Fluvial Landforms

The landforms created by rivers are known as fluvial landforms. It generally involves two processes - erosion and deposition. The Erosional landforms are formed due to the wearing off of soil and land due to the flow of water e.g. valleys, gulleys, entrenched meanders, river terraces and plungepools. The depositional landforms are created when rivers deposit sediments on plain areas e.g. deltas, alluvial fans, flood plains and meanders. These landforms are generally nutrient rich and provide great fertile lands for agriculture.The study of fluvial landforms helps in understanding the modern day issues like infertility of soil, land degradation, water-related problems etc.

Fluvial landforms have evolved with the geological evolution of the earth. River systems began to form during the Precambrian eon, over 4 billion years ago. Processes, such as erosion and deposition started shaping the landscape as water flowed across the surface. Other geological activities like plate movements, tectonic activities and volcanic activities have influenced the formation and evolution of fluvial landforms. In the recent times, human activities also had a profound impact on fluvial landforms through activities such as dam construction, river channelization, urbanization, and agriculture. Overall, the history of fluvial landforms is a story of dynamic change and adaptation over geological time scales.

The creation of landforms by rivers is a fundamental geological process that shapes the Earth's surface.The landforms which are created by rivers are known as Fluvial landforms. These are mainly of two types - Depositional and Erosional. Depositional landforms are created by the aggradational work of running water while the erosional landforms are formed due to degradational action.

1. Erosion : Rivers wear away and remove sediments and rocks from the land through the process of erosion. Rivers erode the land vertically by undercutting riverbanks and horizontally by widening their channels. This erosion leads to the formation of valleys, canyons, and gorges over time. The erosion occurs through hydraulic action (the force of flowing water), abrasion (the mechanical action of sediment particles rubbing against the riverbed and banks), and corrosion (the chemical dissolution of rock materials).
2. Transportation : The eroded sediments are transported by the flow of water from one place to the other. The transportation of eroded material occurs through traction (rolling and sliding along the riverbed), saltation (bouncing along the riverbed), and suspension (being carried in the water column).
3. Deposition : When the velocity of flow of river water decreases, the sediments start to deposit. Deposition occurs in areas of reduced flow energy, such as the inside of meander bends, the mouths of rivers, or where the river enters a larger body of water. Various landforms get created due to the deposition of sediments such as floodplains, alluvial fans, river deltas.
4. Channel evolution : As rivers erode, transport, and deposit sediment, they undergo changes in their channel morphology and behavior. Factors such as changes in discharge, sediment supply, tectonic activities and anthropogenic activities like dam construction and river engineering can change channel morphology and behavior which give rise to new landforms.

Depositional landforms get formed when the speed of running water in the rivers get decreased in the plain areas and sediments start to deposit. Many landforms get created due to deposition -

