Presentation about rejuvenation all related processes. Provides valuable information for A-level Geography Students.

2. Rejuvenation
Definition = Rejuvenation is the renewal of a
river’s energy in response to a relative fall in
base level.
Rivers are always attempting to reach a
state of dynamic equilibrium* where
erosion, deposition & transportation
are in perfect balance.

3. Base level
• Dynamic & eustatic change affect the base level
of a river.
• This is the lowest point to which a river can flow
& erode (usually sea level)*
• Local base level occurs when a river meets a
larger river, reservoir or lake

4. Causes…
1. dynamic change
2. eustatic change
3. static change

5. 1. Dynamic change
• Involves an upward movement of the land
raising the height of the river above sea
level (base level).
• This alters the long profile of the river,
increases the gravitational potential of the
river and therefore increases the energy
available (to erode and transport material)

6. Dynamic change
Why does the land rise?
• Orogenesis (Mountain building)
Compression from plate movement at convergent
boundaries builds mountains (eg. Alps, Andes,
Himalayas). Resultant uplift (usually along fault lines)
raises river (relative to base sea level), steepens river
gradient, and increases energy.*

7. Dynamic change
Why does the land rise?
• Isostatic rebound
18000 years BP (the last glacial maximum*) ice sheets covered
much of the northern & southern land masses.
The weight of this ice was enough to depress the crust beneath.**
The mantle aesthenosphere flowed away under this pressure.
The melting of the ice sheets (10000years BP) removed this
weight. The mantle flowed back & caused uplift. *** This
rejuvenated rivers as they rose up with the land. Its still happening!

8. 2. Eustatic change
Eustatic changes affect the volume of water in the oceans.
• This fits to rejuvenation as an actual fall or rise in global
sea level or base level, linked directly to the temperature
of the Earth.
• In warmer periods there is less ice and the water is
warmer, so expands and sea levels rise.
• In colder periods the ocean water is
colder so contracts and sea levels fall. *

9. How does rejuvenation affect
rivers?
The restart of erosion and reduction in
deposition, provoked by a fall in base
level, leads to the formation of landforms
that would not ‘normally’ be found in rivers.

10. Eustatic effects (landforms)
• Sea level rise (from warmer periods)
Fjords & Rias (drowned valleys)

11. Eustatic effects (landforms)
• Sea level fall = raised beaches, abandoned cliffs,
erosion surfaces created at the coast, and adjustments
made to river valleys:
• Knick points & waterfalls
• River terraces
• Incised & entrenched meanders
These occur as the river attempts to regrade its
long profile to a normal concave shape.

12. Knick Points
• A sudden break or irregularity in the long profile
of a river. Can be sharply defined, (such as
waterfalls) or barely noticeable.
• Erosion at the mouth of the river cuts down into
the land and slowly cuts the profile down to sea
level (base level) from the mouth and
subsequently upstream.
• Knick point is the point at which this process has
reached upstream.*

13. Horseshoe Falls, Niagara
The falls are a knick point formed by slower
erosion above the waterfall than below*

14. The waterfall at Glen Maye, Isle of Man
This
waterfall on
the
rejuvenated
Rushen river
has been
formed as a
result of the
uplift of the
west coast
of the island
post
glaciation.*

15. River terraces near Kasbeki,
(Caucasus) Georgia
• A river terrace is a remnant of
a former floodplain which has
been abandoned at a higher
level due to renewed
downcutting of a river.
• The terraces will be cut back
as the new valley is widened
by lateral erosion.
• This process may be repeated
a number of times leading to
terraces at a number of levels.
• Terraces provide useful shelter
from floods and are ideal as
natural route ways for road and
rail.

16. Incised Meanders
(NB: These may be entrenched or ingrown)
• If a rejuvenated river
occupies a valley with
well developed
meanders, renewed
downcutting results in
them becoming incised
(deepened).
• If incision is rapid, the
valley will be symmetrical
with steep sides and a
gorge like appearance –
this is known as an
entrenched meander.
The city of Durham and its ‘Bailey’
colleges, ‘trapped’ within this
entrenched meander on the River
Wear.

17. Entrenched Meanders
San Juan River, southeastern Utah
The Goosenecks of the
San Juan River in
southeastern Utah are
textbook examples of
entrenched river
meanders, where a lazy
meandering river was
"trapped" as the
surrounding plateau (the
Monument Upwarp) rose
up over millions of years
while the canyon kept
cutting down.

18. Ingrown Meanders
• When incision is slower and more lateral
erosion is occurring, an ingrown meander
may be produced.
• The valley becomes asymmetrical, with
steep cliffs on the outer bends and more
gentle slip-off slopes on the inner bends.

19. Ingrown Meander on the River
Wye, Tintern Abbey
(Note the asymmetrical valley profile)
River Cliff
Slip-off slope

20. 3. Static change
• Not linked to base level change.
• These are the result of the river’s ability to
erode more due to :
– Changes in load transported by the river
– Increase in discharge due to increased rainfall
– Increased discharge through river capture

21. Static change – changes in load
• In glacial times meltwater streams carried large
volumes of material and deposited them in wide
open valleys.
• Huge discharge and large volume of
unconsolidated debris meant deposits grew
high.
• Post glaciation, deposits fell & rivers’ energy
could be re-deployed for eroding.*
• The lack of load enabled valleys to be slowly
eroded – cutting through the deposits**

22. Static change – increase in discharge
• Climate change may bring increased discharge
• Increased precipitation or rivers merging could
link to a progressive lowering of the long profile
• Additional flow brings additional energy which
resulting in greater erosion and deposition

23. Ok, that’s it – now you really do
know all about rivers!