Erosional Landforms – FLUVIAL GEOMORPHOLOGY

Fluvial landforms are shaped through the erosional and transportation processes of running water, as well as the depositional activities it carries out.

Fluvial Erosional Landforms

Fluvial erosional landforms arise from the erosive actions carried out by rivers. Various facets of fluvial erosion encompass:

  • Hydration: The force exerted by flowing water as it gradually wears away rocks.
  • Corrosion: Chemical processes that contribute to the breakdown of rock structures.
  • Attrition: Particles within the river’s load colliding and fracturing as they interact.
  • Corrosion or abrasion: Solid particles in the river load impact rocks and gradually wearing them down.
  • Downcutting (vertical erosion): Erosion occurring at the stream’s base, resulting in the deepening of valleys.
  • Lateral erosion: Erosion affecting the stream’s banks, leading to the widening of valleys.
  • Headward erosion: Erosion originating at the beginning of a stream channel, causing the channel’s source to retreat against the flow direction and consequently elongating the channel.

River Valley

River-carved valleys are significant eroded landforms. The shape and size of these valleys change as the fluvial erosion cycle progresses. In the youthful stage, they’re V-shaped with steep, convex valley sides. These valleys are deep and narrow, with water touching both sides. They’re formed by rapid vertical erosion. With time, lateral erosion widens the valleys, making them broad with flat floors and uniform slopes in the mature stage. In the old stage, they become even broader and shallower with gentle concave slopes. V-shaped valleys are categorized as gorges and canyons.

Gorges

Gorges and canyons are narrow valleys with very steep, wall-like sides. Distinguishing between them can be challenging. Generally, a deep, narrow valley is termed a gorge, while a more extended version is called a canyon. Gorges form due to active vertical erosion, often involving pothole formation, during the youthful stage of river erosion. They can also develop as waterfalls retreat. Many Himalayan rivers have created deep gorges, including notable ones like Hundrughagh gorge on the Subarnarekha River, the Raru River gorge below Johna or Gautamdhara falls, Dassamghagh gorge below Dassamghagh falls on the Kanchi River, Phurghagh gorge on the South Koel River, Chachai gorge on the Bihar River (Rewa, M.P.), Kevti gorge on the Mahana River (Rewa, M.P.), and the gorge of the Odda River (Rewa, M.P.).

Canyons

Canyons are extended versions of gorges. They are very deep, narrow, and long valleys. The steepness of the valley sides depends on the rock type. Resistant rocks create steep sides, while alternating resistant and soft rocks result in undulating sides. The Grand Canyon of the Colorado River in Arizona, USA, is a significant example, stretching 482.8 kilometers long and reaching a depth of 2088.3 meters. Another notable canyon is formed by the Indus River, which cuts through the Himalayan ranges, creating a gorge and canyon that is 17,000 feet deep.

Waterfalls

Waterfalls, or falls, occur when rivers experience sudden drops or abrupt changes in their longitudinal course. These can be caused by various factors, such as differences in rock resistance, changes in topography, shifts in sea levels, and geological movements. A waterfall is defined as a massive volume of water plunging vertically from a great height in the river’s long profile. Rapids are smaller than waterfalls and are typically found upstream or independently. In the USA, a chain of waterfalls exists along the Piedmont and Atlantic coastal plain, extending from New England to central Alabama. This is known as the fall line.

India also has a distinct fall line known as the Indian Fall Line, extending from Tons Falls in the west to Sasaram in the east. It runs along the junction of Peninsular India’s foreland and the Ganga plains. This line is marked by numerous waterfalls, ranging in height from 15m to 180m. Notable waterfalls along this line include Purwa or Tons Falls (70m) on the Tons River, Chachai Falls (127m) on the Bihar River, Kevti Falls (98m) on the Mahana River, Odda Falls (145m) on the Odda River, and many more.

Types of Waterfalls

Waterfalls exhibit vast variations in terms of height, size, shape, and water volume, making their classification challenging. Generally, they are categorized into two main types based on their origin: (1) normal waterfalls and (2) minor waterfalls. Normal waterfalls result from varying rock resistance, indicating a youthful stage of stream development and ungraded stream profiles. Minor waterfalls arise due to disruption in erosion cycles caused by rejuvenation, known as knickpoint falls. The following outlines the classification scheme for fluvially formed waterfalls.

