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Process and impact of rejuvenation – knick points, waterfalls, river terraces and incised meanders.

Sea level change
Landforms of REJUVENATION

Sea level change

The long profile of a river is not  static entity and is prone to change due to changes in sea level.  Indeed, a river is already in DYNAMIC equilibrium because things may be balanced over long periods of time but over the short term more or less rainfall can increase or decrease the amount of energy available to the river to move material.  Over very long periods of time large changes in sea level can drastically alter the rivers potential energy and can increase the chances of vertical erosion.  Sea levels change because of 2 principal causes - ISOSTATIC changes and EUSTATIC changes, both of these changes can lead to rises or falls in sea level.  Isostatic changes are local changes where the height of the land changes relative to the global sea level.  An example of this can be found in glaciation.  During glaciations the weight of ice pushing on the Earth can depress the crust, causing the land to fall relative to the level of the sea.  When the ice goes in warmer periods the land rebounds upwards as Scotland and Northern Britain are today - rising relative to the level of the sea.  This process is also seen in deltas where sediment loading depresses the Earth's surface.  Eustatic sea level change is a GLOBAL change in sea 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.

Eustatic changes in sea level 

When sea levels rise Fjords and Rias (drowned valleys) can form.

When sea levels fall we get raised beaches, abandoned cliffs, erosion surfaces  created at the coast, and several adjustments made to river valleys.  These changes to river valleys are attributable to the fact that the rivers graded profile is no longer in balance, and the gradient of the river AT ITS MOUTH is now steeper because of the fall in sea level.  In effect the water has further to fall on its journey from source to mouth.  This means that there is more energy at the mouth and vertical erosion (which under normal conditions would occur at the river's source) will occur.  This leads to the development of knick points, waterfalls, river terraces and incised meanders.

Knick points, waterfalls, river terraces and incised meanders.

These occur as the river attempts to regrade its long profile to a normal concave shape.  There is headword erosion as the river cuts down into the land at the mouth.  This process slowly cuts the profile down to sea and base level from the mouth and then consequently upstream. The point at which this process has reached upstream is known as a knick point.  If sea level falls several times over many thousands of years there can be several knick points over time.  In most extreme cases, this knick point can be nearly vertical and a waterfall can be created at or close to the rivers mouth. 

 

The Benbrytarforsen (bone-breaker) knickpoint on Piteälven (N-Sweden) was triggered at sea level during deglaciation. Glacio-isostatic rebound has since uplifted this landscape more than 200 m above sea level. This knickpoint on the River Coe has retreated headward at an average rate of ~25 m/ka over the past 3000 years.
River terraces are simply where vertical erosion occurs in a floodplain that was previously being formed by the normal conditions of deposition and lateral erosion.  The river cuts downwards and abandons the old flood plain as a river terrace - ideal for building settlements upon!  Over time once the profile is regraded, the river may recommence eroding laterally and depositing sediment, resulting a secondary flood plain formation.  Where this occurs and the river is meandering, the river can maintain its meandering form but its energy is focussed upon vertical erosion.  This can result in spectacular incised meanders, as found in the Grand Canyon. Incised meanders of the San Juan River are spectacular at Goosenecks State Reserve in SE Utah

Incised meanders in the Grand Canyon
River terraces
River Terraces
Kilt rock waterfall
Kilt Rock
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Fully explain the formation of Kilt rock waterfall shown Here.