CHANGES IN THE EARTH'S CRUST


          The changes in the earth's crust are caused by certain forces or agents. These forces may be divided into two groups, namely, external and internal. (1) External forces include climate, running water, wind, glacier, underground water, sea waves, plants and animals. The changes caused to earth's surface by the internal forces are slow but continuous. (2) Internal forces include earthquake, volcanic activity etc.

 RIVER

  Running water on the surface of the earth is called river. Rivers are one of the most important agents causing change to land form. The rain water, or the water from melting snow, or the water that remains collected in the rocks and soils flow down the slopes. The water from these sources keeps flowing down certain channels. The small channels containing water are known as springs or streams. As a river flows down, smaller ones join in its course. These are known as tributaries. The place where a tributary meets the main river is called a confluence. The area from which a river and its tributaries receive water is called a river basin or a catchment area. The land boundary between two river basins is known as the divine or water-parting.

1.1 Sources of River
a.     Springs
b.     Rainfall
c.      Glacier
d.     Lakes

1.2 Kinds of River
a.     Perennial Stream and River
b.     Intermittent Stream and River
c.      Ephemeral Stream and River

1.3 Three Definite Parts (Stages) of a River
a.     The Upper or Mountain Course
b.     The Middle or Plain Course
c.      The Lower or Deltaic Course

a. The Upper or Mountain Course

          In the upper mountainous course, a river passes through a steep slope. Its water therefore, rushes down with great speed. Under such a condition the water can dig the river bed very deep and carry or roll down heavy boulders and pebbles. Erosion and transportation are the main activities of a river in the upper course. A river has deep and narrow channel in this stage.

b. The Middle or Plain Course

          As a river leaves the hilly area and falls into a plain, its middle course starts. The slope of the river bed along the downstream is less on a plain. Hence, the speed of the current is also less, compared to the upper course. As the river is fed by many tributaries its volume of water increase. The additional sediments carried by the river during the floods are deposited in this part.

c. The Lower or Deltaic Course

The last part of a river near its mouth is called the lower course. In this part the slope of the river bed is not very deep. The current is also very slow. Deposition is the main activity of a river at this stage. The slit and other materials carried by the river are deposited. The slit carried by the river water is deposited at the mouth of the river. In the course of time, the slit thus deposited forms a triangular island at the mouth of the river. This is known as delta.

1.4 Works of River

a.     Erosion
b.     Transportation
c.      Deposition

a.     Erosion

          The river erodes the rock in two ways known as chemical erosion and mechanical erosion. If the river flows through the area of soluble rocks, the dissolved rocks flow along with the river. This type of erosion is known as chemical erosion. With its strong current, the river pushes the rock on the way and breaks it into pieces. This is called mechanical erosion. The pieces of rocks flowing with water dash against each other and are rubbed. So they break and reduce into round and shiny pebbles. The grinding is also mechanical erosion.
          The river cuts the surface in two ways-lateral or sideward cutting and vertical or downward cutting. When the river cuts its sides, it is called lateral cutting. So the river-valley is wide. In the vertical cutting the river depends on its bottom. So the river-valley is deep

b. Transportation

          The river carries with it pebbles, sand, stone and other solid things. This work of the river is called transportation. The things that are carried by the river are called load which are divided into three types:
a.     Load in solution:  salt, lime etc.
b.     Floating and suspended load:  sand-particle, sand etc.
c.      Traction load: piece of rock

c. Deposition

          When the river comes down to plain, its slope becomes gentle. This reduces the energy and the competence of the stream. As the competence decreases, the rock fragments begin to settle down. First, larger fragments settle and, then, they are followed by smaller fragments. Due to the decreasing energy which hampers transportation the rock-flour starts settling down. This activity is known as deposition. The deposition is not only dependent upon slope but also upon the amount of water. Deposition takes place either due to decreasing slope or the fall in the amount of water of the river. Alluvial plain, alluvial fan, flood plain, ox-bow lake, delta, etc are the examples of deposition.

1.5 Importance of River

a.     Plains which are made by deposition of river are very fertile.
b.     We can irrigate the land from river water.
c.      We can get drinking water from the river.
d.     We can generate hydroelectricity from the river.
e.      We can do fishing in the river.
f.       We can extend water transportation in the big rivers.
g.     We can get constructive materials like sand, stone from the river.
River banks are good places to establish big industries.