Normal Waterfalls

  • (i) Step falls
  • (ii) Caprock falls
  • (iii) Barrier falls
  • (iv) Plateau falls

Minor Waterfalls

  • (A ) Falls originated due to endogeneticforces
    • (i) Fault falls
    • (ii) Falls due to upliftment
  • (B ) Falls originated due to changes in the level of valley floors
    • (1) Due to the lowering of the valley floor
    • (i) Hanging valley falls
    • (ii) Glacial hanging valley falls
    • (iii) Falls due to river capture falls
    • (iv) Coastal hanging valley falls
    • (v) Knickpoint falls
    • (2 ) Due to obstructions in the river courses
    • (i) Falls due to landslides
    • (ii) Falls due to lava dams
    • (iii) Falls due to giacial moraines

1. Waterfalls due to structural and lithological variations

Waterfalls resulting from variations in the characteristics of terrestrial rocks are termed normal waterfalls. Different orders of waterfalls (e.g., cataracts, rapids, cascades) are determined by the relative resistance and arrangement of rock layers. Cliffs formed by the presence of hard and soft rocks create large waterfalls known as cataracts. Alternating layers of hard and soft rocks give rise to cascades, a series of small step-like falls. The arrangement of rock beds leads to waterfalls of varying dimensions as explained below.

(i) Rock Beds Dip Upstream:

When alternating layers of hard and soft rocks slope upstream along the river’s course and the uppermost caprock is resistant, the softer rocks beneath erode more swiftly, creating steep wall-like scarps. This setup allows river water to cascade vertically downstream, forming an impressive waterfall. These waterfalls, known as caprock falls, recede quickly due to the cliffing and collapse of the hanging headwalls of the falls. Over time, as the river achieves its balanced graded profile, these falls vanish.

(ii) Rock Beds Dip Downstream:

Caprock rapids emerge when alternating hard and soft rock layers dip downstream, and the uppermost caprock resists erosion. These rapids are characterized by caprock protection.

(iii) Horizontal Rock Beds:

Enormous and impressive waterfalls arise when rock beds are horizontally arranged, and a resistant caprock like quartzitic sandstones or dolomitic limestones overlies softer, more erodible rocks such as shale, volcanic ash, and loose materials. This results in the resistant rocks forming steep wall-like cliffs from which the river water cascades vertically. Examples of this type include Niagara Falls, where the caprock of dolomitic limestone is followed by softer shale, limestone, and sandstone layers. Another example is Kaieteur Falls in British Guiana, where a resistant conglomerate caprock tops the falls, with the Potaro River cutting deeply through the rock to create a 225.5-meter-high waterfall.

Rewa (Madhya Pradesh) and Rohtas (Bihar) plateau waterfalls, as previously mentioned, also fall into this category. These waterfalls feature caprocks of sandstones and quartzitic sandstones overlying weaker shales from the Vindhyan formations. Examples like Chachai Falls (127 m on the Bihar River), Kevti Falls (98 m on the Mahana River), Odda Falls (145 m on the Odda River), Kuaridah Falls (180 m on the Ausane River), and Rakim Kund Falls (168 m on the Gayghat River) illustrate this type of caprock waterfall.

(iv) Vertical Rock Beds:

Waterfalls with steep slopes emerge when alternating resistant and soft rocks are vertically aligned. Soft rocks erode rapidly, leaving behind resistant rock beds that form steep scarps in the river’s path, creating waterfalls. Intrusive dykes also contribute to waterfall formation due to their relatively slower erosion compared to surrounding rocks. These waterfalls are known as vertical barrier falls. An example is the Great Fall of the Yellowstone River in Yellowstone National Park, USA. In areas like the ‘Patlands’ of Ranchi and Palamau (Bihar), similar waterfalls have formed due to structural and lithological influences, with heights ranging from 3m to 30m.

(v) Plateau Waterfalls:

Waterfalls are formed when rivers originating from plateaus descend over precipitous escarpments and enter lower regions. For instance, the Congo River creates the 275m Livingstone Falls as it descends the African Plateau, and the Orange River forms the 140m Aughrabies Falls at the plateau’s edge. Many northward-flowing streams and their tributaries have produced waterfalls at the northern edge of the Rewa Plateau, such as Chachai Falls (127m) on the Bihar River, Kevti Falls (98m) on the Mahana River, and Odda Falls (145m) on the Odda River. Scarp falls are created, like the 17m Pheruaghagh Falls along the southern edge of the Ranchi Plateau by the Karo River. Scarp falls or knick-line falls include examples like Hundru Falls (75m) on the Subarnarekha River near Ranchi, Dasam Falls (39.62m) on the Kanchi River east of Ranchi, and Sadni Falls (60m) on the Sankh River on the Ranchi Plateau.

