A river is any natural stream of fresh water that flows in a definite channel for all or part of the year. It flows toward another river, ocean, lake, or other large body of water. In some areas a river’s water flows underground or dries up before reaching another body of water. A river’s water can come from a multitude of sources: runoff from rainfall or melting snow/glaciers, a spring, or the overflow from a lake. The beginning of a river is called its source.
Rivers grow as other water sources (tributaries) join it (other rivers, streams, creeks, brooks, etc.). Many rivers resemble the branches and trunk of a tree. The branches are the river’s tributaries which flow into the trunk (the main river flowing to its end). The end of a river is called the river’s mouth; usually another river, lake, or ocean. When a river ends at a lake or ocean its flow is slowed down suddenly and it is no longer capable of carrying the entire load of sediment it picked up in its journey. A lot of this sediment is deposited at the mouth of the river forming a fan shaped delta. Sometimes, when the lowest portion of the river is below the level of the sea/ocean, a delta does not form. This is called a “drowned mouth.”
There are three types of rivers: perennial, periodic, and episodic. A perennial river is a river which exists all year. Its level may rise and fall during the different seasons, but there is constantly water in its bed. All of the world’s major rivers are perennial rivers. A periodic river exists on an intermittent basis. Primarily appearing in arid climates these rivers run dry on occasion when evaporation is greater than the water flow. An episodic river is rainfall run-off in a very dry area that gets limited rainfall. Water in the episodic river exists only as long as it takes to be evaporated.
There are rivers on every continent with the exception of Antarctica. The Amazon River in South America is the largest river in the world (based upon amount of water flow) with the output constituting almost 20 percent of the total amount of water flowing to the oceans in the world. The next three largest rivers are the Congo River in Africa, the Ganges-Brahmaputra in India and the Yangtze in China.
The four longest rivers in the world are the Nile River in Africa followed by the Amazon River in South America, the Yangtze in China, and the Mississippi/Missouri in the United States.
Rivers are transportation routes, both on the water and along the adjacent banks. They are suppliers of water and energy and carry off waste products. Not only do rivers supply water for home consumption, they also supply water for industry as well as being a source of food and recreation. They are used by farmers for irrigation. Cities, towns, and civilizations tend to be built along rivers. Many rivers are used as international boundaries between countries.
Flooding occurs when large storms or rapid melting of the snow pack increase the water flow to beyond the river’s banks. Many rivers flood on an annual basis. During ancient times Egyptians came to depend upon the annual flooding of the Nile to leave behind rich soil when the flood retreated.
Rivers can be classified as young, mature, or old. These names do not describe the age of the river, but rather the characteristics of the river. A young river is a rapidly flowing river whose bed is steep and irregular and its flow is often interrupted by rapids and waterfalls. In a mature river most of the rapids and waterfalls have been worn away and the river flows more slowly on a more gradual river bed. The river also tends to meander across the floodplain. Old rivers meander slowly back and forth over wide floodplains and the sides of the bank are worn down to gentle slopes. An old river also forms a delta at its mouth. It is possible for a river (such as the Mississippi River) to have all three characteristics at various points of its length. The source of the Mississippi River (actually it’s the source of the Missouri River) is young with numerous rapids and the Falls of St Anthony. The Mississippi is mature during its middle with the river bed meandering and is old at the mouth with a large delta.
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