Interruptions to Normal Cycle of Erosion

2 years ago

Atiqur Rahman

17 minutes

Fluvial System

The fluvial system may be divided into the following three zones:

Zone 1 is primarily a watershed basin and from here the sediments are collected (primarily the zone of sediment collection).

Zone 2 is primarily involved in the transformation of sediments from zone 1 to zone 2. So this is a transportation zone.

Zone 3 is where these sediments finally sink and gets deposited so this is primarily a zone of deposition.

In all the zones the interacting processes and the landforms found in different zones creates complex landscape structures. The fluvial system is influenced by various variables like time, initial relief, geological structures, climate, type and density of vegetation, relief or volume of the system above base level, Hydrology within zone 1 and 3, drainage network, hillslope morphology, sediment characteristics that is channel and valley morphology and channel characteristics of depositional system morphology.

Cycle of Erosion

American Geographer William Morris Davis (1850-1934) was the first geomorphologist who proposed the model of the cycle of erosion. He develops a model showing sequential changes in landforms through time. Along the lines of Charles Darwin, Davis tried to study the evolution of landforms as an organic form passing through the stages of Youth, Maturity and Old.

Davisian Cycle of Erosion

Davis gave the most complete and ideal cycle erosion most simply and persuasively, naming each stage after the stages of human life- Youth, Maturity and Old.

For simplicity, he made the following assumptions:

uniform lithology or mass of land
a considerable mass of land available which was, in effect, a stable mass, which has rapidly uplifted from beneath the earth’s surface due to the earth’s movements.

Stages in the Ideal Cycle of Erosion:

Youth Stage

It comprises the region of broad and poorly defined water divides and consequent streams. The region has numerous trunk rivers and few large tributaries, which have a direction of flow, velocity and erosional capabilities.

These streams from infancy are aggressive and capable of headward cutting. They would rapidly cut downwards together with the vertical incision of the whole drainage network in due course of time to form deep “V” shaped valleys of 30°.

On these slopes slumping would be slow compared to the speed of the incision. The stream course is mostly irregularly marked by falls and rapids.

Maturity Stage

At this stage, the region achieves maximum relief. The area spreads over a well- an integrated drainage network with maximum association with its bedrock and geological structure. There is a deepening of the “V” shaped valley.

The intricate, complex network of tributaries and streams gives way to a more organized well-integrated network of drainage, and the streams are gentler and their velocity reduced. The base level will have reduced importance, and the drainage network will have a maximum relationship with the geological structure.

This primarily involves the removal of previously accumulated sediments from the bedrock and the rejuvenation of the existing rock pediment.

Lateral cutting predominantly leads to extensive floodplains, which do not generally exceed the width of the meander belts. The valley floor is generally graded, and the relief in general shows a progressive decrease.

Old Stage

By this stage, there is a gradual reduction of the river gradient and velocity. The valley slopes ad the sides are massively degraded and are mostly broad gently sloping broad floodplains. Sometimes the widths of the associated meander belt are surmounted by slowly lowering the rounded divide. The base level will have a thick soil cover, and the whole surface will be closer to the base level.

The integration of the drainage, which progressed through maturity, is complete. There are visible topographical residuals like monadnocks with general outcrops over the peneplain in later old age.

Peneplain can be renewed through various upliftment processes, which causes it to redissect. A new set of landforms is formed as a result. In this process, different stages of the cycle are superimposed on one another.

Rejuvenation

There are numerous incidences where the normal cycle of erosion has been interrupted several times due to various reasons. Multicyclic evolution of the landform is more common as compared to the monocyclic. In such cases where the youthful stage of the cycle is superimposed on the old stage is termed rejuvenation.

It is, therefore, very rare that the landform development in a cycle is ever complete. The ultimate level to which, but not down to which, the river can lower its valley due to fluvial erosion is called the base level.

The ultimate base level is sea level. The base level can vary according to the surface it is flowing on. Any change in the base level can change the local character of the landform. It is very unlikely that the base level will not be interrupted during the course of an erosional cycle.

There are various reasons due to why rejuvenation takes place or there is an interruption in the cycle of erosion.

