Erosion & Deposition

As you know, the river is a body of flowing water. It flows down the slope under the effect of gravity, in a valley. The origin of the river is called source and the place where it ends, merging with a large water body is called the mouth.
Force of flowing water causes a dragging effect on river bed causing erosion and deepening of the valleys. Rivers also erodes the banks and widens the valleys. Sometimes soluble components are dissolved in water causing further erosion.
In the upper part of the river, as they are flowing from a great height, the velocity of flow will be higher and there will be a faster erosion of river bed. As a result, there will be the formation of deep and narrow ‘I’ shaped valleys called Gorges. Waterfalls or rapids are formed when a softer rock which is easily eroded comes after a layer of the hard rock layer.
When rivers erode the branches and widen the valleys the bank will be gently sloping leading to ‘v’ shaped valleys.
The river transports the eroded materials along with their flow and the number of materials carried depends on the speed of flow. Some materials are carried by the river in dissolved form while some are carried as suspended particles(suspended load). Coarse and heavy materials are dragged along the river bed as bed load.
In the later course of a river, it slows down and the carrying capacity also comes down. Excess load gets deposited on the riverbed and along the banks. Mainly it raises the river bed and the river becomes shallow and wide. When the deposits reach the level of flow of water the river gets bifurcated into multiple channels and riverine islands may be formed when some of these channels rejoin. Such a channel is called a braided channel.
Rivers deposits heavier particles at an earlier point and lighter particles further downstream. When river bed raises due to deposition of sediments, the river will show a tendency to overflow their banks. Such areas where rivers overflow, depositing rich and fertile soil is called flood plains. As the flow reduces, the amount of deposition also reduces away from the channel leading to accumulation of deposition along the banks. They are called natural levee.
In the flood plains, there can be the formation of broad curves due to flooding and changing course of rivers. This tendency is called meandering tendency. Such a meander loop can become cut off from the river stream by further deposition leading to the formation of an Oxbow Lake.

When the river reaches the mouth, if it is sufficiently loaded with sediments, can deposit them at the mouth causing the formation of triangular land forms, They are called deltas. As the delta gets enlarged the mouth of river shifts further and some area which was earlier under the oceans or seas are reclaimed as land. Because of depositional activity, the river stream may split into multiple channels and they are called distributaries.
The water seeping underground becomes ground water and it also causes some erosion and formation of landforms. This is more clearly seen in limestone region and is called Karst topography.
Water seeping underground fills the gaps and spaces in rock strata and flows down till it reaches a zone of saturation where all the spaces are filled with water. This area is called water table. The depth of water table differs from area to area.
While water seeps underground through the fractures in limestone, it dissolves the stone in due course of time due to carbonisation. This results in large underground caves and passages. Some of them may carry underground streams and some of these streams may reappear on the surface as springs.
As groundwater containing dissolved lime or calcite drips from the roof of the caves, the water evaporates leaving calcite behind. These deposits hanging from the cave ceilings are stalactites. When similar deposition happens upwards from the floor of the cave, they are called stalagmites. Some of them may join together to form pillars inside the caves.
Wind action of gradation is mostly limited to dry arid areas especially deserts. Erosion happens when loosely bounded particles are carried away by the wind causing wind eroded basins. These particles are deposited over other areas when there is an obstacle lying in its path creating sand dunes of varying sizes and shapes. Sand dunes usually have a gentler slope towards the windward side and are convex from that side.
Glaciers are formed on top of high mountains and in subarctic zones. When the glacier is huge, the lowest layers will be slightly plastic and they slowly flow in the direction of the slope. A glacier behaves like a slow moving river of ice.
Glaciers are categorised by their morphology, thermal characteristics, and behaviour. Alpine glaciers, also known as mountain glaciers or cirque glaciers, form on the crests and slopes of mountains. An alpine glacier that fills a valley of a former stream is sometimes called a valley glacier. A large body of glacial ice astride a mountain, mountain range, or volcano is termed an ice cap or ice field. Ice caps have an area less than 50,000 km2 (19,000 sq mi) by definition.
Glacial bodies larger than 50,000 km2 (19,000 sq mi) are called ice sheets or continental glaciers. Several kilometres deep, they obscure the underlying topography. Only nunataks protrude from their surfaces. The only extant ice sheets are the two that cover most of Antarctica and Greenland.
As the glaciers expanded, due to their accumulating weight of snow and ice, they crush and abrade scoured surface rocks and bedrock. The resulting erosional landforms include cirques, glacial horns, arêtes, U-shaped valleys, over deepenings and hanging valleys. Cirque is the starting location for mountain glaciers.U-shaped valleys are created by mountain glaciers. When filled with ocean water so as to create an inlet, these valleys are called fjords. Arête is a spiky high land between two glaciers, if the glacial action erodes through, a spillway forms.

Please follow and like us:

Leave a Reply

Your email address will not be published. Required fields are marked *