What is the Gological-Action of River?

Rivers carry excess water from the land to the sea. In doing so, they erode valleys and help in shaping the earth’s surface. They transport rock debris and dissolved materials and eventually they deposit most of their sediments in the oceans.

River activity in combination with weathering and mass-wasting predominates by far over other types of erosion such as wind, ice or marine. Accordingly rivers are considered as the most important of the geomorphic agents in bringing about the degradation of the land surface.

The geological activity of river is divided chiefly into three parts as:

1. Erosion.

2. Transportation.

3. Deposition.

Erosion

The term ‘erosion’ is applied for rock breakdown by the dynamic action of any geomorphic agent like moving water, blowing wind, glaciers etc. It is usually defined as the sum total of the process of wearing away of the rocks by the physical forces and chemical factors associated with the natural agencies.

Rivers flowing over various rocks break them down purely mechanically. The chemical action of rivers is minimal. A wide variety of processes are included in river erosion as:-

(A) Hydraulic action.

(B) Abrasion.

(C) Attrition.

(D) Cavitation.

(E) Corrosion.

Of the above processes, the first four are the types of mechanical erosion and the last one belongs to the type of chemical erosion. Erosion is mainly responsible for the development of river valleys through river bed erosion (i.e. down-cutting of the river bed) and lateral erosion.

While river-bed erosion is predominant during the initial stages of development of a river, lateral erosion becomes signifi­cant during the later stages. It may, however, be remembered that the processes of erosion operate in their own ways, both for down cutting or lateral cutting of river-valleys.

Hydraulic Action

It is the process of mechanical loosening or removal of the material by the action of the water alone. As we know, it is the turbulent flow of water which can loosen rock and soil particles along the river channel and move them away.

Maximum turbulance gives maximum capacity for erosion. It is, therefore, that maximum erosion takes place in the belt of greatest turbulance along the outside of a stream bend. The effectiveness of hydraulic action of a river is dependent on the following factors:

Gradient

It is the angle of inclination of two points along the stream divided by the horizontal distance between these two points.

(b) Velocity of the stream.

(c) Width, depth and shape of the channel.

(d) Discharge i.e. the amount of water flowing in the river at a given time.

The stream channel and banks are eroded by moving water due to the forces inherent in them. Hydraulic action’s effectiveness is best observed where the flow of water is responsible for undercutting banks of unconsolidated alluvium.

(b) Abrasion

The material which are being carried away by the running water act as tools of destruction and during their transportation, because of their rubbing against the surface of the bed-rock, they bring about a scrapping of the surface.

This process of wearing-away of surfaces by mechanical processes such as rubbing, cutting, scratching, grinding, polishing etc. is known as Abrasion or Corrosion.

Without the presence of rock particles, rivers cannot scratch and scour their channel or in other words, the rock-wastes which are being carried in the river current as load are responsible for the abrasive action of the river. Three types of situations may arise, as follows on the basis of the hardness of the transported material:

(i) if the rock-waste is hard and the bed-rock is soft abrasion of the bed-rock is more pronounced.

(ii) if rock-waste is hard and bed-rock is also hard, it results in the polishing of the bed-rock.

(iii) if rock-waste is soft and bed-rock is hard, abrasion of the bed­rock is not remarkable, since the rock-waste, in such cases, is itself eroded away.

Abrasion may be vertical or lateral. While vertical abrasion causes deepening of the channel, lateral abrasion gives rise to valley- widening.

The abrasive action of the river is considered to be the most important means of erosion in bed-rock which are too strong to be affected by simple hydraulic action.

(c) Attrition

The products of abrasion and hydraulic action are carried away in the river-flow, which often collide among themselves, with the bed-rock and also with the sides during their transit and in turn get themselves teared.

This process of mechanical wear and tear of the transported rock fragments through which big boulders are gradually reduced in size and finally reach the size-grade of sand and silt, is known as attrition.

Since, in this process rock-fragments in transit suffer mutual collision and repeated impacts, their irregularities and angularities are worn out and they become spherical and rounded.

(d) Cavitation

It is sometimes considered as a type of hydraulic action. It is particularly observed where river water suddenly acquires ex­ceptionally a high velocity such as at water-falls. Rapid increase of velocity reduce the internal pressure in the water.

It has now been established that where stream velocity exceeds 12-14 m/sec, the water pressure at that point equals vapour pressure of water and small bubbles of water vapour form and the water foams.

As soon as the velocity is decreased due to friction against the floor or sides of the channel, the internal pressure increases again and the bubbles become unstable. The bubbles then collapse suddenly and violently resulting in shock waves which deliver hammer-like blows to the adjoining surface (bed and banks) producing a crop of rock-fragments to be carried away by the river flow.

This process, thus, produces hollows in river beds which, in due course, are developed into pot-holes.

Sometimes, highly turbulent rivers in rocky channels erode their beds by hydraulic plucking, in which blocks of bed-rocks etc. are lifted out by suction in strong eddies spiraling up around vertical axes.

This sucking out of the rock pieces produces depressions or holes within the rock, which may develop into pot-holes in due course of time.

Thus the rate of erosion is speeded up due to the process of cavitation.

(e) Corrosion

The chemical processes of rock-erosion by river-water are known as Corrosion or Solution. In the presence of some aiding substances like alkali matter and gases like carbon-dioxide etc, river water is capable of dissolving matter from minerals that constitute the bed-rock in the channel and the rock particles in transport It has been observed that whenever the water containing carbon-dioxide comes in contact with limestone, it gets dissolved into the water easily.

The effectiveness of the process of corrosion depends much on the composition of the rock constituting the river bed (i.e. its solubility) and the composition of the river-water.