Yenna Falls (180m) on Mahabaleshwar Plateau, Gokak Falls (54m) in Belgaum district (Karnataka), Gersoppa Falls (260m) on the Sharavati River in North Kanara, Sivasamudram Falls (90m) on the Cauvery River, and others are also instances of scarp falls.

(vi) Step Falls:

When alternating horizontal layers of hard and soft rocks are arranged in the river’s course, it results in a series of low waterfalls due to varying rates of erosion. These falls are essentially rapids.

(2) Fault and Fracture Waterfalls:

Waterfalls form along fault scarps resulting from faults cutting across river valleys. A prime example is Victoria Falls (110m high) on the Zambezi River.

(3) Uplift-Induced Waterfalls:

Various-sized waterfalls emerge due to localized uplift in river courses. These falls disappear as rivers regrade their longitudinal profiles. In Palamau District (Bihar), a series of waterfalls along the junction of Palamau upland and the northern flat plain resulted from Palamau’s southern uplift during the Tertiary period. Notable examples include Patam Falls (45.72m) and Datam Falls (30.45m) on the Patam River in Bihar.

(4) Hanging Valley Falls:

Waterfalls of varying sizes occur when tributary streams join their main streams from higher levels, forming hanging valleys. The Rajroppa Falls (10m) at the junction of Bhera nadi and Damodar River, and Gautamdhara or Johna Falls (25.9m) on the Gunga River, are examples of this.

(5) Glacial Hanging Valley Falls:

During ice ages, glacial modification of fluvially originated valleys leads to tributaries hanging over main valleys. After ice ages, these tributaries occupy valleys and create waterfalls. Such falls can be seen in Norway, Sweden, Finland, Canada, etc.

(6) River Capture Waterfalls:

Waterfalls form when streams flowing over flat but higher lands are captured by lower streams, causing waterfalls at the point of capture. This phenomenon is abundant in the Himalayas.

(7) Coastal Hanging Valley Falls:

Rivers descending sea cliffs or cliffed coasts form vertical waterfalls before reaching the sea, creating coastal hanging valley falls.

(8) Knickpoint Falls:

Knickpoint falls arise from breaks in channel gradient caused by rejuvenation due to uplift or changes in sea level. These sudden changes create vertical drops in river profiles, forming waterfalls of varying sizes. Hundru Falls (76.67m) on the Subarnarekha River, Johna or Gautamdhara Falls at the Raru-Gunga confluence, Dasam Falls (39.62m and 15.24m) on the Kanchi River, Burhaghagh Falls (148m) on the Burha River, Dhunwadhar Falls on the Narmada River, and major falls on the Rewa Plateau (such as Chachai Falls, Kevti Falls, Tons or Purwa Falls, Oda Falls) exemplify knickpoint waterfalls.

(9) Obstruction Waterfalls:

Temporary waterfalls can emerge due to various natural obstructions in river flow:

(i) Lava-dammed waterfalls result from lava barriers formed across valleys, creating almost permanent falls.

(ii) Landslide-dammed waterfalls occur when debris slides down from hillslopes, obstructing river flow and forming barriers.

(iii) Moraine-dammed waterfalls are created by rivers flowing over morainic debris barriers in valleys.

Recession of Waterfalls:

Waterfalls and rapids are not permanent features. They vanish as rivers achieve equilibrium profiles through vertical erosion. Waterfalls disappear mainly due to horizontal recession through backwasting and lowering of height through downwasting. For example, Niagara Falls recedes at about 1.2 to 1.4 meters annually and has receded approximately 11 km. The recession of waterfalls in India has not been closely documented.

