Rivers deposit sediments in different parts of their courses and thus form three major types of landforms which are called constructional landforms such as alluvial fans cones, natural levees and deltas.
Depositional Landform # 1. Alluvial Fans and Cones:
Alluvial fans (fig. 18.14) and cones (fig. 18.15) due to accumulation of materials are always formed at the base of foothills where there is abrupt drop (decrease) in the channel gradient. The transporting capacity of the streams decreases enormously at the foothill zones while they leave the mountains and enter the plain topography because of substantial decrease in their velocity consequent upon decrease in channel gradient.
Consequently, load consisting of finer to coarser and big-sized materials coming from upstream is deposited at the point of break in slope or foothill zone and thus alluvial fans are formed. There is sorting of materials in the alluvial fans. The size of sediments decreases outward from the apex (which is towards the hills) of the fans towards their outer margins (distal side).
Morphology of alluvial fans:
The shapes of alluvial fans are usually semi-circular or arcuate, the appex of which is located at the mouth of narrow opening through which the stream comes out of the hills and enter the surface of low height and gentle slope. The shape of alluvial fans is described in two perspectives, planimetric and volumetric.
The planimetric consideration involves two dimensional shape whereas volumetric perspective involves three dimensional shape of the fans. The alluvial fan, in planimetric perspective, is similar to the shape of a sector of a circle. In volumetric perspective an alluvial fan has a shape of a part of a cone. The longitudinal profile of an alluvial fan is concave at its apex while the transverse profile (which is parallel to the mountain front) is convex.
Though the size of alluvial fans varies significantly but generally the diameter of fans ranges from a few kilometres to several hundred kilometres. A.B. Mukerji has studied the Chandigarh Dun Alluvial Fans developed at the foothills of Siwalik Range.
According to him the Balad, Kiratupur and Banasar alluvial fans have the area of 20.75 km2, 26.5 km2, and 22 km2 respectively. Several examples of palaeo-alluvial fans are seen in the Dun Valley of Uttaranchal. Similarly, numerous alluvial fans are found along the Himalayan piedmont in the northern part of the Brahmaputra valley.
The slopes of fans are much gentler than those of alluvial cones. Larger alluvial fans have average slope of less than one degree but smaller fans are characterized by gentle to moderate slopes (5 degree). Alluvial cones have average slopes of about 15 degree. Alluvial cones are made of coarser materials than the alluvial fans. The belts of alluvial fans and cones developed at the foothill belt of the Siwalik Himalayas (sub-Himalayan belt) are composed of gravels, pebbles, cobbles, shingles and coarse sands. These depositional, features at the foothills of the Himalayas are called ‘bhabar’.
Structure and origin of alluvial fans:
Alluvial fans and cones are more or less similar except difference in their gradients. Alluvial fans have gentler slopes than the cones. Alluvial cones are formed when the slope of hill or mountain front through which the stream descends is steep (fig. 18.15) so that debris collect at the foot of the hills. The formation of alluvial cones requires more debris but little water. On the other hand, alluvial fans are formed when the slope of hill or mountain front is gentle and water is more than debris so that debris spread away to assume actuate shape. The origin and development of alluvial fans is simple.
The velocity of streams while descending through the hill or mountain front is remarkably reduced due to sudden decline in slope gradient. This phenomenon causes substantial decrease in the transporting capacity of the streams. Consequently, streams become overloaded in relation to their reduced transporting power and thus they unload enormous quantity of debris at the foothill zone (at the point of break in slope) and thus fans are formed.
There is gradual growth and development of alluvial fans in two ways. Firstly, the fans grow in size due to continuous increase in their areal extent because of regular supply of debris from upstream section. Secondly, the alluvial fans grow in height because of gradual deposition of debris upon debris.
The gradual increase in height causes increase in slope gradient. An alluvial fan may be transformed into an alluvial cone if the slope gradient is significantly increased. Sometimes, a series of alluvial fans are formed along the piedmont zone. They grow in size and are ultimately coalesced to form an extensive fan which is called compound alluvial fan. The most extensive compound alluvial fans form undulating and sloping alluvial plain in front of peidmont zone. Such plain is called piedmont alluvial plain.
Depositional Landform # 2. Natural Levees:
The narrow belt of ridges of low height (fig 18.16) built by the deposition of sediments by the spill water of the stream on its either bank is called natural levee or natural embankment. It may be pointed out that not all the streams build natural levees. Levees are formed due to deposition of sediments during flood periods when the water overtops the river banks and spreads over adjoining flood plains. Long ridges of low height are formed parallel to the river valleys. Average height of natural levees is within 10 metres.
The natural levees of the Mississippi river ranges between 6m and 7.6m. Natural levees limit the lateral spread of river water except during severe and widespread floods. Natural levees are more or less stabilized landforms which attract human settlements. Sometimes, natural levees are also used for agricultural purposes because water table of groundwater is very high.
