River Rejuvenation

What is a smooth long profile?

Rivers achieve a smooth, concave long profile over a period of time. This is astate of balance, or dynamic equilibrium , where slope, width and other channelcharacteristics have adjusted to the volume of water and load carried by a riverunder the prevailing conditions.

All factors are in balance, all kinetic energy is used to transport the water andsediment load, with no excess for erosion or deficit for deposition.

What is rejuvenation?

Over a long period a river assumes a generally smooth long profile. This is themost 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, ariver will always be trying to achieve this smooth concave profile.

Occasionally an event occurs that de-stabilisies the situation, and causes the riverto actively erode its channel in order to re-establish its smooth long profile. Thisrenewed period of erosion is called rejuvenation.

How does a river become rejuvenated?

River rejuvenation is associated with increased energy of a river which leads tothe amount of erosion increasing This is due to the sudden fall in the base levelof the river.

Base level is the height or altitude to which a river flows before it either joinsanother river or reaches the sea. If base level falls, a nick or step called a knickpoint is formed in the river’s long profile. Therefore, the down stream gradientof the river increases.

The increase in the gradient caused by the fall in base level means that the rivernow has more energy available. This is used to actively erode the irregularity, sothat the river once again achieves a state of equilibrium by creating a smoothlong profile. Like a normal waterfall, erosion will cause a knick point to retreatupstream.

Causes Explanation

Dynamic changeThe upwards movement of the land which raises theheight 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. Thiscauses an actual fall in sea level. The growth of the icesheets at the beginning of the last glaciation about25,000 years BP removed large amounts of water fromthe oceans and sea levels where 125m lower thantoday’s.

More about sea level change

Since the end of the last ice age, about 8000 years, ago, sea levels havefluctuated considerably. Melting ice initially caused sea levels to rise (eustaticchange).

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 relativesea level.

Whereas, in the north and west of the UK there has been a slight drop in relativesea level, creating conditions that have led to the process of rejuvenation andthe development of specific landforms.

What landforms are created?

Causes Explanation

TerracesThe following type of terrace is a consequence ofrejuvenation – paired terraces.

Knick pointsThe following types of landforms are a consequence ofrejuvenation – waterfalls and rapids

Incised meandersThe following types of meanders are a consequence ofrejuvenation – entrenched and ingrown.

Tom Abbott, Biddulph High School and made available through

www.sln.org.uk/geography and only for non commercial use in schools

River in grade

Flood plain

Tom Abbott, Biddulph High School and made available through

www.sln.org.uk/geography and only for non commercial use in schools

Waterfall

retreats

cutting

a lower valley

Fall in sea level

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

New flood plain forms

River bluffs

Waterfall

decreases in size

to form rapids

(knick point)

Terraces

A river terrace is a remnant of a former floodplain, which has been left at ahigher 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 theold floodplain above the level of the present river.

Lateral erosion caused by erosion on the outside of river meanders cuts backinto the terrace widening the new floodplain.

The river Thames has created terraces in its lower course by several stages ofrejuvenation. The build up areas of London and oxford are mainly located onthese terraces.

Your report should include a located example and why explain the terrace wasformed.

Knick points

A knick point is a sudden break or irregularity in the gradient along the longprofile of the river. Some knick points are sharply defined, for example waterfallsand rapids.

When a river is rejuvenated, (a new base level is created) adjustments to thenew base level usually start at the sea and gradually work there way up therivers course. The river starts to gradually erode vertically back up the riverscourse. Where the new river long profile meets the old long profile of the riverthere is sudden change in gradient. This where waterfalls or rapids caused byrejuvination 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.

Incised meander

There are two types of incised meanders formed by rejuvenation. They areentrenched meanders and ingrown meanders.

Entrenched meanders have a symmetrical valley cross profile.

Ingrown meanders have a asymmetric valley cross profile.

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 themiddle of a winding gorge or valley.

Uplift of the land is more rapid than the vertical erosion of the river. As a resultthe 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 hardrock 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 isentrenched.

Incised meander (entrenched)

Ingrown meander

Ingrown meanders have a asymmetrical valley cross profile. On one side of thevalley has a gentle slope with an expanse of flat land on the valley floor. Theopposite side of the valley has steep sloping sides.

This is caused as an ingrown meander is formed by a combination of vertical andlateral erosion river erosion.

They form when uplift or incision is gradual and this gives the meander time tomigrate across of the valley. The inside of the meander deposits its load to formthe expanse of flat valley floor. The outside of the meander erodes the side ofthe 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 isingrown.

Ingrown meander