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What is a catchment?

A catchment is an area of land where water collects when it rains, often bounded by hills.  As the water flows over the landscape it finds its way into streams and down into the soil, eventually feeding the river.  Some of this water stays underground and continues to slowly feed the river in times of low rainfall.  Every inch of land on the Earth forms part of a catchment.

Catchments can range greatly in size from small urban sub-catchments such as Prospect Creek that feeds part of the larger Parramatta River Catchment, to massive catchments such as the Murray-Darling Basin that spans three states.

Catchments are complex and something happening in one part of the catchment can have a big impact on other parts.  In Australia, the past 200 years has seen big changes to our catchments.  Natural landforms such as bushland and small creeks have been replaced in many areas by houses, roads, footpaths and stormwater pipes.  This has had a large impact on our creeks and rivers.

THE WATER CYCLE
The term ‘water cycle’ is used to describe the process that begins with energy from the sun evaporating water from oceans, rivers and lakes to become water vapour. Water is also evaporated from plants – a process known as transpiration.

As the water vapour rises, it cools and condenses into billions of tiny droplets to form clouds. When the air cools, the droplets that make up the clouds merge until they are so big and heavy that they fall back to Earth as rain, hail or snow (precipitation).

Some of this precipitation evaporates, some seeps into the ground to become groundwater and some stays on or near the surface to form streams, and ultimately rivers. Once the water has fallen to Earth, the cycle starts over again.

Why are catchments important?
The idea of catchments is useful, as it is the standard functioning unit of the landscape: water, soil, plants and animals are all linked together within a catchment, and any activity that occurs within a catchment will affect the whole catchment.  Healthy catchments are important for human survival, as it is where our food is grown and where all the water we drink comes from.

What is a healthy catchment?
A healthy catchment is one that is still able to function as a catchment should.  It should be able to filter and clean water as it flows overland and seeps through the ground, and there should be lots of opportunities for water to seep into the ground so that it can be used by plants.

The PRCG gratefully acknowledges the Georges River Combined Councils Committee for the above information.