The streams have a huge capacity to erode the rock over which they flow. In fact, the formation of the river channel is the result of the erosional capacity of the stream. The erosional capacity of the stream depends on its volume of water and velocity of flow. The river performs three types of work. They are erosion, transportation and deposition.
1. Erosion: The breaking of rocks by the river in along its course is called erosion. Erosional work of a river is performed mechanically and chemically. River erosion is carried out in the following ways:
· Hydraulic action: It refers to the physical force of the moving water which breaks the rocks in its course.
· Corrasion ( abrasion): It refers to the breaking of rock in the bed and on the bank by fragments carried by the stream.
· Corrosion( solution): It refers to the dissolving process of soluble minerals by the splashing of stream water.
· Attrition: It refers to the eroded materials carried by the stream strike against each other.
2. Transportation: Stream carrying the fragmented materials broken by the stream is called transportation. After erosion, the eroded materials get transported alont with the running water. This transportation of eroded materials is carried in four ways:
· Traction: The heavier and larger rock fragments like gravels, pebbles etc are forced by the flow of the river to roll along its bed. These fragments can be seen rolling, slipping, bumping and being dragged. This process is called as traction and the load transported in this way are called traction load.
· Saltation: Some of the fragments of the rocks move along the bed of a stream by bouncing continuously. This process is called as saltation.
· Suspension: The holding up of small particles of sand, silt, and mud by the water as the stream flows is called suspension.
· Solution: Some parts of the rock fragments dissolve in the river water and transported. This type of transportation is called solution transportation.
3. Deposition: When the velocity of the stream decreases, the stream deposits sand, silt and other fragments. It is called as the deposition. When a river moves in a gentle slope, its speed reduces and river begins to deposit its load. The river starts depositing larger materials first and smaller and finer materials are carried further down to the mouth of the river.
The course of a river includes the upper stage, the middle stage, and the final stage. Each stage of the river is dominated by a kind of work. Let’s discuss the stages of a river, the main work and the landforms made in each stage.
The upper stage of a river is also called the youthful stage or mountain stage. The velocity and speed of the stream are very high because the slope here is steep. The vertical erosion is the most dominant work here. The valley is formed here. The place where a river starts is called a source. In the mountain stage, the number of small streams originates from different locations. They are called Tributaries.
The place where two rivers join is called as the confluence. The mountain which has two river systems draining on either side of the slope is termed as the water divide.
Middle stage is the matured stage of a river. Vertical erosion or deepening of the valley is significantly reduced. Lateral erosion is the dominant work. Due to the lateral erosion of this stage, the widening of the valley occurs. The volume of the river water increases and the slope of river is moderate. The depth of the river is deep here.
This is the final stage of a river where the valleys are extremely broad and it has generally gentle slope. The valley becomes almost flat which is called a peneplain. Most of the peneplain forms low residual hills with steep slopes which are called as Monadnocks.
Look at the diagram, read the table of content carefully and fill in the columns with suitable words.
The main work of the river in this stage is the deposition. The depth of the river is shallow here. When the main river splits into many small rivers, they are called as the distributaries. The place where the river ends is called mouth of the river. (for example: Sea coast, Lake.)
The significant landforms resulting from erosion by rivers include gorge, canyon, V-Shaped Valley, waterfall, pothole, structural bench, river terrace, river meander, ox-bow lake, peneplain, etc.
Gorges are formed due to active down cutting of the valleys. So, a Gorge is a narrow and deep river valley which has steep slopes.
Canyons are extended form of gorges. Canyons represent very deep, narrow but long valleys. The steepness of the valley sides depends on the nature of the rocks. The Grand Canyon of the Colorado River in the state of Arizona, USA having a length of 482.8 kilometers and depth of 2088.3 meter is the largest canyon in the world. The Canyon of Gandikota is situated on the Pennar River in Andhra Pradesh is known as the Grand Canyon of India.
V-Shaped Valley The valleys made by the rivers are erosional landforms. The valley is formed in the youthful stage of the river erosion. Due to the steep slope and large volume of water, the river cuts its bed vertically forming narrow and deep river valley. This is called as V-shaped valley.
