State of the Environment

2006

Indicator: IW-24 Extent of sedimentation (incl sand slugs)

Data

Gully, riverbank and sheetwash erosion deliver over 120 million tonnes of sediment to streams each year.

30,000 km of streams (including 30% of southern Western Australia and 20% of the Murray - Darling Basin) have sand and gravel derived from gully and streambank erosion, to the extent that in-stream ecological health is significantly impaired.

14 million tonnes of sediment is transported to the Queensland coast and 3 million tonnes to the New South Wales coast each year. However, only 20% of sediment delivered to streams actually reach the coast (Table below). The remainder is deposited within the rivers, on floodplains or in reservoirs. Even if source erosion was stopped today, large areas of sand deposition would continue to progress through river systems until sediments are stabilised, extracted or flushed.

River sediment loads are generally 10 to 50 times greater than pre-European loads in intensively used river basins.

Ninety per cent of the suspended sediment loads reaching estuaries are derived from 20% of catchment areas, forming a basis for targeted sediment control.

Deposition of sand and suspended sediments in streams and rivers are greatest in the Murray-Darling Basin, coastal regions of New South Wales, south-east Queensland and the Glenelg region of Victoria and areas of significant vegetation clearance and high intensity rainfall events.

Components of sediment supply (million tonnes per year)
Gross sheetwash and rill erosion 666*
Delivery to stream from sheetwash and rill erosion 50
Gully erosion 44
Streambank erosion 33
Total sediment supplied to rivers 127
Total suspended sediment stored 66
Total bed sediment stored 36
Sediment exported from rivers 25
Total of stores and losses 127

* Does not include sheetwash and rill erosion estimates for the Gulf, Western Plateau or Northern Territory as river budget assessments were not undertaken in these areas.

Source: National Land and Water Resources Audit 2001, Erosion and sediment transport, Land and Water Australia, Canberra, viewed 13 Apr 2005, http://audit.ea.gov.au/ANRA/land/land_frame.cfm?region_type=AUS&region_code=AUS&info=soil_erosion.

What the data mean

Sedimentation of river systems is extensive and will continue to be so for some time, even if erosion is controlled.

Data Limitations

NLWRA data has not been updated since 2001.

Issues for which this is an indicator and why

Inland Waters - Habitat scale influences - In-stream habitat - woody debris and sand slugs 

Extent of streams and water bodies affected by sedimentation is a direct measure of this pressure.

Other indicators for this issue:

Inland Waters - Catchment scale influences - Land and vegetation condition - Erosion 

Soil erosion has the potential for downstream impacts on creeks, rivers, reservoirs, lakes, and estuarine and marine environments. Water-borne erosion increases the supply of sediment to rivers. Extent of streams and water bodies affected by sedimentation is an indirect measure of the pressure of erosion on riparian vegetation.

Other indicators for this issue:

Inland Waters - Catchment scale influences - Land and vegetation condition - Nutrients and sediments - sources and loads 

Nutrients and sediments from soil erosion often as a result of vegetation removal are an indirect indicator of the pressure of land clearing. Nutrients and sediments in turn have the potential for downstream impacts on creeks, rivers, reservoirs, lakes, and estuarine and marine environments, and ultimately on terrestrial vegetation and other biodiversity. Extent of streams and water bodies affected by sedimentation is therefore also a direct measure of the area potentially affected by excess nutrients.

Other indicators for this issue:

Inland Waters - Habitat scale influences - Water Quality (for surface and groundwater) - Sediment and turbidity 

Soil erosion also has the potential for downstream impacts on creeks, rivers, reservoirs, lakes, and estuarine and marine environments. Water-borne erosion increases the supply of sediment to rivers. Extent of streams and water bodies affected by sedimentation is a measure of the area likely to be affected by turbidity.

Other indicators for this issue:

Further Information

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