River

A river is a long, flowing body of fresh water that moves across land. Rivers usually start in high places like mountains or hills. They flow downhill until they empty into a lake, a sea, or an ocean. Almost every continent has rivers. Even Antarctica has small ones that flow during the warmest weeks of summer.

Rivers begin in many ways. Some start where snow melts on a mountain. Others bubble up from springs in the ground. A few flow out of large lakes. The starting point of a river is called its source. The place where it ends is called its mouth. The path the river takes between the two is its course.

Smaller streams that join a river along the way are called tributaries. As tributaries flow in, the river grows bigger and stronger. The Mississippi River in the United States has more than 250 tributaries. By the time it reaches the Gulf of Mexico, it carries water that fell as rain or snow in 31 different states.

Rivers are powerful shapers of land. Over thousands of years, flowing water wears away rock and soil. This slow grinding process is called erosion. The Colorado River carved the Grand Canyon this way, cutting more than a mile down into solid rock. Rivers also build new land. They carry tiny bits of dirt, called sediment, and drop them off where the water slows down. Over time, this sediment can pile up at a river's mouth and form a wide, flat area called a delta.

People have lived near rivers for as long as there have been people. Rivers give us drinking water, water for crops, and fish to eat. They are roads for boats. The first big civilizations grew up along rivers like the Nile in Egypt, the Tigris and Euphrates in the Middle East, the Indus in South Asia, and the Yellow River in China.

The longest river in the world is either the Nile or the Amazon. Scientists actually disagree about which one wins. The Nile in Africa is usually listed at about 4,130 miles. Some explorers say the Amazon in South America is a few miles longer if you measure from a different starting point. The argument has gone on for years, and new measurements keep changing the answer.

Wherever a river flows, life follows. Fish, frogs, birds, plants, and people all gather at its banks.