Transcript The Graded River and Base Level Changes in base level Base level

The Graded River and Base Level
Changes in base level
Base level: is the lowest point to which erosion by running
water can occur. In the case of rivers the theoretical limit
is the sea, although there are exceptions where a local
base level may exist, such as
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with the great Lakes of the USA. Or River Jordan flowing
into the dead sea. i.e. deposition will exceed therefore
erosion is reduced.
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where tributaries join a main river,
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or where there is a resistant band of rock crossing the
valley.
Task: Make a copy of figure 3.3a (Page 81 of Waugh)
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Grade: The concept is one of a river forming an
open system, in a state of dynamic equilibrium
where there is a balance between the rate of
erosion and the rate of deposition. A graded river
has a gently sloping profile, with the gradient
decreasing towards the mouth. This is transitory as
the slope profile has to adjust to changes in
discharge and sediment load. This can cause short
term increases in either the rate of erosion or the
rate of deposition, until the state of equilibrium has
again been reached. This is illustrated by:
PTO
•The long profile happens to contain a waterfall and
a lake. I.e. erosion will occur at the waterfall and
deposition at the lake but over a period of time both
will be eliminated.
•There is a lengthy period of heavy rain within the
river basin. As volume of water rises velocity and
load increase, so does the rate of erosion. Therefore
the extra load will lead to extra deposition further
down the valley or out to sea.
Therefore grade is a balance. Also width, depth and
gradient are adjusted to the discharge and load of
the river at a given point in time. If volume and load
change, morphology must adjust accordingly.
This may take lengthy periods of geological time.
Task: Make a copy of figure 3.43bPage 81 of Waugh
Changes in base level result from:
1. Eustatic or Sea Level change:
Glaciations and changes in rainfall.
2. Isostatic or Tectonic change
(crustal uplift): Where land is uplifted
after plate movement or volcanic
activity.
Changes can be positive changessea level rises in relation to the land, or
negative changes sea level fall in
relation to the land = rejuvination
Two types:
Positive changes: Occurs when sea level rises in relation to
the land(or the land sinks in relation to the sea. This results
in a decrease in the gradient of the river with an increase
in deposition and potential flooding of coastal areas.
Negative change: occurs when sea level falls in relation to
the land (or the land rises in relation to the sea) This
movement causes land to emerge from the sea, steepening
the gradient and therefore increasing the rate of fluvial
erosion. This process is called rejuvenation.
Two types of change
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POSITIVE CHANGE
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The sea level rises in relation to the land (or land sinks).
The result is a _______ in the gradient of rivers and an increase
in __________ and or flooding of coastal areas.
Landforms include……
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NEGATIVE CHANGE
The sea level falls in relation to the land (or land
rises).
As a result land emerges from the sea, steepens
the river gradient increasing fluvial _____. This
process is known as
_ _ _ _ _ _ _ _ _ _ _ _.
Rejuvenation
If land emerges from the sea following a negative
change in base level, the potential energy of a river for
erosion is revived and a re-grading of the river can
occur.
The impact of rejuvenation on
a river valley
River in grade
Flood plain
Waterfall
retreats
cutting
a lower valley
River bluffs
New flood plain forms
Fall in sea level
River bluffs
Waterfall
decreases in size
to form rapids
(nick point)
New flood plain forms
Terrace 1
This mini river has
terraces. It is on
Studland beach. The
river had cut down
into its floodplain as
the tide went out,
leaving old sections
of braided channel
‘high & dry’.
Terrace 2
And again, this time in Spain. Note the stranded river channel
on the right of the picture
As shown in the diagram above river terraces offer superb sites for
the location of settlement. They are the remains of former flood
plains that lie far above the extent of present day flooding as a
result of vertical erosion caused by rejuvenation.
River terraces are old landscapes of former floodplains
which following rejuvenation have been left high
above the water therefore have a slight chance of
flooding.
Incised meanders
If land is uplifted for a considerable period of time a
river may cut downwards and form incised meanders.
These are old landscapes of former meanders. There
are two types:
Entrenched meanders
Ingrown meanders
Symmetrical cross section,
and occur if valley sides
are resistant to erosion, or
rapid incision by the river
Less rapid uplift of land,
allowing river to shift
laterally across its
floodplain
Task: Make copies of diagrams 3.46 and the
valley cross sections in 3.48.