Rejuvenation

Ah … to be made young again …

RejuvenationDefinition = 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.

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

Causes…

1. dynamic change

2. eustatic change

3. static change

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)

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.*

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!

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. *

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.

Eustatic effects (landforms)

• Sea level rise (from warmer periods)

Fjords & Rias (drowned valleys)

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.

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.*

Horseshoe Falls, Niagara

The falls are a knick point formed by slower erosion above the waterfall than below*

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.*

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.

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.

Entrenched Meanders

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.San Juan River, southeastern Utah

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.

Ingrown Meander on the River Wye, Tintern Abbey

(Note the asymmetrical valley profile)

River Cliff

Slip-off slope

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

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**

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

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

Yippee – I’m re

juvenated!