- 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.
- The depositional action of a stream is influenced by stream velocity and the volume of river load.
- The decrease in stream velocity reduces the transporting power of the streams which are forced to leave some load to settle down.
- Increase in river load is effected through accelerated rate of erosion in the source catchment areas consequent upon deforestation.
Various landforms resulting from fluvial deposition are as follows:
Alluvial fans and cones
- Alluvial fans and cones 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).
- 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.
- Alluvial fans and cones are more or less similar except difference in their gradients. Alluvial fans have gentler slopes than the cones.
- 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.
- The narrow belt of ridges of low height built by the deposition of sediments by the spill water of the stream on its either bank is called natural levee or natural embankment.
- 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.
- 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”.
- 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).
- Conditions for Delta Formation: The ideal favourable conditions for the formation and growth of delta include:
- Suitable place in the form of shallow sea and lake shores.
- Long courses of the rivers (i.e. long rivers so that they bring enough amount of sediments).
- 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).
- 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).
- Large amount of sediment supply.
- Accelerated rate of erosion in the catchment area of the concerned river.
- 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.
- 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.
Classification of Deltas:
Deltas are divided into following six types on the basis of shape and growth: 1. Arcuate Delta 2. Bird-Foot Delta 3. Estuarine Delta 4. Truncated Delta 5. Growing Delta 6. Blocked Delta.
Type 1: Arcuate Delta:
- Such deltas are like an arc of a circle or a bow and are of lobate form in appearance wherein middle portion has maximum extent towards the sea whereas they narrow down towards their margins.
- Such deltas are formed when the river water is as dense as the sea water.
- The arcuate or semi-circular shape is also given to such deltas by sea waves and oceanic currents.
- The Nile Delta is the best example of arcuate deltas , which is also called as Nile type of delta.
- Arcuate deltas are formed of coarser materials including gravels, sands and silt. The main river is bifurcated into numerous channels known as distributaries.
- Such deltas are very often formed in the regions of semi-arid climate.
- Significant examples of arcuate delta include Ganga delta, Rhine delta, Niger delta, Yellow (Hwang Ho) delta, Irrawaddy delta, Volga delta, Indus delta, Danub Delta, Meekong Delta, Po delta, Rhone Delta, Leena delta etc.
- Arcuate delta is an example of growing delta as it grows towards the sea every year but the annual rate of growth varies from one delta to another. This process of seaward growth of deltas is called progradation.
Type 2 : Bird-Foot Delta:
- Bird-foot deltas resembling the shape of foot of a bird are formed due to deposition of finer materials which are kept in suspension in the river water which is lighter than the sea water.
- The rivers with high velocity carry suspended finer load to greater distances inside the oceanic water.
- The fine materials after coming in contact with saline oceanic water settle down on either side of the main channel and thus a linear delta is formed.
- It is interesting to note that the distributaries of the main channel also form linear segments of delta.
- These linear bars of sediments on either side of the distributaries of the main channel resemble the fingers of human hand.
- Such delta is, thus, also called finger delta. The Mississippi delta exhibits the best example of bird-foot delta.
Type 3 :Estuarine Delta:
- The deltas formed due to filling of estuaries of rivers are called estuarine deltas.
- Those mouths of the rivers are called estuaries which are submerged under marine water and sea waves and oceanic currents remove the sediments brought by the rivers.
- There is continuous struggle between the rivers and sea waves wherein the former deposit sediments while the latter remove them.
- Whenever rivers succeed in depositing sediments at their submerged mouths, long and narrow deltas are formed.Such deltas are called estuarine deltas.
- The deltas of Narmada and Tapi (formerly Tapti) rivers of India are the examples of estuarine deltas.
- The other significant examples of estuarine deltas include Mackenzie delta, Vistuala delta, Elb delta, Ob delta, Seine delta, Hudson delta etc.
Type 4: 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.
Type 5: Blocked Delta:
- Blocked deltas are those whose seaward growth is blocked by sea waves and ocean currents through their erosional activities.
- The progradation of deltas may also be hampered due to sudden decrease in the supply of sediments consequent upon climatic change or management of catchment areas of concerned rivers.
Type 6: Abandoned Delta:
- When the rivers shift their mouths in the seas and oceans, new deltas are formed, while the previous deltas are left unnourished. Such deltas are called abandoned deltas.
- The Yellow (formerly Hwang Ho) river of China has changed its mouths several times and thus has formed several deltas.
- For example, the present delta of the Yellow river is to the north of Shantung Peninsula while the previous delta was deposited to the south of the peninsula. The western part of the Ganga delta, which is drained by the Hoogli River is an example of abandoned delta.