1. Deltas
   • Formation: Deltas are formed when rivers with less flow of water reach the mouth of large water bodies and deposits the sediments there. The deposition of sediment creates a network of distributary channels, which branch out across the delta plain. The sediments get accumulated overtime gradually extending the delta's reach into the water.
   • Components: Deltas consist of three main components: distributary channels, the delta plain, and the delta front.
     • Distibutary channels are the branches through which the river water spreads sediments across the delta plain.
     • Delta plain is a flat, low-lying area of land composed of sediments between the distributary channels and the surrounding body of water.
     • Delta front is the edge of the delta on the sea side where sediment is actively deposited as the river meets the sea or ocean.
   • Examples of Deltas in India: Ganga-Brahamputra delta (also known as Sunderbans delta) ,Godavari delta (Andhra Pradesh), Krishna delta (Andhra Pradesh), Mahi delta (Gujarat), Cauvery delta (Tamil Nadu) etc.
   • Significance :
     • Deltas play an important role in coastal protection, inland areas protection from flooding and storms.
     • Deltas provide breeding grounds, feeding areas, and shelter for numerous species of fish, birds, and other organisms.
     • Due to the deposition of nutrient rich sediments, deltas provide fertile land for agriculture and provide livelihoods for millions of people worldwide.
2. Floodplains
   • Formation: During floods, rivers transport sediment downstream and deposit it on adjacent flat areas. Floodplains have flat topography and close proximity to rivers and streams.
   • Components: There are two main parts of Floodplains - Floodway and Flood fringe.
     • Floodway is the main stream of a river creating the floodplains. It can dry up depending on the season.
     • Flood fringe refers to the outermost areas of a floodplain that are periodically inundated during floods but are less frequently affected by floodwaters compared to floodway.
   • Examples of Floodplains in India: Indo-Gangetic plains ( encompassing states such as Uttar Pradesh, Bihar, West Bengal, and parts of Punjab and Haryana), Yamuna floodplains (in Uttar Pradesh, Haryana, and Delhi), Brahmaputra fllodplains ( in North East India) etc.
   • Significance :
     • Floodplains provide important habitat for a variety of plant and animal species.
     • Floodplains serve as natural reservoirs for sediment and help regulate the flow of rivers.
     • Floodplains play an important role in water storage and groundwater recharge.
3. Alluvial fans
   • Formation: Alluvial fans get created when rivers deposit sediments by transporting them from high-gradient mountainous regions to lower-gradient areas. Alluvial fans are dynamic environments influenced by periodic flooding, debris flows, and sediment transport.It is named as alluvial fans as sediments form a broad low to high cone shape on deposition.
   • Examples of Alluvial fans in India: Ganga alluvial fan, Sutlej alluvial fan, Brahmaputra alluvial fan, Tapti alluvial fan etc.
   • Significance :
     • Alluvial fans provide a habitat for a variety of plant and animal species adapted to arid or semi-arid environments.
     • These are often associated with heavy mineral deposits.
     • These act as natural water reservoirs and are important for groundwater recharge.
4. Meanders
   • Formation: Meanders form through a combination of erosion, transportation, and deposition processes. As a river flows , it erodes its outer banks through hydraulic action and abrasion while depositing sediment on the inner banks (point bars) through the process of deposition. Meanders are dynamic features that constantly evolve and change over time in response to fluctuations in water flow, sediment transport, and channel morphology.
   • Meanders are formed by all the rivers in India.
   • Ox-bow lakes: Ox-bow lakes are the meanders of rivers which got detached from the main channel overtime. Erosion and deposition processes cause the neck of the loop to narrow until it eventually cuts off from the main channel, leaving behind a crescent-shaped lake known as ox-bow lake.
   • Examples of ox-bow lakes in India :
     • Chilika lake, Odisha: It is the largest brackish water lake in India. It is believed to be formed out of meandering of Mahanadi river.
     • Kanwar lake, Bihar: It is world’s largest ox-bow lake formed out of meandering of Gandak river, a tributary of Ganga. It is a residual ox-bow lake.
     • Sambhar lake, Rajasthan : It is the largest inland salt water lake in India. It is formed out of the meandering of the Luni river.
     • Loktak Lake, Manipur : It is believed to be formed out of the meandering of the Manipur river.
     • Pulicat lake, Andhra Pradesh and Tamil Nadu : It is a saline lagoon believed to have formed from an ancient oxbow lake of the Arani river.
   • Significance :
     • Meanders provide diverse habitats such as riffles, pools, and backwaters to support a high level of biodiversity.
     • These play important role in nutrient cycling, water filtration, and floodplain connectivity.

Erosional fluvial landforms are shaped by the gradual removal of material from the Earth's surface through the processes of erosion by river water. It occurs due to different processes-

• Hydraulic pressure: It is the mechanical force exerted by moving water against the bed and banks of a river or stream.
• Corrosion: It is the process of chemical dissolution where certain types of rock and sediment are gradually broken down and dissolved by the chemical properties of water.
• Abrasion: It is the process in which force of flowing water transports sediments that act as abrasive agent, wearing down the surfaces of landforms over time.