Pot Holes

The small depressions in river valleys resembling kettles are known as potholes. These are typically cylindrical and form in coarse-grained rocks like sandstones and granites. Potholing, also called pothole drilling, occurs when rocks caught in swirling water create circular grinding motions that gradually drill and enlarge holes in the valley bed, similar to a drilling machine. These holes, called potholes, increase in diameter, perimeter, and depth over time. Some larger ones are termed plunge pools, often found at waterfall bases due to the impact of falling water. Many river valleys, particularly in Chotanagpur highlands, exhibit numerous potholes due to uplift during the Tertiary period. The Gaur Nadi’s basaltic bed near Bhadbhada showcases a stunning array of various-sized potholes. Pothole drilling is a key process in deepening valleys.

Structural Benches

Terraces are flat surfaces on the sides of valleys. They result from differential erosion of hard and soft rock bands. When erosion rates vary due to different rock types, benches or terraces, known as structural benches, are formed. This is due to lithological control affecting erosion rates and the subsequent development of benches.

River Terraces

  • River terraces are narrow flat surfaces on valley sides representing former valley floors and old flood plains.
  • They often appear as step-like arrangements on both sides of river valleys.
  • Formation occurs due to dissection of deposited floodplain sediments along the valley floor.
  • There’s significant variation in their morphology and origin.
  • Paired terraces result from rapid vertical erosion, creating terraces on both sides at the same level.
  • Unpaired terraces form due to concurrent vertical erosion and lateral channel movement.
  • River terraces are classified as rock terraces (bedrock platforms with fluvial deposits) and aggradational terraces (thick fluvial sediment deposits).
  • They form from erosion of former flood plains after land uplift or sea level fall.
  • River terraces are created through a process of rejuvenation, where increased erosive power due to changing sea levels leads to valley deepening.
  • Rivers first form narrow valleys within former terraces, then broaden them through lateral erosion to create new flood plains.
  • Subsequent rejuvenations lead to the formation of additional terraces and narrow valleys.

River Meanders

  • River meanders are bends in a river’s longitudinal course.
  • The term “meander” originates from the Meander River in Asia Minor due to its numerous bends.
  • Each meander bend has two types of valley sides: concave (erosion-prone) and convex (deposition-prone).
  • The concave side forms vertical cliffs and is called the cliff-slope side.
  • The convex side receives deposits, often sands and gravels, and is called the slip-off slope side.
  • Meander shapes are usually semicircular or circular; their length relates to the channel width.
  • Meandering is a natural process influenced by factors like lithology, topography, vegetation, precipitation, and river development stage.
  • Streams meander across various terrains, with the degree of meandering influenced by slope and materials.
  • Meandering is most significant in flat, gently sloping regions with alluvial deposits and ample stream flow.
  • Despite the theoretical expectation of straight paths, streams consistently deviate, characterized by a sinuosity index.
  • A sinuosity index of 1 to 1.3 indicates a sinuous stream, while values above 1.3 signify a meandering stream.
  • Highly meandering streams have gradients of 20 cm to 10 m per kilometer.
  • Alluvial streams in the Northern Plains of India display meandering courses.
  • The Gomati River exemplifies such meandering due to its low gradient (9 cm per km).
  • The Ganga River also meanders significantly with a gradient of 6 cm per km.
  • Other meandering rivers include Ramganga, Sai, Rapti, Ghaghra, Punpun, Burhi Gandak, and Kosi.
  • Meanders result from both erosion and deposition.
  • Two major meander types are based on erosion nature: normal meanders (lateral erosion) and incised meanders (vertical erosion).
  • Misfit meanders form a third type.
  • Morphologically, river meanders fall into three categories: wavy, horse-shoe, and ox-bow (bracelet) types.
  • Wavy meanders have wide meander necks and are common in Himalayan major streams.
  • Horse-shoe meanders feature highly curved beds, with closely positioned arms.
  • Ox-bow meanders have nearly circular bends with high curvature.
  • Northern Plains’ alluvial rivers often display horse-shoe and ox-bow meanders.

Simple Meanders

  • Meanders formed by lateral erosion during a stream’s first erosion cycle.
  • Three types are based on morphology: wavy, horse-shoe, and ox-bow meanders.
  • Conditions for meander formation: alluvial plains, gentle slope, sufficient precipitation, and limited vegetation.
  • Overloaded and youthful streams don’t form meanders due to sediment transport and valley deepening activities.
  • Mature streams with lateral erosion capability are ideal for meander formation.
  • Minor obstructions divert stream courses, initiating minor bends.
  • Channel currents erode concave sides of bends, sharpening meanders (wavy meanders).
  • Continuous erosion on concave sides and deposition on convex sides results in high curvature (horse-shoe meanders).
  • Curvatures become more circular over time (ox-bow or bracelet meanders).
  • River courses become highly meandering with ox-bow lakes forming.

