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Rivers, lakes, and wetlands

Rivers

Rain drains off the land in streams that join together to form rivers of fresh water. Most of these flow into the sea.

Lakes

In some places, streams and rivers flow into hollows in the ground, filling them up to create ponds and large lakes.

The pure, fresh water that falls as rain forms flowing streams and rivers, broad lakes and ponds, and various types of watery wetlands.

Swamps

The edges of lakes and rivers can become choked with reeds and other vegetation, creating swamps and marshes.

Bogs

Heavy rain encourages the growth of bog mosses, which form thick, spongy masses of acid peat dotted with bog pools.

03: Winds blow the clouds over the land.

04: There, they tend to spili their moisture as rain.

05: In cold regions, rain falls as snów. This can build up to form thick ice sheets that lock up the water for thousands of years.

Some areas of the land hardly get any rain or snów. Parts of the Atacama Desert in South America

have had no rain at all

06: The rain drains off the land in rivers that flow back to the sea, carrying minerals with them.

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Hidden water

• Rain falls to the ground, where it usually soaked up by porous rocks to form reservoirs of ground water.

• Water keeps draining down until it reaches a waterproof layer of rock that stops it sinking any further.

• The layers above fili up with water—the top of the water-saturated layer is called the water table.

• If you dig a hole down to this point, the bottom of the hole will fili with water.

Ice ages

Earth has passed through many cold phases in its history, mainly because of regular changes in its orbit around the Sun.

During these cold phases, temperatures in many areas are too Iow for snów to melt, so it gets thicker each year and is compressed into ice.

The colder it is, the bigger the area that is covered with ice. Such periods of Earth history are known as ice ages.

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The most recent ice age began around two million years ago. It hasn’t finished yet—we are just living in a relatively warm spell!

At the peak of the recent ice age, around 20,000 years ago, so much water was locked up as ice that sea levels fell by 330 ft (100 m).

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