Major rivers and watersheds

Look at any world map and you will see rivers branching across every continent like the veins of a giant leaf. Rivers are how Earth carries fresh water from the high places where rain and snow fall down to the oceans where it began. They have also been the highways, water supplies, and fertile farmlands of human history.

Source, course, and mouth

Every river has three parts. The SOURCE is where it begins — usually high up in mountains, melting glaciers, or a spring. The COURSE is the long path the river travels — over waterfalls, through valleys, sometimes joined by smaller rivers called tributaries. The MOUTH is where the river finally empties into a larger body of water, usually an ocean or a sea. At the mouth, the river often spreads out into a fan-shaped DELTA of fertile soil.

Watersheds

A watershed (also called a drainage basin) is the entire region of land where all the rain that falls eventually drains into the same river system. Watersheds are separated by ridges of higher land called divides — if you stood on the Rocky Mountains in North America, rain on one side would eventually reach the Pacific, while rain on the other side would reach the Mississippi and the Gulf of Mexico. The largest watershed on Earth is the Amazon basin, which collects water from roughly 7 million km² of South America.

The world's great rivers

• Nile (Africa) — at about 6,650 km, often called the longest river in the world. It flows north through Egypt and Sudan to the Mediterranean Sea.
• Amazon (South America) — by far the largest river by water volume; it carries more water than the next seven biggest rivers combined.
• Yangtze (Asia) — the longest river in Asia, flowing across China from Tibet to the East China Sea.
• Mississippi-Missouri (North America) — the major river system of the United States, draining most of the country between the Rockies and the Appalachians.
• Yenisei and Ob (Asia/Russia) — vast Siberian rivers that flow north into the Arctic Ocean.
• Volga (Europe) — the longest river in Europe, running through Russia to the Caspian Sea.

Rivers and early civilizations

It is no accident that the world's first cities grew on the banks of great rivers. Rivers provided drinking water, irrigation for crops, fish, transport, and yearly floods that left rich silt behind. The Egyptians built their civilization along the Nile. The Sumerians and Babylonians flourished between the Tigris and the Euphrates in Mesopotamia. The Harappan civilization grew along the Indus in modern-day Pakistan and India. The earliest Chinese dynasties grew up along the Yellow River. Wherever rivers met farmland and trade, the first civilizations followed.

Rivers today

Modern civilizations still depend on rivers. They provide drinking water to billions of people, irrigate crops, generate hydroelectric power, support fisheries, and move freight on barges. They also need protecting: pollution, dams, and drought caused by climate change are damaging river systems all over the world. The health of a river is a direct measure of the health of the watershed around it.