At any one moment about 1250 km3 of water flows through all the rivers on the earth’s surface. Actually the rivers transport only 0.003% of all fresh water, but the work performed by them makes it the most important agent of landmass denudation, since erosion by running water is the normal means of land sculpture over the major portion of the land surfaces of the earth.
It is undoubtedly true that major land-forms of the earth’s surface have their form and distribution from the direct effects of tectonic movements. However, even these land-forms, to a large extent, are modified by the agents of weathering and erosion.
Even though a valley may occupy a structural depression, the power of running water to excavate a valley with complete disregard for tectonic movements and their direct effects cannot be doubted.
As soon as any new landmass is uplifted with reference to sea level, the work of rivers begins. After uplifting of a new landmass by tectonic forces running water on the uplifted land begins strong vertical cutting down towards the base level.
A stream flowing on such an uplifted land will gradually enlarge its channel in all directions by deepening and widening it through the erosion of underlying rocks. It is well to remember that fluvial erosion is performed in the following ways: (i) hydraulic action, (ii) abrasion or corrasion, (iii) attrition, and (iv) corrosion.
Hydraulic action involves several hydraulic processes which cooperate in causing mechanical loosening, lifting and removal of material by running water. Loose deposits are swept away, the initial lifting force being provided by turbulence. Sometimes in the flowing water bubbles of water vapour form in the highly turbulent eddies.
When such bubbles suddenly collapse, shock waves are produced which shatter the adjoining surface liberating particles that are carried away by the flowing water.
This is called the process of cavitations which accelerates the rate of erosion. Actually cavitations produces hollows in the river beds which later on develop into pot-holes. It plays major role in the erosion of plunge pools below waterfalls.
Abrasion or Corrasion:
Corrasion is the wearing away of the river bed or banks, using the load as the ‘grinding tool’. The rivers transport boulders, pebbles, sand and silt which together work as the grinding tools. With the help of these so-called tools, even the hardest bedrocks of the stream channel are excavated and smoothed.
Particles produced by this process do their own corrasional work, and are constantly worn down to become smaller in size. This wear and tear on the stream bed load is called ‘attrition’. It is largely because of attrition that large streams normally carry only silt to the oceans.
In other words, attrition is the wear and tear suffered by the transported materials themselves. Thus, such materials as are carried-along as bed-load are broken down, smoothed and rounded. These finer materials are more easily transported by the rivers.
The term ‘corrosion’ involves all the solvent and chemical activities of river water to enlarge and deepen its channel. This process is nothing but the chemical weathering of the stream channel. The corrosion may be accomplished through the processes of hydration, solution or hydrolysis.
From the foregoing discussion of varying methods of erosion, it is clear that there exists a close relationship between weathering and the erosive work of the streams.
In case mechanical and chemical weatherings are rapid, the down-cutting by the running water is bound to be great. If the nature of the rocks does not allow rapid weathering, the valley development is adversely affected. In addition, under the protective cover of dense natural vegetation, stream erosion is retarded.