Rapids are stream sections with extremely strong currents, numerous obstacles, and steps in their streambeds. A waterfall is a vertical drop in a streambed. Both water fall and rapids are formed by vigorous erosion. Series of a waterfall in a river is called as Cascade.
A plunge pool is a deep depression in a stream bed at the base of a waterfall. It is created by the erosional forces of falling water at the base of a waterfall.
Angel Falls, in Venezuela, is Earth’s highest waterfall (979 m). Hogenakal falls, Dharmapuri, Tamil Nadu some times is called as the Niagara of India.
Long and narrow depression at the base of a waterfall made by river runoff is called a groove. The grooves are created by water eroding soil from a hill or mountain in a short period of time.
The swirling movement of the water falling into the plunge pool is called eddying.
An interlocking spur, also known as an overlapping spur, is a projecting ridge that extends alternately from the opposite sides of a V-shaped valley. A river with a winding course flows down the interlocking spur.
The kettle-like small depressions in the rocky beds of the river valleys are called potholes. They are always cylindrical in shape. Potholes are generally formed in coarse-grained rocks such as sandstones and granites.
The narrow step like flat surfaces on either side of the valley floor are called river terraces. They represent the level of former valley floors.
1. Alluvial fan
Alluvial fans are often found at the foot of arid or semiarid mountain ranges where intermittent streams flow. An alluvial fan is a fan shaped deposit of gravel, sand and other smaller particles of sediment.
Alluvial fans are found in Kosi river, Himalayan region, Death Valley National Park and along the sides of the Colorado River at Grand Canyon National Park, U.S.
Peneplains represent low featureless plain having undulating surface and remnants of convex-concave residual hills.
A meander is a winding curve or bend in a river. Meanders are the result of both erosional and depositional processes. They are typical landform of the middle and lower course of a river. This is formed by vertical erosion, lateral erosion, and deposition within the floodplain.
Oxbow lake is a free standing body of water formed when the meander is cut off from the main river. This landform is so named because it resembles horse shoe
5. Levees: Raised bed and a bank of the river due to frequent flooding and deposition of the sediments is called levees.
A flood plain is a flat area of land adjacent to a river. It stretches from the bank of its channel to the base of the enclosing valley walls which experiences flooding during the period of high discharge.
The word “estuary” is derived from the Latin word aestuarium meaning tidal inlet of the sea, which is derived from the term aestus, meaning tide. An estuary is a partially enclosed coastal body of brackish water with one or more rivers flowing into it, and with a free connection to the open sea.
The inflow of both sea water and fresh water provide high levels of nutrients both in the water column and in sediment. Hence, it makes estuaries among the most productive natural habitats in the world. Narmada river estuary is located in Gujarat.
Delta is found in the old stage of a river. It is the triangular shaped landform made up of alluvial deposition in the mouth of the river. It is named after the fourth Greek alphabet called delta. Example, The Ganges Bhramaputra delta is the largest delta in the world.
Types of Delta: Delta is classified into the following based on the shape and kind of the load deposited by the river.
1. Arcuate Delta: A bowed or curved delta with the convex margin facing the body of water. It is also known as fan-shaped delta. Example, River Nile Delta in Egypt and Ganga Delta in India.
2. Estuarine Delta : it is formed at the mouth of submerged rivers depositing down the sides of the estuary. Example, Seine River of France.
3. Birds foot Delta: They are formed due to deposition of finer materials by river water. Deposited alluvial material divides the river into smaller distributaries. Such delta is also called as finger delta. Example, Mississippi river delta, the USA.
4. Lacustrine Delta: It is formed when a river flows into a lake. Example, Lough Leanne river delta, Ireland.
5. Truncated Delta: Sea waves and ocean currents modify and even destroy deltas deposited by the river through their erosional work. Thus, eroded and dissected deltas are called truncated deltas.
6. Abandoned Delta: when the river shifts its mouth, the delta already made is left abandoned. Such a delta is called abandoned delta. Example, Yellow river delta, China and the Western part of Ganga delta made by Hoogly river, India.
7. Cuspate delta is a tooth shaped delta formed when a single distributary flows through and deposits its load on its either side. Example, Tiber River of Italy