River rejuvenation

• Rejuvenation-occurs when there is a negative change in a river’s base level (i.e. when the river’s base level falls) which increases its potential energy and thus enables it to revive its erosive energy in the processes.
• A river’s base level is the lowest point to which a river can erode.
• A negative change in a river’s base level may be brought about by vertical uplift (for example isostatic uplift) or by a fall in the level of the sea for example due to tides.
• This change renews a river’s ability to erode due to an increased gradient.
• A rejuvenated river erodes vertically into the floodplain to produce new features that are different from those typically found in the flood plain.

Before and after rejuvenation features. Image credit Alevelgeography.com

Landforms resulting from rejuvenation

• Rejuvenation produces several features including: incised meanders, terraces and waterfalls/knickpoints.

River terraces and incised meanders

• River terraces are remnants of former floodplains
• Which following vertical erosion brought about by rejuvenation have been left high and dry above the current and present-day flood plain.
• If a river quickly erodes and cuts quickly into the floodplain a pair of terraces of equal height may be seen flanking the flood plain creating a valley into a valley feature known as paired terraces.
• Sometimes the river does  quickly cut into the flood plain, allowing it to meander, resulting in one terrace being removed as the meander migrates downstream.
• This results in the formation of unpaired terraces.
• If uplift continues for sometime, incised meanders may form.
• These are meanders that have been cut deeply into the valley floor.
• Incised meanders are also known as entrenched meanders.

Knickpoint, rapids and waterfalls.

• Sometimes the point where the uplift occurs can be visible especially when the uplift or fall in base level is rapid.
• The point where the river crosses from the old plain into the new plain is known as knickpoint.
• A river may have to descent this knickpoint as either a rapid or waterfall.

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