Table of Contents
Fluvial Erosional Landforms are landforms created by the erosional activity of rivers.
Landforms and Cycle of Erosion
- Fluvial landforms and Cycle of Erosion – Erosional Landforms [This Post] and Depositional Landforms [Next Post: Fluvial Depositional Landforms – Alluvial Fan – Levee – Delta Types].
- Glacial landforms and Cycle of Erosion
- Marine landforms and Cycle of Erosion
- Arid landforms and Cycle of Erosion
- Karst landforms and Cycle of Erosion
Fluvial Landforms and Cycle of Erosion
- The landforms created as a result of degradational action (erosion) or aggradational work (deposition) of running water are called fluvial landforms.
- The fluvial processes may be divided into three physical phases – erosion, transportation and deposition.
Fluvial Erosional Landforms
Various Aspects of Fluvial Erosive Action
- Corrasion or abrasion == solid river load striking against rocks and wearing them down.
- Hydration == force of running water wearing down rocks.
- Attrition == river load particles striking, colliding against each other and breaking down in the process.
- Downcutting == Erosion in vertical direction (downcutting leads to valley deepening) or
- Lateral erosion == Erosion in horizontal direction, especially the walls of the stream.
- Corrosion == Chemical action that leads to weathering.
River Valley Formation
- The extended depression on ground through which a stream flows throughout its course is called a river valley.
- At different stages of the erosional cycle the valley acquires different profiles.
- At a young stage, the valley is deep, narrow with steep wall-like sides and a convex slope. The erosional action here is characterized by predominantly vertical downcutting The profile of valley here is typically ‘V’ shaped.
- A deep and narrow ‘V’ shaped valley is also referred to as gorge and may result due to downcutting erosion and because of recession of a waterfall. Most Himalayan rivers pass through deep gorges (at times more than 500 metres deep) before they descend to the plains.
- An extended form of gorge is called a The Grand Canyon of the Colorado river in Arizona (USA) runs for 483 km and has a depth of 2.88 km.
- A tributary valley lies above the main valley and is separated from it by a steep slope down which the stream may flow as a waterfall or a series of rapids.
- As the cycle attains maturity, the lateral erosion becomes prominent and the valley floor flattens out. The valley profile now becomes typically ‘U’ shaped with a broad base and a concave slope.
- Young rivers (A) close to their source tend to be fast-flowing, high-energy environments with rapid headward erosion, despite the hardness of the rock over which they may flow.
- Steep-sided “V-shaped’ valleys, waterfalls, and rapids are characteristic features.
- Mature rivers (B) are lower-energy systems. Erosion takes place on the outside of bends, creating looping meanders in the soft alluvium of the river plain. Deposition occurs on the inside of bends and on the river bed.
- At a river’s mouth (C), sediment is deposited as the velocity of the river slows. As the river becomes shallower more deposition occurs, forming islands and braiding the main channel into multiple, narrower channels.
- As the sediment is laid down, the actual mouth of the river moves away from the source into the sea or lake, forming a delta.
- Head ward erosion == Erosion at the origin of a stream channel, which causes the origin to move back away from the direction of the stream flow, and so causes the stream channel to lengthen.
- A waterfall is simply the fall of an enormous volume of water from a great height.
- They are mostly seen in youth stage of river.
- Relative resistance of rocks, relative difference in topographic reliefs, fall in the sea level and related rejuvenation, earth movements etc. are responsible for the formation of waterfalls.
- For example, Jog or Gersoppa falls on Sharavati (a tributary of Cauveri) has a fall of 260 metres.
- The kettle-like small depressions in the rocky beds of the river valleys are called pot holes which are usually cylindrical in shape.
- Potholing or pothole-drilling is the mechanism through which the grinding tools (fragments of rocks, e.g. boulders and angular rock fragments) when caught in the water eddies or swirling water start dancing in a circular manner and grind and drill the rock beds of the valleys like a drilling machine.
- They thus form small holes which are gradually enlarged by the repetition of the said mechanism. The potholes go on increasing in both diameter and depth.
- Stepped benches along the river course in a flood plain are called terraces.
- Terraces represent the level of former valley floors and remnants of former (older) flood plains.
- Gulley is an incised water-worn channel, which is particularly common in semi-arid areas.
- It is formed when water from overland-flows down a slope, especially following heavy rainfall, is concentrated into rills, which merge and enlarge into a gulley.
- The ravines of Chambal Valley in Central India and the Chos of Hoshiarpur in Punjab are examples of gulleys.
- A meander is defined as a pronounced curve or loop in the course of a river channel.
- The outer bend of the loop in a meander is characterized by intensive erosion and vertical cliffs and is called the cliff-slope side. This side has a concave slope.
- The inner side of the loop is characterized by deposition, a gentle convex slope, and is called the slip-off side.
- Morphologically, the meanders may be wavy, horse-shoe type or ox-bow/ bracelet type.
- Sometimes, because of intensive erosion action, the outer curve of a meander gets accentuated to such an extent that the inner ends of the loop come close enough to get disconnected from the main channel and exist as independent water bodies. These water bodies are converted into swamps in due course of time.
- In the Indo-Gangetic plains, southwards shifting of Ganga has left many ox-bow lakes to the north of the present course of the Ganga.
Peneplane (Or peneplain)
- This refers to an undulating featureless plain punctuated with low-lying residual hills of resistant rocks. It is considered to be an end product of an erosional cycle.
- Peneplain, gently undulating (wave like), almost featureless plain that, in principle, would be produced by fluvial erosion that would, in the course of geologic time, reduce the land almost to baselevel (sea level), leaving so little gradient that essentially no more erosion could occur.
- The typical shape of a river course as it completes its erosional cycle is referred to as the drainage pattern of a stream.
- A drainage pattern reflects the structure of basal rocks, resistance and strength, cracks or joints and tectonic irregularity, if any.
Dendric or Pinnate
- This is an irregular tree branch shaped
- Examples: Indus, Godavari, Mahanadi, Cauvery, Krishna.
- In this type of pattern the short subsequent streams meet the main stream at right angles, and differential erosion through soft rocks paves the way for tributaries.
- Examples: Seine and its tributaries in Paris basin (France).
- The main stream bends at right angles and the tributaries join at right angles creating rectangular patterns.
- This pattern has a subsequent origin (subsequent drainage – you will study this in Indian drainage systems). Example: Colorado river (USA).
- The tributaries join the main stream at acute angles.
- This pattern is common in Himalayan foothill regions.
- The tributaries seem to be running parallel to each other in a uniformly sloping region.
- Example: rivers of lesser Himalayas.
- The tributaries from a summit follow the slope downwards and drain down in all directions.
- Examples: streams of Saurashtra region and the Central French Plateau, Mt. Kilimanjaro etc..
- When the upland has an outer soft stratum, the radial streams develop subsequent tributaries which try to follow a circular drainage around the summit.
- Example: Black Hill streams of South Dakota.
- In a low lying basin the streams converge from all sides.
- Examples: streams of Ladakh, Tibet, and the Baghmati and its tributaries in Nepal.