Different forms of erosional landforms are -

1. Valleys :
   • Formation: Valleys get formed through various geological processes like fluvial erosion (erosion by rivers), glacial erosin by ice (form U-shaped valleys) and tectonic activities. Howerver, the most common is fluvial erosion. Rivers erode the land through hydraulic action, abrasion, and corrosion, gradually carving out V-shaped valleys with steep sides. Valleys often contain rivers or streams that flow through them, carving out channels and transporting sediment downstream.
   • Examples of Fluvial valleys in India: Kashmir valley, Bhagirathi valley (Uttarakhand), Narmada valley (Central India), Godavari valley ( Deccan plateau) etc.
   • Significance :
     • Valleys provide a flat fertile land for agriculture with access to irrigation.
     • Valleys also have recreational and scenic value, attracting tourists with their natural beauty, hiking trails, and opportunities for outdoor activities such as camping, fishing etc.
     • The presence of water in valleys provide unique habitats for aquatic and riparian ecosystems, supporting diverse plant and animal life.
2. River terraces
   • Formation: River terraces get formed due to combination of both erosion and deposition. Initially, the river erodes downwards into its bed cutting down deep channel. As the river cuts downward, it may leave behind remnants of its former floodplain as elevated terraces along the sides of the valley. Sediments deposited by rivers on the valley floor create a new floodplain at lower elevations. This process repeats multiple times resulting in a series of terraces at different elevations along the valley walls.
   • Examples of Fluvial river terraces in India: Ganga river terraces (Uttarakhand, Uttar Pradesh, Bihar, and West Bengal), Yamuna river terraces (Uttarakhand, Himachal Pradesh, and Uttar Pradesh) etc.
   • Significance :
     • River terraces reveal valuable information about history and evolution of river valleys and landscapes.
     • These provide habitat for a variety of plant and animal species adapted to riparian environments.
     • These are suitable for terrace farming.
3. Plungepools
   • Formation: Plungepools get formed due to erosion by river water typically in environments where there is a significant drop in elevation such as waterfalls or steep sections of rivers. When a waterfall rushes through a narrow channel, it exerts hydraulic pressure on the underlying rock, eroding it to create a plunge pool at the base of the waterfall.
   • Examples of Plungepools in India: Dudhsagar Falls (Goa), Jog Falls (Karnataka), Nohkalikai Falls (Meghalaya), Nohsngithiang Falls (Seven Sisters Falls, Meghalaya), Bhimlat Falls (Rajasthan).
   • Significance :
     • Plungepools provide habitat for aquatic plants and animals adapted to rocky substrates and fast-flowing water.
     • As water plunges into the pool from a height, it erodes the rock and new sediments get formed which get transported downstream and can form a new landscape overtime.
     • Plunge pools with waterfalls form great tourist spots.
4. Gulleys/ Rills
   • Formation: Gullies form through the process of fluvial erosion. As the river water flows downhill, it gains velocity and erosive power to cut into the soil and rock, gradually deepening and widening the channel to form a gully. The rate and extent of gully formation depends upon many factors like slope gradient, soil type, vegetation cover, and the intensity and duration of rainfall or runoff.
   • Example of fluvial gulleys in India : Chambal ravines ( Uttar Pradesh and Madhya Pradesh), Gulleys are also very prevalent in Western and Eastern Ghats.
   • Impacts :
     • Gully erosion has many environmental impacts like land degradation, loss of soil fertility and destruction of habitats.
     • Gullies can also pose hazards to infrastructure, property, and human safety by undermining roads, buildings.
5. Entrenched meanders
   • Formation : Meanders form through a combination of erosional and depositional processes acting on the river channel over long periods of time. Initially, meanders develop on relatively flat or gently sloping terrain but as the river downcuts into the landscape, the erosional process dominates and the river deepens carving into rocks forming the entrenched meanders. Entrenched meanders typically have a pronounced and sinuous shape, with tightly curved bends and steep-walled sides.
   • Examples of Entrenched meanders in India : Entrenched meanders are formed by many rivers in India along its path such as Narmada river, Tapi river, Yamuna river, Chambal river etc.
   • Significance :
     • Entrenched meanders reveal the erosional history and landscape evolution of riverine environments.
     • These provide habitat for a variety of plant and animal species adapted to riparian and canyon environments. They may serve as wildlife corridors and migration routes for species that depend on riverine ecosystems for survival.
     • These are often valued for their aesthetic and recreational qualities, attracting tourists.

Fluvial landscapes are formed by deposition and erosion by flowing water. They create various landscapes from meandering rivers to cliffs of river valleys. They also transport nutrients from the upper regions to lower regions. Understanding fluvial landforms helps in our efforts to manage water resources, mitigate hazards, and preserve the ecological integrity of riverine environments.