What’s a catchment?

The catchment of a river is the area of land that water falls on to feed that river. Every area of land on the planet is part of a catchment. As water moves over the land, it finds its way into streams and down into the soil, eventually feeding the river. Some water remains underground and this slowly recharges the river even when there is no rainfall.

Catchments range greatly in size and use, from small urban catchments such as the Cooks River in Sydney, to massive catchments spanning three states, such as the Murray Darling.

Not all catchments feed rivers that flow to the sea. Some are the source of rivers which flow inland, ending in a dry lake or wetland. The Diamantina river catchment is one example – this river ends up flowing into Lake Eyre, which is dry much of the time. Other rivers like the Macquarie and Lachlan feed huge wetlands and marshes on the way to the sea. Most of Australia’s inland rivers have large floodplains which are an essential part of the river.

Although we don’t often think about it, cities are catchments for rivers as well. For example the entire catchment for the Cooks River is in Sydney. In Melbourne the city and suburbs form much of the Kororoit Creek catchment and it is much the same for Norman Creek in Brisbane.

Where’s your catchment?

Every one of us lives in a catchment. Look at a map or street directory of your area and find the nearest stream, creek or river. If you are in the city it may now be just a drain or even a pipe under the ground. Compare this map with a contour map, which will show which way the land slopes. Using the contours, imagine which direction a drop of water would flow and follow it to the river. Even if the stream is now a drain or a pipe, keep following the contours and you will eventually find the river. You live in the catchment of the river it flows to.

Why are catchments important?

The concept of a catchment is useful, because it is the scale on which many parts of the landscape work. The soil, plants, animals and water all function together in a catchment – anything that affects one of these will also have an impact on the others. The health of our catchments is vital for human existence because they are where all food is grown. The water you drink comes from a catchment near you.  The oxygen you breathe comes ultimately from a growing green plant or algae. So catchments are part of our life support systems, and support the lives of all plants and animals.

How are they doing?

In 2002, an assessment was made of the overall health of Australia’s more intensively used catchments (ie, all catchments except those in the arid interior and the far north west).

The assessment looked at land, water and biodiversity in each catchment, including problems such as salinity, erosion, sedimentation, pesticide use, pollution, roads, native tree-cover, human population and pests.

The survey, done by the National Land and Water Resources Audit, found:

• 5% of catchments are in the poorest condition
• 15% are in low range condition
• 50% are in mid-range condition, and
• 30% are in better condition.

These stats suggest that the condition of 70% of Australia’s most populated catchments is merely average or worse than average.

Why are things so bad today?

Over the last 200 years people have been busy clearing land for farming, mining, building towns and cities and changing catchments which had been in equilibrium for thousands of years.

All this activity has resulted in damage to the health of our catchments, including loss of biodiversity, sedimentation and pollution of rivers and the spreading of weeds and pests.

One reason they are so damaged is because we have not understood that changing something in one part of a catchment can have a huge impact somewhere else. For example, if you remove trees from the top of a catchment, you could make the soils , rivers, wetlands and groundwater further downstream more salty. When trees are removed, more rain soaks into the ground and flows for many kilometres, adding to the groundwater beneath the soil and rock. Eventually the land fills up like a bucket, bringing saline water close to the surface, killing crops and native vegetation and sometimes leaving salt scalds.

Many catchments have been invaded by weeds, but weed removal has been hindered by the same ignorance of the connectedness of catchments. For example, money is often spent to clear weeds such as willow from the lower reaches of a river. Yet upstream, there are many more willows which will re-seed the area, meaning the money was wasted.

Catchments in the cities have taken a hammering too. The big offenders are fertilizers sprayed onto gardens and detergents washed off cars – these run down gutters and stormwater drains into local rivers and harbours, causing algal blooms and providing nutrients for weeds to thrive. Pollutants discharged from industries along rivers have also caused enormous damage to urban rivers.

Unfortunately we still struggle to coordinate different activities within the one catchment and manage them at the right scale. We are still doing things without understanding what the consequences will be. People and the environment down the catchment are still paying the price for the bad management of those further up.

How do we fix it?

Catchments are like people. Each has its own unique qualities and traits, and so each one needs to be understood and managed accordingly. Each catchment must be managed according to its soil types, vegetation and climate, including rainfall. These will determine the type and the scale of any activity, whether it be agriculture, forestry, industry, tourism and places for recreation or urban development.

Catchments will benefit from a more integrated approach to management: where the impact of all activities – farming, development, tourism, and so on – are considered as part of the entire landscape and can be done in greater sympathy with the natural plant, animal, soil and water assets of the catchment.