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River rejuvenation

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Title: River rejuvenation  
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Subject: River morphology, Fluvial geomorphology, Geological processes
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River rejuvenation

In geomorphology a river is said to be rejuvenated when the base level that it is flowing down to is lowered. This can happen for various reasons.

Contents

  • Signs 1
  • Example 2
  • Causes 3
    • Dynamic rejuvenation 3.1
    • Eustatic rejuvenation 3.2
    • Static rejuvenation 3.3

Signs

Rejuvenated terrains usually have complex landscapes because remnants of older landforms are locally preserved. Parts of floodplains may be preserved as terraces along the downcutting stream channels. Meandering streams often become entrenched, so a product of older river systems is found with steep, very pronounced "V" shaped valleys - often seen with younger systems.

Example

One example of rejuvenation is the silt.

Causes

Rejuvenation may result from causes which are dynamic, fluvial terraces derived from old floodplains.

Dynamic rejuvenation

A region may be uplifted at any stage. This lowers the base level and streams begin active downward erosion again.

Dynamic rejuvenation may be caused by the epeirogenic uplift of a land mass. These movements are either associated with neighboring orogenic movements or may be worldwide in nature. Warping or faulting of a drainage basin will steepen the stream gradient followed by the downcutting. The effect of seaward tilting can be felt immediately only when the direction of that stream is parallel to the direction of tilting.

Eustatic rejuvenation

Eustatic rejuvenation results from the causes which bring worldwide decrease in sea level, and two types of such rejuvenation are recognized. Diastrophic eustatism is the change in sea level due to variation in capacity of ocean basins, whereas glacio-eustatism is the change in sea level due to withdrawal or return of water into the oceans, occupying the accumulation or melting of successive ice sheets.rejuvenation can also be caused by tectonic uplift and river capture(letsapa kgosietsile)

Eustatic rejuvenation rejuvenates the mouth of the stream. Regrading of a stream toward a new base level will proceed upvalley. The result may be an interrupted profile with the point of intersection of the old and new base levels.

Static rejuvenation

Three changes may bring static rejuvenation, to the stream.

  1. decrease in load
  2. increase in runoff because of increased rainfall
  3. increase in stream volume through acquisition of new drainage by stream capture

Rejuvenation due to decrease in load took place during post-glacial times along many valleys that formerly received large quantities of glacial outwash. With change to no glacier conditions stream load decreased and valley deepening ensued.

Either way, rejuvenation results in a "knickpoint", as it appears on a river's long profile, which often turns out to be rapids or a waterfall, such as Seljalandsfoss in

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