• Fluvial landforms are generated by running water, mainly rivers
• Fluvial derives from the Latin word ‘fluvius’ meaning river
• Fluvial landforms range in size from small features like rills to large rivers and their drainage basins
• About 68% of the Earth’s land surface is drained by rivers flowing to the oceans
• Rivers generally have their source in upland regions with a slope down for the run-offs
• Uplands form the catchment areas of the rivers and the crest of mountains becomes the divide or watershed from which the streams flow down the slope
• The initial stream that exists as a consequence of the slope is called the consequent stream
• The consequent stream is joined by several tributaries from either side as it wears down the surface
• The drainage basin or watershed is a fundamental landscape unit in fluvial geomorphology
• A drainage basin contains a primary or trunk river and its tributaries
• Various aspects of fluvial erosive action include:
  • Hydration: the force of running water wearing down rocks.
  • Corrosion/Solution: chemical action that leads to weathering.
  • Attrition: wear and tear of transported material when they roll and collide with one another.
  • Corrasion or abrasion: solid river load striking against rocks and wearing them down.
  • Downcutting (vertical erosion): erosion of the base of a stream, which leads to valley deepening.
  • Lateral erosion: erosion of the walls of a stream, which leads to valley widening.
  • Headward 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.
  • Hydraulic Action: mechanical loosening and sweeping away of materials by river water, mainly by surging into the crevices and cracks of rocks and disintegrating them.
  • Braiding: the main water channel splitting into multiple, narrower channels. A braided river consists of a network of river channels separated by small, often temporary, islands called braid bars. Braided streams occur in rivers with low slope and/or large sediment load.
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River course

Youth

• Streams are few during this stage with poor integration and flow over original slopes.
• The valley developed is deep, narrow, and distinctly V-shaped with no floodplains or with very narrow floodplains.
• Downcutting predominates over lateral corrasion.
• Stream divides are broad and flat with marshes, swamps, and lakes.
• Outstanding features developed in this stage include gorges, canyons, waterfalls, rapids, and river capture.

Mature

• During this stage, streams are plenty with good integration.
• Lateral corrasion tends to replace vertical corrasion.
• The valleys are still V-shaped but wide and deep due to active erosion of the banks.
• Trunk streams are broad enough to have wider floodplains within which streams may flow in meanders confined within the valley.
• Swamps and marshes of youth stage, as well as flat and broad inter-stream areas, disappear.
• The stream divides turn sharp.
• Waterfalls and rapids disappear.
• Meander and slip off slopes are the characteristic features of this stage.

Old

• The river moving downstream across a broad level plain is heavy with sediments.
• Vertical corrasion almost ceases in this stage though lateral corrasion still goes on to erode its banks further.
• Smaller tributaries during old age are few with gentle gradients.
• Streams meander freely over vast floodplains.
• Divides are broad and flat with lakes, swamps, and marshes.
• Depositional features predominate in this stage.
• Most of the landscape is at or slightly above sea level.
• Characteristic features of this stage are floodplains, oxbow lakes, natural levees, and Delta etc.

