Naturalists at Large: Rivers watersheds
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Transcript of Naturalists at Large: Rivers watersheds
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The streams found in
mountains are likely to have
formed relatively recently.
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These young streams
typically have V-shaped valleys.
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The rivers tend to move fast in their
upstream portions,
although, some are slowed by the large rocks and boulders in the river channel.
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The young mountain
streams tend to have a
steep slope or gradient. Slope or
Gradient
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Rivers move a great deal of material. The faster the water
moves, the larger material that can be carried down stream.
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Fast moving upstream portions can often move rocks and boulders.
Downstream portions usually only transport small silts and clays.
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This process tends to sort materials by size.
Have you ever wondered why we have so much silt and clay in our
soil?
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Valleys with very steep,
almost vertical sides are
called canyons or gorges.
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How long do you think it took to form the Grand Canyon?
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Most scientists believe it took 1 to 3 million
years!
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A stream or river can not cut its bed any
lower than the body of water into which it
flows.
As the stream approaches this
base level, the slope and speed of the stream decrease.
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The river channel becomes wider, deeper, and the volume is bigger
as there are more tributaries
contributing to the volume.
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The river valley also changes at this
point.
It becomes more open, more of a U shape with a wider
river floor.
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The lower or downstream portion of the river is at its
deepest, widest and slowest speed.
It also has the largest volume and a very
gentle gradient.
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The river valley tends to be broad, flat and bounded by
bluffs.
These bottom lands are often areas that flood.
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Lower portions of rivers often start to meander or form a s-shape river
pattern.
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Erosion is greater on the outside of the bend, deposition more on the
inside.
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The river can eventually cut through the meander, leaving a straighter
section and an ox-bow lake.
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To view an animation of this process click on this web site.
http://www.school-portal.co.uk/GroupDownloadFile.asp?file=21606
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Rivers deposit the sediment they
have carried as they slow down. This sediment load will often
form deltas as the river flows into
the quiet waters of a bay or gulf.
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The river tends to be split into channels by its own deposits
as it drops more sediment.
As the deposits grow, they
resemble the Greek letter ▲(delta).
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Alluvial Fans
An alluvial fan differs from a
delta in several ways.
The deposit is formed on land,
not in water.
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Alluvial Fans
Also, the sediments of
these deposits are coarse sands
and gravels, rather than fine
silts and clays of the deltas.
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Watershed A watershed, or drainage basin
includes all of the land that drains
into a river or bay either directly or
through its tributaries.
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Watershed The high land that
separates one river valley or
watershed from the next, is called
the divide.
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