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What does the word rejuvenation mean?

Rejuvenation means to renew or increase the energy levels.

For RIVERS, the specific meaning is . . .

An increase in the energy of a river caused by either a fall in its base level or an uplift of the land.

What are the processes and impacts of rejuvenation?

Learning Objectives:

To be able to explain the changes made to fluvial (river) landforms due to rejuvenation.

7th November 2013

Learning Outcomes:

Essential – you will know the 2 reasons for rejuvenation.

Stretch – you will explain the new landforms that rejuvenation makes to a rivers course.

Challenge – you will explain in detail the new landforms and contrast this to the original landforms.

Geography

Rivers adjust to create a balance between the energy available (erosion) and the energy used (deposition).

This adjustment creates the downstream gradient that we call the Long Profile of a river

Dynamic Equilibrium = balance between erosion and deposition

Geography

The long profile of the river is the study of how a river’s height changes from it’s source to mouth. This has an

impact upon the landscapes and processes that occur at each point along a river’s course

This is the long profile of a typical

river. It is concave in shape because

irregularities have been removed by

erosion and deposition

Geography

e.g. if there is a period of time with heavy rainfall the discharge of the river will rise along with the velocity and rate of erosion, also increasing the load. This extra load leads to

extra deposition further down the valley or at the delta.

We call this the graded profile –

where the rates of erosion and

deposition are balanced.

The work rivers do is governed by the energy they possess. In terms of the erosive power, this is related to 2 types of energy.

What are the 2 types of energy that a river possess?

1. The potential energy – the height that the water has to fall downhill to reach the sea.

2. The kinetic energy – the amount of water in motion.

The long profile of a river evolves over 1000’s of years and graded appearance, from steep in the uplands to relatively gentle near the mouth of the river, reflects the fact that water does not have so far to fall as it nears the sea so has less erosive power.However, over time the situation can change if the relative heights of the land and the sea alter.

Why might the height of the land or sea change over time?

Isostatic changes = the land rising relative to the sea, as a result of crustal movements. E.g. in Scotland, the land has “bounced back” after the weight of the ice cap was removed at the end of the last ice age, around 10,000 years ago.

Eustatic change = change in the relative height of the land and the sea resulting from a rise or fall in sea level.

Sea level fall may result from many causes, one of the most common being the growth of ice caps during a glaciation. As water is locked up in the ice, less is available to be released into the oceans.

The effect of a relative rise of the land means that water has further to fall and thus has more erosive power.

Geography

River in grade

Flood plain

Rejuvenation

If a river landscape is uplifted this allows the river flowing to start eroding vertically instead of laterally. Therefore it starts to

act more like it did in it’s “youthful” stage and we can say it is rejuvenated

What landforms can result from river rejuvenation?

Since the river is returning to it’s youthful stage with lots of vertical

erosion we find landforms associated with this… eg waterfalls

Geography

NICK POINTS

A knick point is a sudden change in gradient along the long profile.

When a river is rejuvenated adjustment (vertical erosion) to the new base level starts at the sea or lake and works its way up the river’s course.

INCISED MEANDERS

If the river has well developed meanders before rejuvenation occurs we get incised meanders. This is when they become deeper (vertical erosion)

RIVER TERRACES

This is just the remains of a former floodplain. When vertical erosion takes over the river cuts down leaving the old floodplain above the new level of the river. The terraces are cut back as the new floodplain forms when lateral erosion starts to occur again.

Geography

Waterfall retreats cutting

a lower valley

Fall in sea level

River bluffs

New flood plain forms

Geography

New flood plain forms

River bluffs

Waterfall decreases in size to form rapids (nick point)

Nick point = marked changes in in slops caused by rejuvenation

Geography

Geography

May 2012

Rejuvenation occurs when the river has renewed/increased energy, as a result of a change in the river height relative to base level, caused by fall in sea level or land uplift.

2 x 1 (2 marks)

(2 marks)