Fluvial Processes and Landforms: Erosional & Depositional

1 year ago

Atiqur Rahman

15 minutes

Fluvial landforms are those landforms which are shaped and modified by running water. Running water has sculpted most of the land surface across the world in comparison to other agents of erosion like wind or glaciers.

Streams are in a constant process of shaping the land surface into newer forms. The running water erodes even the loftiest of mountains and carves deep valleys or gorges into it.

Besides erosion, it transports heavy loads from one place to the other. The present chapter focuses on the landforms produced by fluvial processes. This encompasses both the erosional and depositional landforms produced by the running water.

Whenever any landform is created due to tectonic forces, it is attacked constantly by various agents of gradation, and thus it undergoes transformation taking new shapes. The running water due to exertion of force on the exposed surface erodes it and carries particles of varying size (ranging from very fine to the size of a boulder) depending on the slope and amount of water discharge.

Fluvial Processes

Erosion is a process in which the earth’s surface is worn away by various agents of erosion like wind, water or glaciers. The removed material is carried away and deposited elsewhere. Erosion by streams occurs through several processes going together. These processes are Abrasion, Hydraulic action, Solution and Attrition.

Abrasion

Running water, when armed with sand, silt, and other sediments, acts as a very effective means of erosion of the river bed.

In the upper reaches, the river with a steep gradient carries a lot of such materials which act as a tool and are effective in carving huge valleys, gorges and canyons. In contrast, if the river is free of these sediments, the erosion by abrasion is minimal, and the river would take a much longer time to erode its bed.

Solution

Pure water seldom exists in natural conditions. It contains various gases and acids in them. The water is charged with the acids and acts as an important solvent. It may not be effective on all rocks, but when it comes in contact with limestone, dolomite or chalk, it becomes the most powerful agent of erosion as it dissolves them much faster. So solution also forms an equally important process of erosion.

Attrition

When the stream loads (pebbles, sand, silt etc.) move together, they cause their own wear and tear by colliding with one another. This rounding and shaping of these pebbles and boulders amongst themselves are called attrition.

Hydraulic Action (Pressure)

Running water has enormous force in it, when it strikes the stream bed on its banks, it may loosen the rocks, lift them and easily transport them. It acts as a wedge when it strikes the crack or fractures in the rock bed by widening and loosening it. Thus, Hydraulic pressure is also reckoned as an important means of erosion.

The above-discussed processes, coupled with other factors, give rise to diverse landforms occurring in different parts of the stream course. A brief description of them is given below.

Fluvial Erosional Landforms

V Shaped Valley

Deep cutting and erosion by a rapid flow of the stream carve out a valley that resembles the English letter V. The V-shaped valley has a deep, narrow bottom (or valley) floor with steep valley sides. The shape is an outcome of a number of other factors at interplay; Mass movement and weathering of rocks are effective in shaping the valley.

Gorge

It is a narrow chasm with a very steep precipitous wall. These are common features found very often in mountainous regions.

The Himalayas are home to a number of gorges located at different places in their ranges. The Kali Gandaki Gorge is one of the deepest gorges in the world. River Satluj enters India through a huge gorge near Shipkila Pass.

The Brahmaputra also moves through various gorges and enters the Indian Territory. Besides the Himalayas, the Gorges have a presence across the world on most continents.

Waterfalls

It refers to a sudden descent of the stream or river flow caused by various factors. They are very picturesque and have beautiful features. Waterfalls may result due to variations in the resistance of rocks, crust deformation, changes in the sea level etc.

The different mode of origin has made its classification rather difficult. However, they can broadly be classified into 5 to 6 major categories considering their origins and distribution worldwide.

Formation of Waterfalls Due to Structural Variation:

Resistant Rock Lain Horizontally

When a hard and resistant rock overlies a very weak rock, the latter wears down quickly, and the resistant rock shows deep undercutting. The river bed gets steepened at the point giving rise to waterfalls of larger dimensions.

Niagara Falls is a perfect example of this type of fall where the caprock is a strong dolomite limestone of the Silurian age.

