River Systems

A river is a natural flowing watercourse, usually of fresh water, that moves toward an ocean, sea, lake or another river. Rivers form a central part of the water cycle: precipitation falling on the land gathers into small streams that join together, growing into larger channels as they descend toward lower ground. A river together with all the streams that feed it makes up a river system or drainage basin, the whole area of land whose water flows into that river.

Rivers are powerful agents of erosion and deposition. As water flows, it wears away rock and soil, carries sediment downstream, and lays it down where the current slows, gradually reshaping the landscape. Where a river crosses a sudden drop or a band of hard rock, it may form a waterfall, while along gentler stretches it can wind in broad curves across flat plains. Human-made reservoirs store river water behind dams for supply, irrigation and power, and the study of how water moves over and through the land is known as hydrology.

River systems support rich ecosystems and have long been vital to people. They provide drinking water, irrigation for crops, routes for transport and trade, and habitats for fish and other freshwater life. Major rivers such as the Yellow River have shaped the history and settlement of entire regions. Because rivers connect uplands to the sea, the health of a river often reflects conditions across its whole catchment.

Frequently asked questions

What is a river system or drainage basin?

It is a river together with all the tributary streams that feed it and the entire area of land that drains into it, collecting precipitation and channelling it toward the river's mouth.

How are waterfalls formed?

Waterfalls often form where a river flows over a sudden change in elevation or where softer rock erodes faster than a band of harder rock above it, leaving a steep drop.

What is hydrology?

Hydrology is the scientific study of water on and below the Earth's surface, including how it moves through rivers, the water cycle and the land.