Transcript Deposition - Geography

Deposition
By Tom,
Joe and
Ryan
Deposition
• Deposition is the release of the particles that a
river is carry due to a decrease in the velocity of
the river.
• The Hjülstrom Curve shows the relationship
between the velocity of the river and the size of
particles that the river will deposit, transport or
erode. It shows that the river begins to deposit
boulders first, then gradually deposits smaller
particles as the rivers velocity decreases. Ending
with the deposition of particles any less than
0.01mm at 0.1cm/s.
Deltas
• A delta forms when the rivers velocity drops due to it
entering a slower moving body of water. As the river
begins to slow down it deposits the largest of the
transported particles first, then as the velocity decreases
ever more the size of the particles that the river deposits
slowly gets smaller. Therefore the large sediment is
found at the front of the delta with the sediment size
decreasing as the delta nears the sea. Deltas are
normally triangular in shape and there are two types;
Arcuate and Bird-foot.
• Distributaries are also form part of delta formation.
These occur when the river splits and becomes more
than one individual river. A good example of this is the
Mississippi delta, a picture of this is shown on the
following slide
Deltas; Flocculation
• Flocculation refers to a process by which a
solute comes out of a solution. As the
sediment is dropped the charged
particles, such as clays and polymers,
clumps together to form flocs.
An arcuate delta that has
formed on the south-west
coastline of Greenland, near
Narsarsuaq.
A close up image of the currently
active Mississippi delta, which is a
bird-foot delta.
Change in the Mississippi Delta
Braided river
•A braided river is a channel that consists of a network of
smaller channels separated by small and often temporary
islands called braid bars. However some braid bars become
that stable that they become almost permanent, with some
having settlements on them. Braided streams are common
wherever a drastic reduction in velocity causes the rapid
deposition of the stream's sediment load. Braided channels are
also common in river deltas.
•The channels and braid bars frequently move, with the river
layout often changing significantly during flood events.
Channels move sideways via differential velocity: On the
outside of a curve, deeper, swift water picks up sediment
(usually gravel or larger stones), which is re-deposited in
slow-moving water on the inside of a bend.
Levees
• The ability of a river to carry sediments is dependent on
it velocity. When a river floods over its banks, the water
spreads out, drops in velocity , and deposits its load of
sediment. Over time, the river's banks are built up above
the level of the rest of the floodplain. The resulting
ridges are called natural levees. The sediment that is left
on the floodplain is known as alluvium.
• When the river is not in flood it may deposit material
within its channel, raising its level. The combination can
raise not just the surface, but even the bottom of the river
above the surrounding country. Natural levees are
especially noted on the Yellow River in China near the
sea where ocean-going ships appear to sail high above
the plain on the elevated river. Natural levees are a
common feature of all meandering rivers in the world.
Levees
The largest sediment
will be deposited
closest to the bank
because the larger
sediment requires
the greater velocity.
As the velocity
decreases as the river
floods, the sediment
will decrease in size
as the distance from
the river increases.
Flood Plains
• A floodplain is flat or
nearly flat land
adjacent to a stream
or river that
experiences
occasional flooding.
Floodplains are
formed by the
deposition of
sediment carried by
river as it floods.
Flood Plains
• As the river continuously floods, the sediment
that is deposited accumulates in depth and
therefore as the depth of the sediment increases,
the age of the sediment at the base of sediment
will increase.
• Also the river will constantly change its course,
this will cause lateral erosion of the banks of the
valley. This means that the width of valley will
increase over geological time.
Bibliography
• http://www.web.umr.edu/~rogersda/levee
s/Should%20I%20Tru...
• http://www.wikipedia.org.
• http://www.google.ca
• http://www.internal.eawag.ch/~uehlinge/
Pictures.html