The resistant rock’s inclination determines the waterfall’s nature and size. If the overlying hard rock dips downstream, it results in the formation of rapids (a waterfall of small dimensions). When it dips upstream, it gives rise to a precipitous wall and a relatively larger waterfall.

Vertical Falls

Granitic intrusions (or vertically arranged hard rocks) lying vertically in the course of the river are least affected by erosion as compared to the adjoining rocks. The differential erosion accentuates the development of a steep slope causing a high waterfall.

Yellowstone River fall is an example of this type of fall; here, a dike stands adjoining weaker rocks, and Yellowstone River has made a stupendous fall when it flows over it, cutting deep gorges as it descends. Such falls do not recede upstream compared to the falls produced on horizontal rocks because there is no undercutting of soft rocks.

Fault and Fracture Falls

Rivers flowing over faulted rock structures give rise to such types of falls. The fault scarps created during faulting cause the water to fall from a vertical height resulting in waterfalls.

The Victoria Falls of river Zambezi is often cited as an example of this type of fall because the river flows over a basaltic plateau and crosses the prominent fractures in the plateau, thus giving rise to a waterfall.

Falls due to Descend from Uplifted Highlands

When a river flows from the uplifted hard and resistant rocks (like igneous) to the plain region, It gives rise to waterfalls. Such types of falls are noticeable in the Appalachian regions, where streams have developed numerous falls because of their descent into the Atlantic coastal plain.

This type of fall may include other examples of rivers descending from the plateau to the lowlands. E.g. the Livingstone Falls, Depicts the abrupt break in the flow of the river Congo giving rise to an exhilarating waterfall.

Hanging Valley Falls

The main glaciated valley eroded deeper than their tributary valleys when they were formerly occupied by the glaciers. Post-glaciation, the glacial valleys were occupied by water. As the tributaries meet the deeply eroded and steep wall of the main glacial valleys, they make waterfalls because of their sudden descent into the main valley.

There are a lot of waterfalls present in the Scandinavian region. Yosemite Falls in California lies in this category of the waterfall. The stream (a tributary of the main river) exhibits a fall over the cliff into the Merced River Valley, which was previously scoured and deepened by the glacier. This deep valley lies well below the valley of its tributary, thus giving rise to hanging valley fall.

Besides the classification of waterfalls based on their mode of origin, there are numerous other types of waterfalls, which are classified based on their size, height and water flow rate.

Sediment Transportation

The stream is an important means of transporting sediments of varying size and shape to distant lands much away from their source of origin. The sediments are transported in various ways depending on the material’s size, composition, and water volume.

Movement in Solution

Materials like sulphate, carbonate, and chloride are dissolved in water and carried out till the end of the stream. Rivers flowing in arid and semi-arid regions show more material in solution.

Movement in Suspension

The rivers transport very fine particles consisting of sand, silt and mud in huge quantities. The river keeps these particles in suspension as the volume of water and its speed does not allow particles to settle. They are always kept in suspension while travelling and therefore are carried for a fairly long distance than the larger boulder or pebbles which roll along the stream bed.

Movement by Traction

Large boulders and pebbles, which are heavier for the stream to carry by means of suspension or saltation, move by rolling along the stream bed. This rolling causes abrasion on the bed, and they are in constant contact with the bed. They emerge as one of the chief erosive agents of the stream bed.

Movement by Saltation

The particles jump and bounce when uprushing water comes with force lifting them from the bed to some distance. They cannot be carried long as their weight is more and therefore cannot be kept in suspension. This type of movement is called saltation.

Fluvial Depositional Landforms

Alluvial Fans

When a stream carrying a heavy load descends from a narrow mountain valley onto a plain, it leaves behind its load in the form of a fan or a cone. This deposit at the base of a mountain occurs due to a sudden drop in the velocity of the stream which cannot carry further such a heavy load on a plain region as it lacks a gradient that earlier provided velocity to the stream enabling it to carry an enormous load with much ease.

