The rain hammering your window this morning may once have splashed on a Roman soldier, floated as a cloud over the Sahara, or frozen inside a glacier. Water is never made and never destroyed — it is simply moved around, again and again, in a giant loop called the water cycle (or hydrological cycle). The very same water has been circling the planet for billions of years.
The stages of the cycle
- Evaporation — the Sun heats seas, lakes and rivers, turning liquid water into invisible water vapour that rises into the air.
- Transpiration — plants draw up water through their roots and release vapour from their leaves; together with evaporation this is called evapotranspiration.
- Condensation — as vapour rises it cools, turning back into tiny droplets that gather to form clouds.
- Precipitation — droplets join until they are heavy enough to fall as rain, snow, sleet or hail.
- Surface run-off — water that flows over the ground into streams and rivers.
- Infiltration — water soaking down into the soil.
- Groundwater flow — water moving slowly through rock underground, eventually seeping back to rivers and the sea.
These stages are not a neat straight line: a single drop might evaporate and fall dozens of times, or spend thousands of years locked in an ice sheet or deep underground, before it ever reaches the sea. That is why we call it a cycle — there is no beginning and no end, only endless movement powered by the Sun and gravity.
Drainage basins
The area of land drained by a river and its tributaries is a drainage basin (or river catchment). Learning its parts helps you read any river map:
| Term | Meaning |
|---|---|
| Source | Where a river begins, often high ground |
| Mouth | Where the river meets the sea or a lake |
| Tributary | A smaller stream joining the main river |
| Confluence | The point where two rivers meet |
| Watershed | The high edge separating one basin from the next |