Generally, natural levees help in checking the floods but when breached they cause severe catastrophic floods inflicting heavy loss of human health and wealth. Since the channel is more or less confined within the natural levees and hence there is continuous sedimentation which causes gradual rise of the river beds (valley floor).
Consequently, sometimes the bed of the stream becomes higher than the adjoining flood plain. Breach of natural Levees in such situation causes sudden catastrophic floods because the river water gushes in the flood plains and settlements with high velocity beyond imagination. Such cases of breaches of natural levees and consequent severe floods are very often reported from the Yellow river (formerly Hwang Ho) of China. This was the reason that the Hwang Ho was called “Sorrow of China”.
Depositional Landform # 3. Delta:
The depositional feature of almost triangular shape at the mouth of a river debouching either in lake or a sea is called delta. The word delta, derived from Greek letter, was first used by Greek historian Herodotous (485-425BC) for the triangular depositional feature at the mouth of the Nile River. Whether small or large, almost every river forms delta. The size of delta of major and small rivers all over the world varies from a few square kilometres to thousands of square kilometres (e.g. Ganga delta in India and Bangladesh).
The size of delta depends on the rock characteristics, vegetal cover, rate of erosion, amount of annual rainfall etc. The depth of sediments has been reported to be hundreds of metres. For example, the average depth of sediments in Mississippi delta is about 610m. The shape of delta also varies from one river to the other. Common shapes of delta are arcuate shape, bird-foot shape, elongated shape etc.
1. Conditions for Delta Formation:
The ideal favourable conditions for the formation and growth of delta include:
(1) Suitable place in the form of shallow sea and lake shores.
(2) Long courses of the rivers (i.e. long rivers so that they bring enough amount of sediments).
(3) Medium size of sediments (because if the sediments are very fine, they would be carried in the sea in suspension for longer distances and if they are very coarse, they would soon settle down at the sea bottom, and hence no delta would be formed).
(4) Relatively calm or sheltered sea at the mouths of the rivers (so that ocean currents, strong waves or high tidal waves do not interfere with the natural process of gradual sedimentation and delta formation).
(5) Large amount of sediment supply.
(6) Accelerated rate of erosion in the catchment area of the concerned river.
(7) Almost stable condition of sea coast and oceanic bottom (because sea coast subjected to frequent emergence or submergence caused by tectonic movements does not allow regular sedimentation and thus disfavours delta formation) etc.
2. Delta Formation:
The formation of delta starts with the deposition of sediments if the aforesaid favourable conditions are available. The sedimentation takes place regularly at the mouth of the river, on the sides of stream channel, in the bed of the river and in front of river mouth where the river debouches in the sea. Thus, an extensive fan is formed which slopes towards the sea. Several such fans are formed at the mouth of the river.
These fans gradually grow towards the sea. Ultimately these fans are coalesced and a delta is formed. These deposits obstruct the free flow of main river and hence it is divided into several branches. This process of segmentation of main stream is known as bifurcation. Thus, the main channel is bifurcated into numerous small and narrow sub-channels which are called distributaries and the stream with numerous distributaries is called braided stream.
3. Structure of Delta:
The deposition of sediments or say materials takes place in such a way that larger materials (e.g., gravels, pebbles, cobbles etc.) are deposited towards the coastal land and the size of sediments gradually decreases with increasing distance from the coastal land towards the sea.
An average delta consists of three beds of sediments e.g.:
(1) Topset beds,
(2) Foreset beds, and
(3) Bottomset beds.
The topset beds represent the uppermost bed of sediments of a delta. These are quite extensive, wide and gentle in slope. These represent delta plains. The topset beds are relatively higher than sea level. The series of steeply dipping beds inclined towards the sea are called foreset beds which are always under sea water.
The lowest beds are called bottomset beds because they rest on sea bottoms.
Deltas undergo subsidence because of:
(1) Gradual sedimentation and consequent increase in the weight of delta materials,
(2) Compaction of sediments caused by load of sediments,
(3) Enormous thickness of sediments, and
(4) Isostatic adjustment etc.
4. Growth of Delta:
No doubt, there is growth in all types of delta towards the sea but the rate of growth varies considerably from one situation to the other.
The nature and rate of delta growth depends on a variety of factors e.g.:
(1) Velocity of the stream flow,
(2) Nature of sea waves,
(3) Supply of sediments,
(4) Oceanic currents, and
(5) Slope and height of deltas etc.
Most of the sediments are unloaded at the mouths of the rivers if their velocity is extremely low and thus the growth of deltas toward the sea becomes sluggish.
On the other hand, streams with greater velocity transport their load far greater distance in the sea and thus allow faster rate of delta growth, but deltas formed in such situation are narrow and long. Strong sea waves and oceanic currents retard the growth of deltas because they erode and remove the sediments away. The sliding of materials from higher deltas towards the sea also encourages the seaward growth of deltas.