The Graded River and Base The Graded River and Base LevelLevel

Changes in base levelChanges in base levelBase levelBase 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 with the great Lakes of the USA. Or River Jordan flowing into the dead sea. i.e. deposition will exceed therefore erosion is reduced.where tributaries join a main river, or where there is a resistant band of rock crossing the valley.

Task: Make a copy of figure 3.3a (Page 81 of Waugh)

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 changes- sea 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 changeTwo types of change

POSITIVE CHANGE POSITIVE CHANGE The sea level rises in relation to the land (or land The sea level rises in relation to the land (or land

sinks).sinks). The result is a _______ in the gradient of rivers The result is a _______ in the gradient of rivers

and an increase in __________ and or flooding of and an increase in __________ and or flooding of coastal areas.coastal areas.

Landforms include……Landforms include……

NEGATIVE CHANGE NEGATIVE CHANGE The sea level falls in relation to the The sea level falls in relation to the

land (or land rises).land (or land rises). As a result land emerges from the As a result land emerges from the

sea, steepens the river gradient sea, steepens the river gradient increasing fluvial _____. This process increasing fluvial _____. This process is known asis 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 The impact of rejuvenation on a river rejuvenation on a river

valleyvalley

River in grade

Flood plain

Waterfall retreats cutting

a lower valley

Fall in sea level

River bluffs

New flood plain forms

New flood plain forms

River bluffs

Waterfall decreases in size to form rapids (nick point)

Terrace 1

Terrace 2

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’.

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 meandersIf 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 meander Entrenched meanderss

Symmetrical cross Symmetrical cross section, and occur if section, and occur if valley sides are valley sides are resistant to erosion, or resistant to erosion, or rapid incision by the rapid incision by the riverriver

Ingrown meandersIngrown meanders Less rapid uplift of Less rapid uplift of land, allowing river to land, allowing river to shift laterally across its shift laterally across its floodplainfloodplainTask: Make copies of diagrams 3.46 and the

valley cross sections in 3.48.