Catchments

A catchment is an area where water is collected by the natural landscape.

For example, imagine cupping your hands together in a downpour of rain and collecting water in them. Your hands have become a catchment, just like the natural landscape.

The outside edge of a catchment is always the highest point. Gravity causes all rain and runoff in the catchment to run downhill where it naturally collects in creeks, rivers, lakes or oceans. This means that rain that falls outside the edge of one catchment will fall into a different catchment, before flowing into other creeks and rivers.

Some water doesn’t get caught in a catchment, instead it seeps below ground and is stored in the soil or in the space between rocks. This is called groundwater.

What do catchments do in the water supply system?

We use the water collected in catchments and the natural landscape to help supply water for our needs by building dams and weirs or tapping into groundwater. This is called the water supply system.

Rain in Sydney doesn’t necessarily mean that dam levels will increase. The rain must fall into the drinking water catchment for it to travel into the corresponding dam or weir.

Where is the Greater Sydney drinking water catchment?

The Greater Sydney drinking water catchment is made up of five water catchments- Warragamba, Shoalhaven, Upper Nepean, Woronora and Blue Mountains. They stretch from north of Lithgow at the head of the Coxs River in the Blue Mountains, to the source of the Shoalhaven River south of Braidwood - and from Woronora in the east to the source of the Wollondilly River west of Crookwell.

Did you know? The five catchments in Greater Sydney cover 16,000 square kilometres of land. This is only 2% of the land area of NSW, but it supplies drinking water to 60% of the state's population - over 5 million people who live in Sydney and the Blue Mountains, Illawarra, Southern Highlands, Goulburn and Shoalhaven regions.