles, the lower the velocity to
move them Sand is easily eroded and carried
silt and clay size particles less so because the
particles are bound or stuck together Gravel is
hard to move simply because of its size Settling
velocity The velocity at which grains are
deposited Varies according to grain size
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3) Suspended load Finer particles than bed load
supported by water Once these particles are
picked up, very little energy is needed to
transport them, less than the energy needed to
pick them up. Highest concentration is near the
bottom Larger particles are near the bottom
smaller ones evenly distributed Greater the
stream discharge the greater the suspended load
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2. Stream Regime
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Stream regime Knowing the load of a stream or
river is important for planning storage
dams. Sediment that flows into the reservoir
behind the dam settles to the bottom. It is
trapped behind the dam and will eventually fill
in and end the useful life of the reservoir. At
the same time it deprives the river of sediment
further down the river past the reservoir. This
can upset the rivers regime. For example, if the
sediment load is reduced, when the same amount of
water is put back into the river below the dam it
can erode a lot more material, since the river is
not carrying anything this can scour the river
bottom or sides, wiping out land and settlements.
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Physical Geography, Strahler and Strahler
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3. Floods and Erosion
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Floods and Erosion Floods increase the capacity
of a stream to carry load (both suspended and bed
load) because the volume of water flowing
downstream increases. Floods are usually also
associated with increased stream velocity, and
this also increases a rivers capacity to move
material. For example, the capacity to move bed
load goes up about 3rd to 4th power of the
velocity. If a streams velocity is doubled in a
flood, its capacity to carry bed load goes up 8
to 16 times, and the overall load will increase
even more due to increased volume. Rivers,
therefore, can change dramatically in floods.
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Physical Geography, Strahler and Strahler
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4. Stream Equilibrium
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Stream Equilibrium A stage of river maturity,
when downcutting is completed and there is a
smoothly graded river course In equilibrium the
supply of load to a stream (from tributaries and
from overland flow) equals the average rate of
stream transport In other words, capacity is
satisfied by the load supplied. The river cannot
cut downwards without upsetting its equilibrium
and therefore it tends to meander, cutting
sideways at the banks. As it erodes material from
one bank (on the outside the pool) it deposits
material on the other bank (the inside the
riffle) and the load remains constant.
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Physical Geography, Strahler and Strahler
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4. Rejunvenation
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Rejuvenation The re-establishment of a youthful
stream when it starts to downcut. Due to a change
in the base level or an increase in either the
amount of water a stream carries or its load. Can
occur three ways Eustatic rejuvenation caused
by the lowering of the sea level during an ice
age Dynamic rejuvenation caused by the uplift
of the land during tectonic activity Static
rejuvenation caused when either the load is
reduced therefore increasing the capacity or when
rainfall increases thereby increasing capacity
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Physical Geography, Strahler and Strahler
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5. Stages of a Stream
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The 3 stages of a stream or river
C old age or lower stage
A youthful or upper stage
B mature or middle stage
Physical Geography, Strahler and Strahler
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6. Old Age or Lower Stage Stream Features
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Old age or lower stage features
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• 1. Meander
• occurs in fairly flat areas.
• speed is greater on the outside of the bend.
• therefore erosion occurs on the outside of the
  bend, and deposition on the inside.

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2. Oxbow lake

• erosion on outside of a meander causes a neck
  to form.

• eventually erosion breaks through the neck, and
  deposition occurs where the meander used to go
• the cut off portion is an oxbow lake.

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2. Oxbow lake
If you thought those diagrams were good...

• erosion on outside of a meander causes a neck
  to form
• eventually erosion breaks through the neck, and
  deposition occurs where the meander used to go
• the cut off portion is an oxbow lake

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3. Floodplains Valley area through which a river
flows in the lower (old age) stage Built by river
depositing material on the insides of the bends
as it meanders and when finer materials are
deposited on the valley floor when the river
floods. Finer materials traditionally added
nutrient value to the soil and thus helped to
make floodplains very fertile areas, supporting
large populations Unfortunately, large floods
also usually meant a loss of life as well. A few
floodplains support a large portion of the
worlds population Hwang Ho, Yangtze, Indus,
Ganges and the Nile.
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A Landsat 5 image taken Sept. 1992 shows a
section of the Missouri River. The oblique
perspective of this image is looking upstream.
This image has been color enhanced and modified
to show an exaggerated topographic relief. Bare
soil and plowed land appears red, vegetation
appears green, and water is dark blue. A flat
river flood plain can be seen in the center of
the image. Because of the season, most of the
farmland located on the rich and fertile soils of
the floodplain is plowed and devoid of
vegetation.
http//www.geog.ouc.bc.ca/physgeog/contents/11j.ht
ml
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4. Levees Form on floodplains when rivers
flood. Water quickly loses its velocity as it
spreads out from the channel and deposition
occurs Heavier sediments tend to settle out
immediately close to the channel and an area of
slightly higher ground is built up on both sides
of the river Because of levees, the river channel
is often built up higher than the floodplains,
which is then very susceptible to flooding Yazoo
streams are tributary streams that cant reach
the main river because of levees, and therefore
flow parallel to the main channel in the river
valley
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Natural levees have been built up by man for
flood control.
http//www.spk.usace.army.mil/what/envir/fldpln/fl
dpln.html
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Meanders and levees, Red River flood, Manitoba,
1997
http//sts.gsc.nrcan.gc.ca/page1/landf/smanitoba/l
evees.htm
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5. Deltas Where a river empties into a large
body of water, such as a sea, ocean, or lake, it
loses speed. It loses energy and deposits
suspended material Since it is no longer
constrained by riverbanks, and since it gradually
loses speed as it reaches further into the sea,
the result is a triangular formation, called a
DELTA.

• Mississippi

• Mackenzie