Fluvial Depositional landforms
In geography, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. Landforms are small to medium tracts or parcels of the earth s surface.
There are two types of landforms created by the fluvial process - Fluvial Erosional Landforms and Fluvial Depositional Landforms. Fluvial Depositional landforms are made by river sediments brought down by extensive erosion in the upper course of the rivers.
Fluvial Depositional Landforms
- Rocks and cliffs are continually weathered and eroded in the youth stage or upper course of the river.
- The river moving downstream on a level plain brings down a heavy load of sediments from the upper course.
- The decrease in stream velocity in the lower course of the river reduces the transporting power of the streams which leads to deposition of this sediment load.
- Coarser materials are dropped first and finer silt is carried down towards the mouth of the river
- This depositional process leads to the formation of various depositional landforms through fluvial action such as Delta, Levees and Flood Plain etc.
- An alluvial fan is a cone-shaped depositional landform built up by streams, heavy with sediment load.
- Alluvial fans are formed when streams flowing from mountains break into foot slope plains of low gradient.
- Normally very coarse load is carried by streams flowing over mountain slopes. This load gets dumped as it becomes too heavy to be carried over gentler gradients by the streams
- Furthermore, this load spreads as a broad low to a high cone-shaped deposit called an alluvial fan that appears as a series of continuous fans.
- 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.
- Floodplain is a major landform of river deposition.
- Deposition develops a floodplain just as erosion makes valleys.
- Rivers in the lower course carry large quantities of sediments
- Large sized materials are deposited first when stream channel breaks into a gentle slope.
- Sand, silt and clay and other fine sized sediments are carried over gentler channels by relatively slow-moving waters
- During annual or sporadic floods, these materials are spread over the low lying adjacent areas. A layer of sediments is thus deposited during each flood, gradually building up a floodplain
- In plains, channels shift laterally and change their courses occasionally leaving cut-off courses which get filled up gradually by relatively coarse deposits.
- The flood deposits of spilt waters carry relatively finer materials like silt and clay.
- Active Floodplain - A riverbed made of river deposits is the active floodplain.
- Inactive 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.
- Delta plains - The floodplains in a delta are called delta plains.
- This is an important landform associated with floodplains.
- They are found along the banks of large rivers.
- They are low, linear and parallel ridges of coarse deposits along the banks of rivers on both sides due to deposition action of the stream, appearing as natural embankments.
- At the time of flooding, the water is spilt over the bank. As the speed of flow of the water comes down, large sized sediments with high specific gravity are dumped along the bank as ridges.
- They are high nearer the banks and slope gently away from the river.
- Generally, the levee deposits are coarser
- When rivers shift laterally, a series of natural levees can form.
- Artificial embankments are formed on the levees to minimize the risk of the floods.
- But sudden bursts in the banks due to the pressure of water can cause disastrous floods.
- An example of such flood can be seen in Hwang Ho river which is also called China s sorrow .
- Point Bar is also associated with floodplain
- Point bars are also known as meander bars.
- A point bar is a depositional feature
- It is formed by alluvium that accumulates in a linear fashion on the inside bends of streams and rivers below the slip-off slope.
- They are found on the convex side of meanders of large rivers.
- They are almost uniform in profile and in width and contain mixed sizes of sediments.
- Long and narrow depressions can be found in between the point bars where there is more than one ridge
- Rivers build a series of them depending upon the water flow and supply of sediment.
- As the point bars are built by the rivers on the convex side, erosion takes place on the concave side of the bank.
- In large flood and delta plains, rivers rarely flow in straight courses. Loop-like channel patterns called meanders develop over flood and delta plains
- Normally, in meanders of large rivers, there is active deposition along the convex bank and undercutting along the concave bank.
- If there is no deposition and no erosion or undercutting, the tendency to meander is reduced.
- 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 the slip-off bank
Factors responsible for meandering of the rivers
- 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
- Coriolis force acting on the fluid water deflecting it like it deflects the wind
- In the lower course of a river, meanders become very much more pronounced
- As meanders grow into deep loops, the same may get cut-off due to erosion at the inflexion points and are left as independent water bodies, known as ox-bow lakes.
- Through subsequent floods that may silt up the lake, oxbow lakes are converted into swamps in due course of time. It becomes marshy and eventually dries up
- A braided channel consists of a network of river channels divided into multiple threads and separated by small and often temporary islands called eyots .
- Braided channels are commonly found where water velocity is low and the river is heavy with sediment load
- Deposition and lateral erosion of banks are essential for the formation of the braided pattern.
- There is the formation of central bars due to selective deposition of coarser material which diverts the flow towards the banks causing extensive lateral erosion
- As the valley widens due to continuous lateral erosion, 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.
- Deltas are fan-shaped alluvial areas, resembling an alluvial fan
- This alluvial tract is, in fact, a seaward extension of the floodplain
- The load carried by the rivers is dumped and spread into the mouth of the river at sea. Further, this load spreads and piles up as a low cone
- Unlike in alluvial fans, the deposits making up deltas are very well sorted with clear stratification. The coarsest sediments are deposited first and the finer sediments are carried out further, into the sea.
- Deltas extend sideways and seaward at an amazing rate
- As the delta grows, the river distributaries continue to increase in length and Delta continues to build up into the sea.
- Some deltas are extremely large. For example, the Ganges delta is as big as the whole west of Malaysia
Types of Deltas
There are great variations in size, shape, growth and importance of Deltas. A great number of factors influence the eventual formation of deltas such as depth of the river, sedimentation, sea-bed, character of tides, waves and currents etc. owing to these factors several types of deltas can be found.
Bird s foot delta It s a kind of delta featuring long, stretching distributary channels, which branch outwards resembling the foot of a bird. Deltas that are less subjected to wave or tidal action culminate to a bird s foot delta. Example the Mississippi River has a bird s foot delta extending into the Gulf of Mexico
Arcuate delta Arcuate is the most common type of delta. This is a fan-shaped delta. It s a curved or bowed delta with the convex margin facing the sea. Arcuate deltas have a smooth coastline due to the action of the waves and the way they are formed. Examples - The Nile, Ganges and Mekong river deltas
Cuspate delta A few rivers have tooth-like projections at their mouth, known as the cuspate delta. Cuspate deltas are formed where the river flows into a stable water body (sea or ocean). The sediments brought down by the rivers collide with the waves. As a result, Sediments are spread evenly on either side of its channel. Example Ebro river delta in Spain
Estuarine delta some rivers have their deltas partly submerged in the coastal waters to form an estuarine delta. This may be due to a drowned valley because of a rise in sea level. Example Amazon river delta
Conditions favourable for the formation of delta
- Active vertical and lateral erosion in the upper course of the river to provide extensive sediments to be eventually deposited as deltas
- The coast should be sheltered preferable tideless
- The sea adjoining the delta should be shallow or else the load will disappear in the deep waters
- There should be no large lakes in the river to filter off the sediments
- There should be no strong current running at right angles to the river mouth, washing away the sediments