Fluvial Landforms – Erosional

• Splash erosion:
  • the first stage in the erosion process caused by the bombardment of soil surface by rain drops.
  • Rain drops displace soil particles and destroy soil structure.
• Sheet erosion:
  • occurs as a shallow sheet of water flowing over the ground surface, resulting in the removal of a uniform layer of soil from the soil surface.
  • Sheet flow is the cause of sheet erosion.
• Rills:
  • shallow channels created by the erosion of flowing water in soil.
• Gullies:
  • large rills that cannot easily be removed by tilling the soil.
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• Rivulet:
  • a small stream
• Ravine:
  • a narrow landform often formed by streambank erosion, larger than gullies but smaller than valleys
• River valley:
  • an extended depression on the ground where a stream flows throughout its course
  • Valleys start as small, narrow rills and gradually develop into long, wide gullies
  • Gullies deepen, widen, and lengthen to form valleys
  • Different types of valleys can be recognized, such as V-shaped valleys, gorges, canyons, etc.
• I-shaped valley/gorge:
  • valley sides are almost parallel, forming an “I” shape; gorges have very steep to straight sides and are almost equal in width at the top and bottom
  • Gorges are formed in hard rocks
  • Example of a gorge: Indus Gorge in Kashmir
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• Canyon:
  • a wider variant of the gorge with steep step-like side slopes
  • Canyons commonly form in horizontal bedded sedimentary rocks
  • Grand Canyon carved by Colorado River, USA is an example of a canyon
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• V-shaped valley:
  • deep, narrow and distinctly V-shaped, formed by vertical corrasion
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• Interlocking spurs:
  • projections of high land that alternate from either side of a V-shaped valley
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• Waterfalls & Rapid:
  • Waterfalls occur when a river plunges down from a height
  • They are formed due to several factors like relative rock resistance and differences in topographic relief
  • The force of water from the waterfall can create a plunge pool beneath it
  • Rapids are formed by an abrupt change in the river’s gradient
  • Waterfalls and rapids are transitory landforms and will recede gradually over time.
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• Potholes & Plunge Pool:
  • Potholes are circular depressions on rocky beds of hill-streams.
  • They are formed by stream erosion and abrasion of rock fragments.
  • Small and shallow depressions are formed initially.
  • Pebbles and boulders rotate in the depressions due to flowing water, leading to growth in dimensions.
  • Depressions join eventually, deepening the stream valley.
  • Large and deep potholes form at the foot of waterfalls due to the sheer impact of water and rotation of boulders.
  • These are called plunge pools.
  • Plunge pools help in deepening of valleys.
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• Cataract:
  • Waterfalls on large rivers
  • Usually applied to sections of rapidly flowing rivers with a sheer drop
  • Powerful and dangerous waterfalls
• Ait/Eyot:
  • Small islands found in rivers
  • Typically formed by sediment deposition in the water
  • Long and narrow in shape, may become permanent islands with vegetation
  • May be eroded and result in another ait, a channel with many aits is called a braided channel
• Incised/Entrenched Meanders:
  • Found cut in hard rocks
  • Very deep and wide
  • Formed from original gentle surfaces in initial stages of stream development
  • Meander loops get entrenched into rocks due to erosion or gradual uplift of land
  • Widened and deepened over time, can be found as gorges and canyons in areas with hard rocks
  • Indication of original land surface over which streams developed
  • Impact of river rejuvenation.
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• Structural Benches:
  • Step-like sequence of geomorphic surfaces
  • Differential erosion of alternately arranged hard and soft rocks forming step-like valleys
  • Structural benches formed due to differential erosion of alternate bands of hard and soft rock beds
  • Lithological control in the rate of erosion and consequent development of benches
• River Terraces:
  • Surfaces relating to old valley floor or floodplain levels
  • Bedrock surfaces without any alluvial cover or alluvial terraces consisting of stream deposits
  • Products of erosion resulting from vertical erosion by the stream into its own depositional floodplain
  • Different terraces found at different heights indicating former river bed levels
  • Paired terraces occur at the same elevation on either side of the river
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• Peneplain:
  • Low-relief plain formed as a result of stream erosion
  • Represents a near-final stage of fluvial erosion during times of extended tectonic stability
• Drainage Basin:
  • Also called catchment, catchment area, catchment basin, river basin, or water basin
  • Includes both streams/rivers and land surface
  • Collects all the water within the area covered by the basin and channels it to a single point
  • In closed drainage basins, water converges to a single point inside the basin known as a sink
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• Drainage Divide:
  • Adjacent drainage basins are separated from one another by a drainage divide.
  • Drainage divide is usually a ridge or a high platform.
  • Drainage divide is conspicuous in case of youthful topography (Himalayas), and it is not well marked in plains and senile topography.
• Drainage Patterns:
  • Drainage pattern refers to the shape of a river course during its erosional cycle
  • It’s determined by topography, resistance and strength of base rocks, and land gradient
  • Types of drainage patterns:
    • Dendritic: most common, branches out like a tree, e.g. Indus River
    • Trellis: primary tributaries flow parallel and joined by secondary tributaries at right angles, e.g. Appalachian Mountains and Seine River in France
    • Parallel: observed in steep sloping regions, tributaries run parallel, e.g. Lesser Himalaya
    • Rectangular: develops on rocks of uniform resistance with two directions of joining at right angles, e.g. Colorado River
    • Angular: observed in foothill regions where bedrock joints and faults intersect at acute angles, e.g. Ladakh and Tibet streams
    • Radial: rivers originate from a hill and flow in all directions, e.g. Amarkantak Range
    • Centripetal: rivers discharge in all directions in a lake or depression, e.g. streams in Ladakh, Tibet, and Loktak Lake in Manipur
    • Annular: streams follow a roughly circular path along a belt of weak rock, e.g. Black Hill streams in South Dakota, USA.
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Fluvial Landforms – Depositional

• Fluvial depositional landforms are created by river sediments from erosion in the upper course of rivers.
• Weathering and erosion of rocks and cliffs occur in the youth stage or upper course of the river.
• The river brings down a heavy load of sediments from the upper course as it moves downstream on a level plain.
• The decreasing stream velocity in the lower course reduces the transporting power of the streams, leading to the deposition of sediment.
• Coarser materials are deposited first, followed by finer silt towards the mouth of the river.
• This depositional process creates various landforms through fluvial action, including Delta, Levees, and Flood Plain.

