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Rivers and River Basins: The Basic Concepts

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The Concepts of a River Basin and Drainage Divide

A river basin (watershed) is the entire area from which water drains into a single river, and the boundary line separating one basin from another is called a drainage divide. Which side of the divide rainwater falls on determines which river it will eventually flow into.

The Structure of Mainstreams and Tributaries

The largest, central waterway of a river system is called the mainstream, and the smaller waterways that flow into it are called tributaries. Multiple tributaries come together to form the mainstream, which eventually flows into a sea or lake, forming a branching, tree-like structure.

The World's Major Rivers

The Nile, at roughly 6,650 km, is often cited as the world's longest river, while the Amazon has a drainage basin of about 7 million kmΒ² β€” the largest of any river in the world. By discharge volume, the Amazon also overwhelmingly outpaces every other river on Earth in the amount of freshwater it sends into the ocean.

Other Major Rivers of Note

The Congo River in Africa, the Mississippi River in North America, and China's Yangtze and Yellow Rivers are also major rivers representing their respective continents, sustaining large populations and farmland across their extensive basins.

How Rivers Shaped the Development of Human Civilization

Mesopotamian civilization developed between the Tigris and Euphrates rivers, Egyptian civilization along the Nile, and the Indus Valley and Yellow River civilizations along the Indus and Yellow Rivers respectively. The fertile soil around these regularly flooding great rivers made irrigated agriculture possible, laying the foundation for the formation of early civilizations.

How River Basins Connect to the Ocean

Most rivers eventually flow into the sea, and the freshwater and sediment carried from their basins have a major impact on estuary and coastal ecosystems. In contrast, some inland basins β€” such as those found across parts of Central Asia β€” never reach the ocean, instead terminating in inland lakes.

Why Does the Concept of a River Basin Matter?

A basin is a geographic unit that connects not just water but the people, ecosystems, and agriculture within it, since pollution or development upstream can significantly affect areas downstream β€” which is why basins are also used as the basic unit for international cooperation on water management between countries.

Learn About the World's Five Oceans, Where Basin Water Ends Up

To learn more about the world's five oceans, where water flowing through river basins ultimately arrives, see our guide to the world's oceans.

Frequently Asked Questions

Are there cases where two countries share a single river basin?

Yes β€” just as the Nile flows through several African countries, international rivers that cross national borders often mean multiple countries share a single basin, which can lead to both cooperation and conflict over water use.

What is the difference between a mainstream and a tributary?

The mainstream is the largest, central waterway of a river system, while tributaries are the smaller waterways that feed into it. Multiple tributaries combine to form the mainstream, which then flows onward to a sea or lake.