Depositional landforms of river Upsc

Depositional landforms of river

The depositional landforms that formed the running waters of the river are:

Alluvial Fans, Deltas, Floodplains, Natural Levees, and Point Bars, Meanders, Braided Channels

The main work of the river is deposition, bed building, by which it forms the flood plains. When there is a large water flow such as floods, the river cuts the bank and takes straighter, shorter, and new routes.

Alluvial Fans

Alluvial fans are developed when streams running from higher levels break into foot slope plains of low gradients.

Usually, the very coarse load is transported by streams running over mountain slopes. This load grows too heavy for the streams to be carried over gentler gradients and gets dumped and spread as a broad low to high cone-shaped deposit called an alluvial fan.

Usually, the streams which flow over fans are not confined to their original channels for long and shift their position across the fan forming many channels called distributaries.

Alluvial fans in humid areas show normally low cones with a gentle slope from head to toe and they appear as high cones with a steep slope in arid and semi-arid climates.

Also, their Alluvial cone. The alluvial cone is a steep, narrow, cone-shaped alluvial fan. This is where a swift stream rapidly slows down due to an upland stream emerging abruptly into a level plain.

Deltas

Deltas are like alluvial fans but develop at a different location. The load carried by the rivers is dumped and spread into the sea. If this load is not carried away far into the sea or distributed along the coast, it spreads and accumulates as a low cone.

Unlike in alluvial fans, the deposits making up deltas are very well sorted with clear stratification. The coarsest materials settle out first and the finer fractions like silts and clays are carried out into the sea.

As the delta grows, the river distributaries continue to increase in length and the delta continues to build up into the sea.

Floodplains

Deposition develops a floodplain just as erosion makes valleys. Floodplain is a major landform of river deposition. Large-sized materials are deposited first when the stream channel breaks into a gentle slope.

Thus, normally, fine-sized materials like sand, silt, and clay are carried by relatively slow-moving waters in gentler channels usually found in the plains and deposited over the bed and when the waters spill over the banks during flooding above the bed.

A river bed made of river deposits is an active floodplain. The floodplain above the bank is an inactive floodplain. Inactive floodplain above the banks basically contains two types of deposits — flood deposits and channel deposits.

In plains, channels shift laterally and change their courses occasionally leaving cut-off courses that get filled up gradually.

Such areas over flood plains built up by abandoned or cut-off channels contain coarse deposits. The flood deposits of spilled waters carry relatively finer materials like silt and clay. The flood plains in a delta are called delta plains.

Natural Levees

Natural levees and point bars are some of the important landforms found associated with floodplains. Natural levees are found along the banks of large rivers. They are low, linear, and parallel ridges of coarse deposits along the banks of rivers quite often cut into individual mounds.

During flooding as the water spills over the bank, the velocity of the water comes down and large-sized and high specific gravity materials get dumped in the immediate vicinity of the bank as ridges.

They are high nearer the banks and slope gently away from the river. The levee deposits are coarser than the deposits spread by floodwaters away from the river. When rivers shift laterally, a series of natural levees can form.

Point Bars

Point bars are also known as meander bars. They are found on the convex side of meanders of large rivers and are sediments deposited in a linear fashion by flowing waters along the bank.

They are almost uniform in profile and in width and contain mixed sizes of sediments. If they’re more than one ridge, narrow and elongated depressions are found in between the point bars.

Rivers build a series of them depending upon the water flow and supply of sediment. As the rivers build the point bars on the convex side, the bank on the concave side will erode actively.

Meanders

In large flood and delta plains, rivers rarely flow in straight courses. Loop-like channel patterns called meanders develop over flood and delta plains. Meander is not a landform but is only a type of channel pattern.

This is because of the propensity of water flowing over very gentle gradients to work laterally on the banks, unconsolidated nature of alluvial deposits making up the banks with many irregularities which can be used by water exerting pressure laterally and Coriolis force acting on the fluid water deflecting it like it deflects the wind.

When the gradient of the channel becomes extremely low, water flows leisurely and starts working laterally.

Slight irregularities along the banks slowly get transformed into a small curvature in the banks; the curvature deepens due to deposition on the inside of the curve and erosion along the bank on the outside.

If there is no deposition and no erosion or undercutting, the tendency to meander is reduced. Normally, in meanders of large rivers, there is active deposition along the convex bank and undercutting along the concave bank.

The concave bank is known as a cut-off bank which shows up as a steep scarp and the convex bank presents a long, gentle profile and is known as a slip-off bank.

As meanders grow into deep loops, the same may get cut-off due to erosion at the inflection points and are left as ox-bow lakes.

Braided Channels

When rivers carry coarse material, there can be selective deposition of coarser materials causing the formation of a central bar which diverts the flow towards the banks; and this flow increases lateral erosion on the banks.

As the valley widens, 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.

Deposition and lateral erosion of banks are essential for the formation of the braided pattern.

Or, alternatively, when discharge is less and load is more in the valley, channel bars and islands of sand, gravel, and pebbles develop on the floor of the channel and the water flow is divided into multiple threads.

These thread-like streams of water rejoin and subdivide repeatedly to give a typical braided pattern.

Conclusion

This is Ncert’s notes for the topic ‘Depositional landforms of the river’, written Upsc Exam. This article is also useful for Tnpsc and other state service exams. This covers the landforms formed by the deposition of rivers such as natural levees, point bars, braided channels, meanders, etc.