Oceans cover around 71 percent of the earth’s surface and the remaining 29 percent is the land area. They are estimated to contain 1.3x10 18 tons of water. There are five oceans: the Atlantic, Pacific, Indian, Arctic and Antarctic. There are smaller bodies of water called seas, sounds, bays and gulfs. All of these are connected to each other.
The Pacific Ocean is the largest and the deepest. It covers an area of about 16, 62, 40,000 sq. km. the second largest is the Atlantic ocean which has an area equal to half of the Pacific Ocean. The Indian oceans are the third largest. It extends from Kanyakumari (Cape Comorin) in India to the South Pole, Antarctica. The Arctic Ocean surrounds the North Pole. It is completely frozen and unnavigable. Some people consider Antarctica also to be an ocean.
The ocean water is saline and not if for drinking. The average salinity is 3.5 percent. The main source of salinity of oceans is the water of the rivers that carry with them billions of tons of dissolved minerals every year.
Oceans exercise a great influence on climate. They are the main source of rainfall on the continents. Ocean currents regulate the temperature in the coastal regions. The sea breeze and the land breeze keep moving in their set directions. Due to the heat of the sun, the water in the oceans evaporates into smoke and converts into clouds that cause rainfall to make the soil fertile.
A river is a large stream of water that flows from high land to low land. The water in rivers comes from rain, snow melt, lakes, springs and water falls. The river water eventually flows into oceans.
There are many kinds of rivers such as- the swift flowing rivers, the slow moving rivers, the straight rivers, the meandering rivers, the large and the small rivers. The speed of flow of water in a river depends on the steepness. The rate of flow at the mountain slopes is higher than that in the plains. The course of river near its source is narrow but it widens as it moves downstream.
Some rivers owe their origin to melted ice in the mountains while others to the glaciers. A river is in youthful stage in mountainous region upper course, in mature stage in flat valleys mid course and in old stage in delta region lower course.
In the upper course the river flows zig-zag through the mountain blocks. Further downwards it develops land forms like gorges and canyons. In mid course, the river slows down but gathers larger volume of water from its tributaries and takes many turns. In the lower course, the river widens its bed and wends its way leisurely. Here, it forms flood plains, ox-bow lakes and deltas. The delta region consists of very fertile soil and silt. Rivers are a very useful means of transport and communication. Ships and boats sail on the rivers. Dams are constructed on big rivers to generate electricity, popularly known as hydroelectric power. Canals are cut from these big rivers to divert the water for irrigation purposes.