Rejuvenation …

Ah … to be made young again …

True Definition:

• Rejuvenation occurs when there is either a fall in sea level relative to the level of the land or a rise of the land relative to the sea.

• This enables a river to renew its capacity to erode as its potential energy is increased.

• The river adjusts to its new base level – at first in its lower reaches and then progressively inland.

• This can create a number of landforms: knick points, waterfalls and rapids, river terraces and incised meanders.

River in grade

Flood plain

Waterfall retreats cutting

a lower valley Fall in sea level

River bluffs

New flood plain forms

Waterfall decreases in size to form rapids (knick point)

Waterfall retreats and the river uses it’s energy to try to reach a state of equilibrium again (graded profile). Eventually the river will erode the knickpoint point, so it no longer exists.

If is difficult to tell if a waterfall is due to geological changes in the rivers course or rejuvention…

Knick Points

• A knick point is a sudden break or irregularity in the long profile of a river. Some knick points are sharply defined, such as waterfalls – others are barely noticeable.

• Although a number of factors may cause such features to occur – many can be attributed to rejuvenation.

Features of Rejuvenation

Waterfalls (in the lower course)Incised MeandersRiver Terraces

Rapid vertical erosion after an increase in potential energy after uplift of land…

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.

Long profile with knick points

They are the remains of former flood plains that lie far above the extent of present day flooding as a result of vertical erosion caused by rejuvenation.

The terraces themselves are often made from fluvial material previously dumped by the river to create the earlier floodplain.

These terrace deposits are broadly horizontal layers of gravel, sand and finer sediments. As base level has changed, the river has gained more potential energy and its erosive power increased vertically

River Terraces

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 usefel shelter from floods and are ideal as natural routeways 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.

Or toenails

?

Ingrown Meander on the River Wye, Tintern Abbey

(Note the asymmetrical valley profile)

River Cliff

Slip-off slope

Tom Abbott, Biddulph High School and made available

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

schools

Tom Abbott, Biddulph High School and made available

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

schools

River terraces

Tom Abbott, Biddulph High School and made available

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

schools

Incised meander

Tom Abbott, Biddulph High School and made available

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

schools

Durham

Tom Abbott, Biddulph High School and made available

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

schools

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

Yippee – I’m rejuvenated!