Over millions of years erosion has shaped and changed the appearance of the earth's surface. Erosion is constantly wearing away the surface of the earth. Through weathering, erosion and deposition the topography changes, at times very rapidly and other times very slowly. The agents responsible for these changes are wind, ice and moving ice and most importantly flowing water. Almost one sixth of the total amount of rainwater that reach the surface runs back to the sea by means of rivers. It is these rivers that act as an important agent for erosion and deposition.
Rivers receive its water from various sources – springs, geyser, rain, melting ice and snow, lakes, swamps, etc. There is a constant link between the availability of water and the hydrological cycle. The atmosphere is a closed system, which means, no water can leave the system nor added. Although, water can temporarily be removed from the hydrological cycle, when it is trapped in lakes, glaciers, dams or underneath the surface.
Not all water is therefore available for run-off for example in streams. Some water are ‘lost’ or retained–
Run-off = Precipitation – (infiltration + evaporation + transpiration + surface retention)
Water will flow in rivers to the oceans. River can be classified into FOUR types of rivers:
i) Permanent rivers
ii) Periodic rivers
iii) Episodic rivers
iv) Exotic rivers