• Alluvial Fans and Cones:
  • Alluvial fans are cone-shaped depositional landforms built up by streams heavy with sediment load.
  • They are formed when streams flowing from mountains break into foot slope plains of low gradient.
  • Normally, very coarse load is carried by streams flowing over mountain slopes, but this load gets dumped as it becomes too heavy to be carried over gentler gradients by the streams.
  • The dumped load spreads as a broad low to a high cone-shaped deposit called an alluvial fan that appears as a series of continuous fans.
  • Alluvial fans in humid areas show normally low cones with a gentle slope from head to toe, while they appear as high cones with a steep slope in arid and semi-arid climates.
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• Floodplains:
  • Floodplain is a landform created by river deposition.
  • Deposition and erosion are responsible for the development of floodplains and valleys, respectively.
  • Rivers in the lower course carry large amounts of sediment.
  • Large materials are deposited first when the stream channel breaks into a gentler slope.
  • Fine sediments like sand, silt, and clay are carried over gentler channels by slow-moving waters.
  • During floods, sediments are spread over low-lying adjacent areas, gradually building up a floodplain.
  • In plains, channels occasionally shift and change course, leaving cut-off courses that are filled up by coarse deposits.
  • Flood deposits carry finer materials like silt and clay.
  • Active floodplain is the riverbed made of river deposits.
  • Inactive floodplain above the banks contains flood deposits and channel deposits.
  • The floodplains in a delta are called delta plains.
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• Doab:
  • Doab is the tract of land between two converging rivers.
  • Doab is a term used in South Asia particularly in India and Pakistan to refer to ” tongue” or tract of land lying between two converging rivers.
• Natural Levees:
  • Natural Levees are low, linear and parallel ridges of coarse deposits found along the banks of large rivers.
  • They are formed by the deposition action of the stream and appear as natural embankments.
  • During flooding, water is spilt over the bank and as the flow speed decreases, large sediments are deposited along the bank as ridges.
  • The levee deposits are coarser and high near the banks, sloping gently away from the river.
  • Series of natural levees can form when rivers shift laterally.
  • Artificial embankments are formed on the levees to minimize the risk of floods.
  • Sudden bursts in the banks due to water pressure can cause disastrous floods, as seen in the example of the Hwang Ho river, also known as China’s sorrow.
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• Point Bars & Cut Banks:
  • Point Bar is a depositional feature associated with floodplain
  • Also known as meander bars, they are formed by alluvium accumulation on the inside bends of streams and rivers below the slip-off slope.
  • Found on the convex side of meanders of large rivers, they are almost uniform in profile and width, and contain mixed sizes of sediments.
  • Long and narrow depressions can be found in between the point bars where there is more than one ridge.
  • Rivers build a series of point bars depending upon the water flow and supply of sediment.
  • Erosion takes place on the concave side of the bank, while point bars are built by the rivers on the convex side.
  • Cut banks are found on the outside of a bend in a river, caused by the moving water of the river wearing away the earth.
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• Meanders:
  • Meanders are loop-like channel patterns that develop over flood and delta plains of large rivers
  • Active deposition happens along the convex bank while undercutting occurs along the concave bank in meanders
  • If there is no deposition or erosion, meandering tendency is reduced
  • The concave bank is called a cut-off bank and appears as a steep scarp, while the convex bank presents a long, gentle profile called the slip-off bank
• Oxbow Lake:
  • Oxbow lakes are independent water bodies formed when deep meander loops get cut-off due to erosion at inflection points
  • Oxbow lakes can get silted up and turn into swamps, eventually drying up over time due to natural processes.
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• Pools:
  • Deep areas with slow current in a stream.
  • Created by the vertical force of water falling over logs or boulders.
  • Provide depth and still water.
• Riffles:
  • Shallow areas with fast, turbulent water in a stream.
  • Created by rocks that agitate the water flow.
  • Provide protection, food deposition, and shelter.
  • Depths vary depending on the size of the stream, ranging from 1 inch to 1 meter deep.
  • High dissolved oxygen concentration due to the turbulence and streamflow.
• Bluff:
  • Small, rounded cliff overlooking a body of water or where a body of water once stood
  • Ridge of land extending into the air
• Braided Channels:
  • Network of river channels divided into multiple threads and separated by small and often temporary islands called eyots
  • Commonly found where water velocity is low and the river is heavy with sediment load
  • Deposition and lateral erosion of banks are essential for the formation of the braided pattern
  • Formation of central bars due to selective deposition of coarser material which diverts the flow towards the banks causing extensive lateral erosion
  • As the valley widens due to continuous lateral erosion, the water column is reduced and more and more materials get deposited as islands and lateral bars developing a number of separate channels of water flow.
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• Delta:
  • Deltas are fan-shaped alluvial areas, resembling an alluvial fan, formed by sediment deposition from rivers at their mouths into the sea
  • Deltas extend sideways and seaward at an amazing rate
  • Deltas are well-sorted with clear stratification, with coarse sediments deposited first and finer sediments carried further out to sea
  • Types of Deltas:
    • Bird’s foot delta: long, stretching distributary channels branching outward resembling a bird’s foot
    • Arcuate delta: a fan-shaped delta with a curved or bowed margin facing the sea
    • Cuspate delta: a delta with tooth-like projections at its mouth, formed where the river flows into a stable water body
    • Estuarine delta: a delta partly submerged in the coastal waters due to a drowned valley or rise in sea level
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  • Conditions favorable for delta formation:
    • Active vertical and lateral erosion in the upper course of the river to provide extensive sediments
    • Sheltered coast with shallow adjoining sea
    • No large lakes in the river to filter off sediments
    • No strong current running at right angles to the river mouth to wash away sediments
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