The alluvial fans show a thick deposit at the mouth of the valley and gradually descend in height away from it. The size of material deposits also shows a gradual decline from large to small away from the mountain front. The slope of fans varies due to variations in the size of the grains. Where there is an abundance of coarse sediments, there is the development of fans with steeper slopes, whereas fans having finer sediments have gentler slopes.

The alluvial fans, in comparison to alluvial cones, have a wider spread making an arc (or fan) in shape, while cones are relatively less extensive in their spread and have a steeper gradient. These two terms are often used together to denote the deposit at the mountain front.

In India, one can see a fan on the river Kosi at the base of the Himalayas. It is a large, gently sloping fan with a width of about 140 kilometres.

Floodplains

Floodplain is a very gently sloping flat region bordering the stream. It is covered with fine silt, mud, sand etc., brought down by the river and deposited in the adjoining region due to its regular flooding. This regular flooding enriches the soil and makes it a very good fertile land suitable for cultivation.

A river flowing across a floodplain may either form meanders in its course or develop numerous channels due to the deposition of the sediment load on its bed. It also gives rise to embankments known as “Natural Levees”, running along it for a fairly long distance.

All the said features of floodplains are discussed below in detail.

a) Meanders

A stream flowing on a flat or gently sloping surface seldom flows straight; it tends to take a sinuous course. While flowing, it makes gentle loops in its flows, commonly referred to as meanders. There is no unanimity on the reason leading to the formation of meanders.

The water flowing in the stream when striking the banks causes erosion at the site, leading to the development of a bend in the channel. The repeated striking of water and resultant erosion accentuates the bend and makes it more prominent. This prominent bend takes the form of a meander.

The meander represents the process of erosion and deposition going together on its banks. The outer side of the bend shows erosion and is referred to as a cut-off bank, while the inner side of the meander, where the flow of the water is slow, gives rise to deposition at the site of the bend, known as the point bar.

These two processes going on together cause the meanders to shift laterally. So meandering also leads to the formation of floodplains.

As has been said, the process of erosion and deposition continues; it tends to accentuate the meander leading to the formation of a complete circle. The river then cuts across the meander loops and takes a straighter course. The abandoned meander loops form oxbow lakes in the flood plains.

Meander and Ox Bow Lake — Meander and Ox-Bow Lake
Source

b) Natural Levees

They are embankments formed on both sides of the river. They owe their origin to the regular flooding of the river. During floods, the river overflowing its bank leaves behind a good amount of material. The material, due to regular occurrence, consolidates into small embankments known as Natural Levees. These Levees sometimes rise very high as they grow with each successive flood.

Braided Streams

The streams, when get overloaded with sediments, do not carry them along and leave the excess material on the river floor in the form of sand bars. These deposits cause the stream to split into several channels. The braided stream is a common occurrence in the region, which is relatively dry and arid and where the supply of water is not steady.

Alluvial Terrace or River Terrace

Alluvial terraces are the sites of former floodplains which are deeply eroded by the stream due to its rejuvenation. Its process of formation is explained in the following stages-

i. A stream is creating its valley by erosion.

ii. The stream with a decline in its gradient tends to deposit its load in adjoining areas creating an extensive floodplain.

iii. Due to falling in the base level (because of the fall in sea level) or upliftment of the region over which the stream flows, rejuvenation of the stream takes place. The rejuvenated stream gets actively involved in the downcutting of the floodplain resulting in the development of terraces on both sides. New terraces can also be created by further erosion on the newly established floodplain. Thus, lowering of the Base Level either due to falling in sea level or upliftment of the land surface causes the formation of alluvial terraces.

Deltas

Deltas are an arcuate or fan-shaped feature at the mouth of the river formed by regular deposition of the sediments. The name owes its origin to the Greek letter Δ.

The process of formation of a delta begins with the deposition of sediments as soon as the river enters the sea or lake. First, the river would shed its coarse sediments, which are heavier than the finer light particles. The distributaries carry forward the finer particles to some distance, where they come in contact with the saline water, get coagulated and settle.

Fluvial Processes and Landforms: Erosional & Depositional