Base Level Change

The change in the base level can be termed as positive if there is a rise and negative if there is a fall, which may lead to accelerating alluviation and renewed erosion. A complete peneplanation may take around 10 to 50 million years.

How early or late it happens depends on the importance of the local isostatic compensatory uplift. It is unlikely that anyone erosional cycle will run its course uninterrupted by base-level changes induced by either tectonic or eustatic causes.

The positive movement of the base level is associated with accelerated alluviation or aggradation. It involves the submergence of the lower or the old stage of the river cycle. The result is the aggradation work done by the stream, which results in the formation of the buildup of the floodplains, which is the result of the backfilling of the upper valley and has effects on the stream gradient. It also forms deltaic outbuildings. All these features give it an appearance of an old stage of the valley. Mississippi River is an example of such a river system.

There may be other causes of aggradation which included uplift of the source area, increase in the debris supply when a certain tributary flush its water in the main stream due to change in the climatic conditions and due to various response of the basin due to a wide range of complex changes superimposed on the drainage system.

The negative movement is associated with renewed erosion. It involves the rejuvenation of the landforms bringing youthful characteristics in the older stage. It may have steeper slopes and headward cutting with waves of incision and plantation.

It first affects the outcrops of the softer rocks and then works on the harder outcrops. It is marked by steeper slopes, terrace edges, and nick points on the river courses. The river course produces a kind of valley-in-valley feature, also called multicyclic or polycyclic features.

Because of the negative movement rise in base level, the peneplain gets dissected. Sometimes the resistant outcropped rocks get preserved and act like summit peneplain remnants.

In the lower terrains, too, we can observe less resistant outcrops on partial peneplains. Sometimes the marine erosional surfaces are confused with the stripped plains, pediplains or exhumed peneplains.

Dynamic Rejuvenation

These are the changes brought in the cycle of erosion due to eperiogenic movements (tectonic movements). It may include upliftment of the land mass accompanied by tilting and warping, lowering the outlets and volcanic activity.

i) Upliftment of Landmass

Such movement can be caused by local orogenic movements. When the river is in its old stage, nearing its old stage and the landmass on which it flows uplifts, the cycle gets interrupted and rejuvenates.

The process of rejuvenation can change a peneplain landscape, which has attained the profile of equilibrium and were aggrading to revive its erosional power and engage in the process of valley deepening.

Sometimes on the pre-existing peneplain “V” shaped valley, which is typical of a youthful stage, may appear, and the existing cycle would not only retard, but a new cycle of erosion will begin.

We may see remnants of the old cycle in the early stages of the new cycle of erosion, especially near hills and plateaus- tops and on the broader interfluves. By the stage of late youth, the old peneplain will be visible as summit heights, but by early maturity, the renewed onset of effective divide wasting will cause the peneplain to disappear altogether on the landscape.

Most peneplains in the British Isles take the form of ‘hill-top surfaces’ and are so fragmented that they are by no means easy to identify, let alone interpret accurately. Their existence, however, is more readily inferred from an analysis of detailed topographical maps.

ii) Tilting of Land and Warping

Tilting of the land, warping, or faulting of the river basin will steepen the gradient of the stream, which leads to increased downcutting with more transporting power than required. When there is a seaward tilt, its effects are noticed along the entire course of the stream as it is rejuvenated and is reflected in the deepening of the valley, especially in the stream where the direction of the course is parallel to the direction of the tilting.

When the tilt is at the right angle to the direction of the river course will respond to the rejuvenation only after the joining stream deepens its valley to leave the tributary out of adjustment, even if the effect is only felt at the mouth of the course.

iii) Lowering of the Outlet

The mouth of the river or where the river drains is the outlet. As the outlet is lowered, the river rejuvenates, and its velocity increases. Higher velocity leads to a higher degree of erosion and downcutting.

If the outlet of the river is a lake, then the level of the lake is its local base level. If this base level suddenly gets lowered then the outlet gets rejuvenated (at the mouth). The river suddenly joins the lake descending abruptly, leading to downcutting, which is typical of the youthful stage.

