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Understanding how the water cycle operates is key to understanding how rivers work. The water cycle is also known as the hydrological cycle. It is called a cycle because water continuously moves around the system. Rivers are part of this cycle. The illustration below shows how water changes state through the cycle. It can be a liquid, a vapour or a solid.

How does the water cycle work?

• Energy from the Sun heats the surface of the Earth.
• Water is evaporated from oceans, rivers, lakes, etc.
• The warm, moist air rises because it is less dense.
• Condensation occurs when water vapour is turned back into water droplets as it cools down. Clouds are formed.
• Precipitation occurs as water droplets get bigger and heavier they begin to fall as rain, snow and sleet, etc.

When the precipitation reaches the surface, some falls directly into the sea but other water falls on land:

• Some water is intercepted by vegetation. Some water may then slowly reach the ground. Some will evaporate from the surface of leaves or be taken up by the plant roots, and some of this water will eventually return to the air as vapour through the process of transpiration. This slows down or prevents some water flowing back to the river.
• Some water flows across the surface of the ground - surface run-off. This happens when the surface doesn't allow water to penetrate. Surface run-off is more likely to occur if the ground is saturated with water or when the rock is impermeable. This water moves quickly to the river.
• Some water infiltrates into the soil. This through flow moves more slowly back to the river than surface run-off.
• Some water percolates deeper into the ground and is slowly transferred back to the river or sea.

Stores and transfers

The major stores of water are the ocean, ice caps, land and the atmosphere. The movement of water between these stores is called transfers.