Transcript Unit 4: Rivers/Watersheds Review

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