Niagara Falls is an example of the lowering of the outlet, where the waterfalls from Niagara and is drained into Lake Erie. It is the condition with the river course having a youthful stage at its mouth near the lake and an old stage and topography at the higher side of the valley.

iv) Volcanic Activity

Volcanic activity can cause an accidental flow of lava in the valley which may interrupt the normal cycle of erosion. Blocked by lava, the river will try to erode the obstacle by intensifying the erosive activity. If the lava flow engulfs the entire valley then an all-new cycle begins on the volcanic surface.

Eustatic Rejuvenation

Processes which result in the worldwide lowering of the sea levels and are not related to the local changes happening to the base level. The eustatic changes can be produced by a decrease in ocean basin capacity as a result of the formation of the mid-oceanic ridges. There can be two kinds of eustatic rejuvenation.

i) Diastrophic Eustatism

When the change in the sea level is due to the change in the capacity of the ocean basin. According to Baulig (1935), who is a modern proponent of diastrophic eustatism. He recognized glacio-eustism but gave more importance to diastrophic eustatism, stating that there is evidence of the worldwide lowering of the sea level during the Pliocene and Pleistocene, which was of far greater magnitude than possible changes made by glacio-eustatism.

According to him, it was the result of epeirogenic movement associated with orogenic movement and not independently orogenic alone.

ii) Glacio Eustatism

When there is a change in the sea level as a result of the withdrawal of water in the ocean, which was earlier accumulated in the form of ice sheets and glaciers. During the glacial period, the sea water gets stored and locked up, resulting in the sea level falling. Fall in the sea level leads to upliftment of the land.

iii) Climatic Interruptions to the Cycle of Erosion

The interruptions to the cycle, which have already been discussed, are either caused by structural or the earth’s movements or by eustatic changes in sea level caused by glacial accumulation or diastrophic movements.

Many times streams that show characteristics of the youthful stage starts by eroding vast volume of unconsolidated glacial debris. Such a heavily loaded stream, when spreads in the lower levels, starts to deposit these sediments.

Over a period of time, the stream has a lesser load. So the river requires less energy to transport the load, and more energy is used for vertical erosion. This may have involved neither upliftment by diastrophic movement nor eustatic lowering of the sea level. Such interruption is called static rejuvenation.

Static rejuvenation of the river course involves an increase in the discharge of the river, which is caused without any upliftment of the base level, nor lowering of the sea level, but because of the climatic changes.

According to Davis, climatic changes have a tremendous effect on the normal cycle of erosion, and in certain circumstances, climatic effects on erosion would totally change the course that is outlined in the ‘normal’ cycle.

An increase in the precipitation is usually a reason for the increase in the volume without having an increase in the load. The stream in such a situation can carry the load over more gentle slopes.

As a result of the decrease in the load as compared to runoff and subsequent rise in the stream volume also through acquiring the new drainage because of diversion or derangement. This kind of rejuvenation was common during the post–glacial times when the river valleys comprised of large volumes of glacial outwash.

It is debatable that only increased rainfall is the only reason for increased stream volume. Many times rejuvenation also occurs due to capturing of one drainage system by another; this is also called river capture, diverting water from the captured river to the captor river.

Ohio Valley is an example of this version, as before heading to southeast Indiana and southwest Ohio, Ohio Valley had a much shorter stream but later included that of Kanawha, Monongahela and Allegheny.

Denudational work in any region is greatly influenced by the climatic regime of the region. Climatic conditions like glacial and arid climates greatly influence and modify the normal cycle. Davis termed these imposed modifications as ‘climatic accidents’.

But it is very difficult to find any remnant of the cycle of glacial erosion since, on the present landscape, there are only one, i.e., the Quaternary glaciations –which has left any mark. It occupied a period of only one-half million years.

Glacial erosion can show their effects on the upland area or the youthful stage of the cycle, which is indicated in Davisia’s view of irregular profiles with basic steps. But its old age glaciated landform is difficult to recognize. But its effect on the heavily glaciated lowland, such as the Laurentian and Fenno-Scandian shields, can be easily regarded as glacial paneplains.

