Geological agent (Water)

Running water

Running water is the important agent that brings changes to the earth's surface. A part of the water resulting from the rain and the melted snow enters the soil by infiltration and contributes to the accumulation of the underground water, a part of it is returned to the atmosphere through evaporation and other part flows over the ground surface as runoff to the lower levels due to the gravity. Here, the runoff is of two types: overland flow and the channel flow.

The river is the body of the running water that carries sediments and flows in the definite paths. All the river and the stream flow along the channel of their own and the amount of water passing through the channel per unit time is the discharge. In the rivers, the natural water flow is categorized by its velocity and the gradient.

The flow of water is categorized as:

Laminar flow: In laminar flow, the water particles travel in the parallel path with the flat gradient and the low velocity,

Turbulent flow: In turbulent flow, the rate of the flow of water is not constant with respect to the velocity.

The geological work of the river can be divided into three following parts:

1. Erosion 2. Transportation 3. Deposition

1. Erosion

The erosion of the running water occurs by means of several processes which are:

• Hydraulic Pressure

The pressure created by the water for the removal of the materials is known as hydraulic pressure. The carrying effect of the water is most effective in those areas where the bedrocks are highly fractured and cemented. The strong current of the water may enforce removing the rock material from the bed of the river.

• Abrasion

The abrasive or the corrosive ability of the river depends on its load and the amount of the material carried by the water at the given time. The sand grain, pebble or the boulder carried by the water becomes the cutting tool capable of deepening and the widening of the river bed. The abrasion is the most effective where the river flow is rapid, river load is heavy and much debris is being rolled along the bottom. The polishing of the bedrock occurs, if the rock and the bedrock are hard but if the rock waste is hard and bedrock is soft, then the abrasion is prominent. If the waste is soft and the bedrock is hard then the rock waste is eroded itself.

• Attrition

Attrition is the process of the mechanical breakdown of the transported rock fragments due to the impacts among themselves. Such mutual collision naturally causes the further wear and tear of their own selves reducing the big boulders to the size of the sand and the silt. In this process, the irregularities and the angularities are worn out and they become spherical and rounded. The abrasion and the attrition always work together to produce the fine rock particles which are transported to the greater distances.

• Cavitation

The rapid increase in the velocity of water reduces the internal pressure when the pressure at that point equals vapour pressure of water, small bubbles of water form and the water foams. As soon as the velocity decreases the internal pressure increases again and the bubbles become unstable and then collapse suddenly and violently resulting in the shock waves thus producing hollow waves in the river beds. Thus, this process is known as cavitation.

• Corrosion

Corrosion is the chemical process of the rock erosion by river water. In such river erosion, carbonic acid is present which can dissolve the mineral present in the bedrock. The rocks such as limestone, dolomite are affected by this process.

2. Transportation

The load transportation from the river can be categorized as:

• Chemical transportation

In chemical transportation process, the matters are transported in the form of the dissolved state. The load which is in the solution becomes the part of the flowing water.

• Mechanical transportation

The mechanical transportation depends upon the velocity of the river current and the density of the transporting load.

a. Suspended load

The fine ground particles like clay and silt that are in the form of the suspension are generally less than 0.5mm.

b. Bed Rock

The coarse-grained materials like sand, pebbles, gravels, cobbles etc are transported close to the channel floor by rolling or sliding which may take place either by saltation or by traction.

3. Deposition

The deposition of the transported materials takes place due to the following reasons as:

• The decrease in the velocity of the river.
• Decrease in slope or gradient
• Decrease in volume of the water.
• Change in the channel.
• Chemical precipitation.

Erosional features associated with rivers

• Potholes

Potholes are the cylindrical or the bowl-shaped depressions of the dimensions ranging from the few inches to the several feet. These are formed upon the valley floor made up of comparatively softer rocks when the boulders and the pebbles of the harder rocks are caught up in the eddies of the water and are allowed to have the swirling motion upon the floor of the channel which results in the formation of depressions.

• Waterfalls

The surface upon which the river flows is uneven and the rocks composing it are of different hardness. The channel rapids are formed within the alternation of the soft and the hard rocks. When the river flows over the surface of an originally broken relief, waterfalls will be formed in its channel.

• River valleys

The rivers originate from highlands and flows down due to the pull of the gravity. The path carved by the flow of the water is known as river valley. During the formation of the river valleys, the processes like valley deepening, valley widening and valley lengthening are essential. During valley deepening, the river- bed undergoes down cutting giving rise to the narrow but deep valley which also gives rise to the geological features like gorges or canyons. When the highlands lower downs, the erosive power of the river cut downward but it starts to cut sideways termed as valley widening. The valley lengthening is usually achieved by the river at the old stage.

• Georges

When the river erosion is confined only downward, it gives rise to the deep cut narrow valley, with the steep walls known as gorge or canyon. The greatest canyon in the world is the Grand canyon of Colorado River. The process of the downcutting of the river stops when the base level of the erosion is reached. The base level of the erosion is the level of the basin into which it falls and at the level of which the river loses its erosive power.

References

Santra, S. (2004). Environmental Science . India: New Central Book Agency (p) Ltd.

T., R. (2008). Towards a Sustainable Future . India: PHI (p)Limited.

Keller, E.A.Environmental Geology. Columbus, Ohio: Charles E. Miller Publishing Company,Belland Howell Company, 1985.