Incised Meanders

  • Formed through vertical erosion, representing rejuvenation and valley deepening.
  • Develop within simple meanders due to accelerated valley incision from rejuvenation.
  • Simple meanders have wide, shallow valleys; incised meanders are narrow and deep.
  • Incised meanders are created exclusively in bedrocks, unlike simple meanders on loose materials.
  • Variations of incised meanders include entrenched, intrenched, enclosed, and ingrown meanders.
  • Meanders in narrow valleys enclosed by rocky walls.
  • Incised meanders formed by downcutting older meanders.
  • Types of incised meanders: entrenched/intrenched (uniform slopes on both sides) and ingrown (unequal slopes).
  • Entrenched meanders have valley floors deeply entrenched by vertical erosion from rejuvenation.
  • Ingrown meanders have one side deeply undercut, forming a hanging cliff.

Misfit Meanders

  • Meanders formed within larger former meanders due to reduced water volume.
  • Rivers create extensive meander loops in alluvial plains, sometimes braided into multiple channels.
  • A significant decrease in river water volume leads to narrower channels.
  • Narrow channels can’t fit within broader former valleys, forming their own meandering paths within larger meanders.
  • These narrow meanders are termed misfit meanders due to their inability to fit within the larger ones.

Ox-bow Lakes

  • Ox-Bow and Horse-Shoe Lakes: Lakes formed from water impounding in abandoned meander loops.
  • Meander loops can become nearly circular due to strong lateral erosion.
  • When meander curvature increases and loop ends approach, streams straighten, forming ox-bow lakes.
  • Ox-bow lake formation involves erosion (by stream straightening) and deposition (filling cutoff ends).
  • Oxbow lakes frequently accumulate sediment during floods, transforming into swamps over time.
  • Ganga River’s shifting course in Uttar Pradesh created palaeochannels and oxbow lakes, now seen as tanks and ponds.
  • Pratapgarh district in Uttar Pradesh displays examples of palaeochannels and palaeo-oxbow lakes.

Peneplains

  • Peneplains are featureless low plains with a gently rolling surface and remnants of residual hills.
  • They result from the normal erosion cycle and feature low residual hills called monadnocks.
  • Monadnocks are left due to less erosion of resistant rocks and were named after Monadnock hills in New England, USA.
  • Different geomorphologists use various terms for the end product of the normal erosion cycle: peneplain (W. M. Davis), endrumpf (W. Penck), panplain (C. H. Crickmay), pediplain (L. C. King), etchplain (Pugh and Thomas), panfan (A. C. Lawson).

TRANSPORTATIONAL WORK OF STREAMS

  • Rivers transport eroded materials within certain limits determined by their transporting power.
  • Load size, amount, and stream velocity affect this transporting power.
  • Stream velocity is influenced by factors like channel gradient, valley features, and water volume.
  • Steeper gradients, less meandering courses, and sufficient water volume increase stream velocity and its transporting power.
  • G.K. Gilbert proposed Gilbert’s sixth power law linking stream velocity and transporting power.
  • According to this law, transporting power is proportional to the sixth power of stream velocity.
  • Suspended load (fine particles in water) and bedload (varied-sized particles on or near the channel bed) make up river sediment load.
  • Bedload moves through traction or saltation, while suspended load remains in suspension within the fluid (water).

Rivers employ various methods for transporting their load, including traction, saltation, suspension, and solution. Traction involves moving larger rock pieces and boulders as bedload, which roll or slide along the channel floor. This bedload remains in contact with the valley floor and typically includes gravels, pebbles, cobbles, and boulders. Saltation, on the other hand, entails the transport of coarse load by water currents, causing them to leap or hop through the valley floors. This process is slow and involves relatively larger particles. Suspended load consists of medium-sized materials that are buoyant in water, allowing them to be suspended in the fluid. This suspended load can be carried by the streams for greater distances. Lastly, soluble materials are dissolved in the water, becoming invisible. Such materials are transported in a dissolved state, and this process is referred to as transportation by solution.

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