But it is also a known fact that these old peneplain surfaces were produced under very different climatic conditions of a very geological time scale. This has been preserved over a long period under layers of deposits. This was exhumed by present-day erosion slightly modified by the Quaternary ice sheets.

When arid climatic conditions differ from glacial, so are the desert landscapes. The desert landscape appears after several periods of the desert cycle run in their full course for a considerably long period, over the earth’s surface. It must be noted that the arid and semi-arid areas today were not necessarily arid in the past; they could be humid and even humid temperate climates.

‘Desert’ erosion was particularly active in permotriassic times, In the region of the British Isles. It may have escaped submergence by the ‘Chalk sea’.

But as envisaged by Davis, the arid and semi-arid cycle is more normal than the humid conditions over a long period on the earth’s surface. Davis, therefore also realized the need to form a separate cycle of desert erosion.

Rejuvenated Landforms

i) Uplifted Peneplain

Due to various reasons, the level of the peneplain gets uplifted higher than the present base level. This makes the peneplain come to the second cycle of erosion.

Various factors can give evidence of such upliftment like the peneplain’s accordance with the summit areas and the inter stream levels, topographical unconformities, truncation of rocks of varying resistance, presence of the rocks with varying resistance to erosion, presence of layers with weathered rock debris and evidence of remnants of former alluvium.

ii) Knick Points

Sometimes in the long profile of the river, there is a break due to a fall or lowering of the base level. Rejuvenation causes the renewal of the downcutting along the long profile of a valley. When this valley intersects with the long profile of the other valley, knick points are formed. It is the starting point in the formation of the river terrace.

iii) River Terraces

River terraces are formed when the river starts cutting downwards and reaching a new base level. They represent the valley floor abandoned by the rivers.

As the streams get rejuvenated, it abandons the old valley and starts cutting the new valley’s base level. It is like cutting a second valley inside the first valley and developing a second valley flat inside and below the first one.

Sometimes terraces may appear at an entire range of different heights on either or one side of the valley. Such valleys are called paired (with the same height) and unpaired with different heights.

iv) Spurs and Benches

When after rejuvenation, valleys have deeper cutting and which leads to the formation of terraces. When the tributaries’ streams join the main river, a series of spurs are created, representing the old system’s valley floor.

v) Incised Meanders

Meandering of steam over the flood plains is rejuvenation. It can cut gorges and canyons into old meanders because of the extra erosional capacity while still retaining the meandering form. These are called incised meanders.

Interruptions to Normal Cycle of Erosion

Fluvial System

Cycle of Erosion

Davisian Cycle of Erosion

For simplicity, he made the following assumptions:

Stages in the Ideal Cycle of Erosion:

Youth Stage

Maturity Stage

Old Stage

Rejuvenation

Base Level Change

Dynamic Rejuvenation

i) Upliftment of Landmass

ii) Tilting of Land and Warping

iii) Lowering of the Outlet

iv) Volcanic Activity

Eustatic Rejuvenation

i) Diastrophic Eustatism

ii) Glacio Eustatism

iii) Climatic Interruptions to the Cycle of Erosion

Rejuvenated Landforms

i) Uplifted Peneplain

ii) Knick Points

iii) River Terraces

iv) Spurs and Benches

v) Incised Meanders

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Fluvial System

Cycle of Erosion

Davisian Cycle of Erosion

For simplicity, he made the following assumptions:

Stages in the Ideal Cycle of Erosion:

Youth Stage

Maturity Stage

Old Stage

Rejuvenation

Base Level Change

Dynamic Rejuvenation

i) Upliftment of Landmass

ii) Tilting of Land and Warping

iii) Lowering of the Outlet

iv) Volcanic Activity

Eustatic Rejuvenation

i) Diastrophic Eustatism

ii) Glacio Eustatism

iii) Climatic Interruptions to the Cycle of Erosion

Rejuvenated Landforms

i) Uplifted Peneplain

ii) Knick Points

iii) River Terraces

iv) Spurs and Benches

v) Incised Meanders

Read More in Geomorphology

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