What are the Important Erosional Features Produced by River Action?

1. Pot-holes

These are cylindrical or bowl-like depressions in the rocky beds of streams, which are excavated in the floors of the streams by extensive, localized abrasion. These are commonly formed in the softer bed-rocks of the stream floor.

The drilling of the pot-holes are usually caused by stones swirled round by eddies. The boulders and pebbles etc. act as boring tools and themselves becomes rounded in the process, while the pot-holes continue to grow in size. Pot-holes may vary in dimensions ranging from a few centimetres to several metres.

2. Water falls

When the river flows over a surface with an originally broken relief, the river literally falls from a height and acquires normal flow against some distance below. Thus, sheer precipices along the course of a river give rise to waterfalls.

As we know, the surface over which a river flows is usually uneven and sometimes the rocks composing the surface are of different hardness.

The harder beds may withstand erosion by river, while the softer ones are relatively quickly worn down causing a local differ­ence of elevation in the channel. Two types of situations may arise.

(i) if a hard bed dips gently down the stream, that the steepness is not so pronounced, the river passing over it generally forms a rapid. These are water-falls of small-dimensions.

A & B = Outcrop of resistant rocks.

X & Y = Slope on which the river flows.

1,2,3, = Soft beds.

(ii) if the hard bed is horizontal or dips gently up the stream, the river will erode away partially the softer rocks beneath it.

In such cases, the hard rock may stand a« a ledge where from water jumps down, falling on lower beds with increased velocity, giving rise to what is known as a waterfall. Thus, when a river falls from a vertical escarpment it forms a waterfall.

Due to recession, gradually there takes place a diminution in height of a waterfall and with continued recession, waterfalls eventually degenerate into rapids and become extinct.

A fall that descends in a series of leaps is referred to as a Cascade, Gerosoppa (Jog) water-falls in the Swarvati River in Karnataka is the highest water-fall-in India.

3. River valleys

The typical river rises in the highlands and flows down due to the pull of gravity. The channels carved out by the flow of running water are commonly known as river-valleys. These are negative land forms of varying size and shape.

Three processes such as Valley- deepening, Valley-widening and Valley-lengthening are mainly responsible for development of valleys.

(a) Valley-deepening

Almost all the processes of river erosion cause valley-deepening in which the river-bed undergoes down-cutting giving rise to a narrow but deep valley.

The down-cutting of the valley floor takes place in the upper part of the course of any river, where it flows down the hill slopes i.e. in the mountain or highland tract. This process of valley deepening gives rise to important geological features like gorges or canyons.

Gorges or Canyons

When the river erosion is confined to down-cutting of its channel only, it gives rise to a deep-cut narrow valley, with steep or vertical walls known as Gorge or Canyon, in which the confined water rushes with tremendous force.

The Grand Canyon of Colorado river is the greatest Canyon in the world. It is 900 to 1800 metres deep, 60 to 90 metres wide and extends for a length of 300 kilometres.

Deep gorges are found in mountainous areas and plateaus where the rocks constituting the river­bed are chemically resistant and mechanically strong.

The process of down-cutting of the valley-floor does not continue forever. Gradually the rate of deepening slows down and commonly it stops when the base-level of erosion is reached at a late stage.

This happens when a river completes its course i.e. meets the sea or lake, whereby it loses its erosive power.

The Base-level of erosion of a river is the level of the basin in to which it falls and at the level of which the river loses its kinetic energy and therefore a river cannot excavate its channel below this level.

It should be noted that when a river falls into the sea or ocean the sea-level must be taken as the base-level; when it falls into a lake, the base-level is the level of the water in the lake. Base-level of erosion is commonly defined as the mean-sea-level produced inland by river erosion.

(b) Valley-widening

Lower down the highland tract there is a gradual reduction in the channel gradient of a river and the erosive power of the river to cut downwards becomes less; but the river starts cutting sideways with wide swinging curves and meanders.

A number of processes may be attributed to the phenomenon of valley-widening which are as follows.

(i) Lateral erosion

Through erosion of the valley sides mostly by hydraulic action and abrasion as well as through slumping of the materials in to the river because of undercutting of the valley-walls, the net effect of which is to broaden the channel.

(ii) Apart from the process of lateral erosion, the processes of rain- wash (or sheet wash), gullying, weathering and mass-was­ting and incoming tributaries etc. lead to the widening of valleys.

(c) Valley-lengthening

Lengthening of river-valley is usually achived by the process of headward erosion (or regressive erosion), where the long profile of the river develops from the base-level towards its sources. This can be illustrated as follows:

In the given figure,

A = Source of the river.

B = Base-level of the river.

AB = the surface along which the river had flowed initially.

It is assumed that along the surface AB, the points a₂ a1, and a represent various tributaries falling into the river. As such, there will be more water for discharge at point a (because here the river receives the water from a number of tributaries also) and erosion will be more intensive.

Thus the river bed in the aB sector is strongly cut and there develops a steep slope occupying the position abB. This enhances the velocity of the current and the bottom erosion above point a gets intensified with the result that the channel deepens on sector aa₁ and the river occupies the position a, b, bB.

The same process is repeated till the river valley is excavated to such an extent that instead of the original position the new smooth curve is in the shape of a parabola. Thus a backward extension or lengthening of the river-valley takes place.

Apart from the above process of head ward erosion, the other proc­esses responsible for lengthening of river valley are as follows:

(i) through increase in the size of their meanders;

(ii) uplift of the land or lowering of the sea-level results in extension of the valley through the newly exposed land etc.