Rivers over long periods of time develop a smooth, concave long profile as they adjust their characteristics to transport water and sediment in a state of dynamic equilibrium with their environment. However, occasional events can destabilize this equilibrium, increasing the river's energy and causing it to actively erode its channel to reestablish its optimal long profile through a process called rejuvenation. Common causes of rejuvenation include changes in base level from tectonic uplift or changes in sea level. During rejuvenation, landforms such as waterfalls from knickpoints, incised meanders, and river terraces may form as the river adjusts to its new gradient.

1. River Rejuvenation

2. What is a smooth long profile?
Rivers achieve a smooth, concave long profile over a period of time. This is a
state of balance, or dynamic equilibrium , where slope, width and other channel
characteristics have adjusted to the volume of water and load carried by a river
under the prevailing conditions.
All factors are in balance, all kinetic energy is used to transport the water and
sediment load, with no excess for erosion or deficit for deposition.

3. What is rejuvenation?
Over a long period a river assumes a generally smooth long profile. This is the
most efficient profile for a river to have in order to transport water and sediment.
It represents a state of equilibrium (balance) with the environment. In theory, a
river will always be trying to achieve this smooth concave profile.
Occasionally an event occurs that de-stabilisies the situation, and causes the river
to actively erode its channel in order to re-establish its smooth long profile. This
renewed period of erosion is called rejuvenation.

4. How does a river become rejuvenated?
River rejuvenation is associated with increased energy of a river which leads to
the amount of erosion increasing This is due to the sudden fall in the base level
of the river.
Base level is the height or altitude to which a river flows before it either joins
another river or reaches the sea. If base level falls, a nick or step called a knick
point is formed in the river’s long profile. Therefore, the down stream gradient
of the river increases.
The increase in the gradient caused by the fall in base level means that the river
now has more energy available. This is used to actively erode the irregularity, so
that the river once again achieves a state of equilibrium by creating a smooth
long profile. Like a normal waterfall, erosion will cause a knick point to retreat
upstream.

5. Causes Explanation
Dynamic change
The upwards movement of the land which raises the
height of the river above sea level which is its base level.
This is caused by either orogenesis or isostatic rebound.
Eustatic change
This involves the amount of water in the oceans. This
causes an actual fall in sea level. The growth of the ice
sheets at the beginning of the last glaciation about
25,000 years BP removed large amounts of water from
the oceans and sea levels where 125m lower than
today’s.

9. More about sea level change
Since the end of the last ice age, about 8000 years, ago, sea levels have
fluctuated considerably. Melting ice initially caused sea levels to rise (eustatic
change).
However, this was followed by a much slower rise in the land (isostatic recovery)
as the weight of the ice cover was removed.
In the south and east of the UK, the trend has been for a slight rise in relative
sea level.
Whereas, in the north and west of the UK there has been a slight drop in relative
sea level, creating conditions that have led to the process of rejuvenation and
the development of specific landforms.

10. What landforms are created?
Causes Explanation
Terraces
The following type of terrace is a consequence of
rejuvenation – paired terraces.
Knick points
The following types of landforms are a consequence of
rejuvenation – waterfalls and rapids
Incised meanders
The following types of meanders are a consequence of
rejuvenation – entrenched and ingrown.

11. River in grade
Tom Abbott, Biddulph High School and
made available through
www.sln.org.uk/geography and only for
non commercial use in schools
Flood plain

12. Waterfall
retreats
cutting
a lower valley
Tom Abbott, Biddulph High School and
made available through
www.sln.org.uk/geography and only for
non commercial use in schools
Fall in sea level
River bluffs
New flood plain forms

13. River bluffs
New flood plain forms
Tom Abbott, Biddulph High School and
made available through
www.sln.org.uk/geography and only for
non commercial use in schools
Waterfall
decreases in size
to form rapids
(knick point)

14. Terraces
A river terrace is a remnant of a former floodplain, which has been left at a
higher level than the new floodplain after rejuvenation of the river.
The river channel starts to vertically erode the floodplain it is running over.
Therefore, the channel starts to cut down into the floodplain, leaving behind the
old floodplain above the level of the present river.
Lateral erosion caused by erosion on the outside of river meanders cuts back
into the terrace widening the new floodplain.
The river Thames has created terraces in its lower course by several stages of
rejuvenation. The build up areas of London and oxford are mainly located on
these terraces.
Your report should include a located example and why explain the terrace was
formed.

21. Knick points
A knick point is a sudden break or irregularity in the gradient along the long
profile of the river. Some knick points are sharply defined, for example waterfalls
and rapids.
When a river is rejuvenated, (a new base level is created) adjustments to the
new base level usually start at the sea and gradually work there way up the
rivers course. The river starts to gradually erode vertically back up the rivers
course. Where the new river long profile meets the old long profile of the river
there is sudden change in gradient. This where waterfalls or rapids caused by
rejuvination may be located.
An example of a waterfall formed due to a knick point is the Beezley Falls on the
River Greata in the Yorkshire.
Your report should include a located example and explain why a waterfall is
formed due to rejuvination.

24. Incised meander
There are two types of incised meanders formed by rejuvenation. They are
entrenched meanders and ingrown meanders.
Entrenched meanders have a symmetrical valley cross profile.
Ingrown meanders have a asymmetric valley cross profile.

25. Entrenched meander
Entrenched meanders have a symmetrical valley cross profile.
The both sides of the valley have steep slopes and the river is found in the
middle of a winding gorge or valley.
Uplift of the land is more rapid than the vertical erosion of the river. As a result
the cross section is symmetrical because there has been no meander migration.
The entrenched meanders of the San Juan River, a tributary of the Colorado ,
show this well.
The valley sides are kept steep by a combination of vertical erosion, the hard
rock and lack of weathering and mass movement in this dry region.
Another example is the River Wear at Durham, (sandstone rock).
Your report should include a located example and explain why the meander is
entrenched.

26. Incised meander (entrenched)

28. Ingrown meander
Ingrown meanders have a asymmetrical valley cross profile. On one side of the
valley has a gentle slope with an expanse of flat land on the valley floor. The
opposite side of the valley has steep sloping sides.
This is caused as an ingrown meander is formed by a combination of vertical and
lateral erosion river erosion.
They form when uplift or incision is gradual and this gives the meander time to
migrate across of the valley. The inside of the meander deposits its load to form
the expanse of flat valley floor. The outside of the meander erodes the side of
the valley to form the steep sloped side.
An example of an ingrown meander is at Tintern Abbey on th eRiver Wye
Your report should include a located example and explain why the meander is
ingrown.

29. Ingrown meander