Australia State of the Environment Report 2001 (Theme Report)
Prepared by: Ann Hamblin, Bureau of Rural Sciences, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06748 5
Accelerated erosion and loss of surface soil (continued)
Active gullying and sheet erosion catchments - case studies * [L Indicators 1.4 and 1.5]
In recent years more studies of selected major rivers in different climatic regions have been undertaken to identify which combinations of climates and land uses are most vulnerable to accelerated water erosion. The application of remote sensing has also increased our capacity to record accurately those irregular, rare storm events that are responsible for the majority of accelerated erosion and sediment transport as plumes issuing from estuaries. This has been dramatically documented for the north-west of Western Australia, where both Landsat TM and NOAA-AVHRR real-time satellite surveillance have been used to record such episodic events as cyclonic damage and flooding (see satellite photo).
The Ashburton River and its tributaries are badly degraded only in the headwaters of the main channel and in the lower reaches; the majority of the catchment is in good to excellent condition. Despite this, a massive flood that occurred after 470 mm of rain fell in 24 hours in February 1997 caused enormous erosion and redeposition of sediment along the flooded extent of the main channel. The peak in-channel water depth was 20 m, and a sediment plume extended 15 km offshore and covered 500 km2. This event was monitored with NOAA-14 AVHRR imagery by DOLA, but because of the low relief and difficulties in discriminating between parallel-oriented rib erosion and deposition features, the actual extent of the erosion could not be accurately assessed.
Such events are rare. The peak flow on this occasion was four times the volume of the previously highest recorded peak in 1975-76, but a system might experience three or four such events in a century.
The Water and Rivers Commission in Western Australia has assessed the state of the northern rivers of the Indian Ocean, Timor Sea and Western Plateau Drainage basins in a recent report (WRC 1997), and concluded that by far the most serious cause of land and river degradation in the north-west is the removal of natural riverine and catchment vegetation. This results in rapid headward extension of gullies and networks of eroded channels in the upper reaches. The WRC considered the problem to be growing in magnitude, because of very intensive grazing pressure, particularly in the Timor Sea and Indian Ocean basins, where over two-thirds of the land area is in pastoral leases. The riparian zones are of critical importance in stabilising the landscape, and the contrast between their condition in most pastoral leases and the condition of vacant Crown land or national parks demonstrates the extent to which pastoralism has caused such degradation.
The tropical north-east of Queensland is also a susceptible region; large volumes of soil can be eroded in the severe cyclonic storms even where there is no land disturbance. However, changes in land cover and use over the past 150 years have exacerbated the potential rate of erosion substantially. Consequently in recent years fears have been raised over the impact of sediments and nutrient exports on other ecosystems in general and in particular the Great Barrier Reef.
Satellite image of sediment plume extending 50 km from the mouth of the Gascoyne River two weeks after a major cyclone.
Source: SPOT image of 2 March 1995 showing the sediment plume from the flooding Gascoyne River, WA. Seriously degraded inland areas can also be seen in this image. Image provided by WA Department of Land Administration-Remote Sensing Services
Most sediments are exported during infrequent, intense storms, including adsorbed phosphorus, organic material and pesticides that may be on suspended clay particles. For example, during cyclone Sadie in 1994, the Herbert River discharged over 100 000 tonnes of suspended sediments, sourced principally from grazing land. This would be sufficient to cover the whole of Sydney in 2 cm of soil (Mitchell and Bramley 1997).
Table 7 summarises the relationship between land use, annual flow and sediment export. Grazing lands are responsible for over 80% of the estimated annual sediment export from these north Queensland rivers. The proportion of each catchment remaining pristine, with full native tropical vegetation, varies from 2% to 76%.
|Catchment||Mean flow'000s ML||Area'000s km 2||(kg/ha)||'000s t||Pristine as % of total area||Grazing contribution'000s t||Cropping contribution '000s t||Urban contribution '000s t||Total sediment '000s t|
|NE Cape York||19 100||43 300||484||130||21||1 963||3||0||2 096|
|Burdekin-Haughton||10 850||133 510||212||12||2||2 741||73||2||2 829|
|Fitzroy||7 100||142 646||130||41||9||1 589||229||2||1 861|
|Herbert||5 000||10 130||543||23||16||462||64||1||550|
|Johnstone||4 700||23||2 436||60||23||271||235||1||567|
|Mossman-Daintree||4 250||2 615||1 024||104||76||111||52||1||268|
|Mulgrave-Russell||4 200||2 020||2 328||66||49||192||212||1||471|
|Burnett-Kolan||2 900||39 470||177||32||17||599||229||2||698|
|Pioneer-O'Connell||2 650||3 925||1 838||32||19||464||233||1||720|
|Barron||40||2 175||1 150||12||76||75||20||4||114|
|Total||61 090||389 409||565||8 818||1 430||17||10 660|
Source: Davis and Hamblin (1998).
Despite these large sediment losses, and the probable impact of extensive grazing areas within these vulnerable catchments, accelerated erosion is difficult to detect and distinguish from natural erosion unless it becomes severe. As a result it is given less prominence in the media and in policy than other land degradation issues such as salinity or pesticide pollution.
Concern for the condition of the Great Barrier Reef in recent years has focused more attention on erosion in this region of Queensland. Monitoring by remote sensing has revealed that high-intensity cyclonic rainfall causes large plumes of fresh water to enter the coastal zone, carrying varying amounts of sediment. The Burdekin River discharge measurements for the period 1966-1995 has recently been coupled to local wind and tidal measurements in a verified three-dimensional hydrodynamic model that simulates such river plumes, in order to assess the overall impact of the river on the central portion of the Great Barrier Reef. These results indicated that the plume regularly extends over 400 km to the north of the river mouth in coastal waters, but individual events follow different paths because of the complexity of coastal topography, islands and reef matrices. The work predicts that shelf edge reefs are not affected by the Burdekin River plumes, but the inner shelf reefs are affected whenever offshore winds prevail during flood events (McAllister et al. 2000).
Sediment samples taken from the coast to the outer reef in the Great Barrier Reef lagoon in the Townsville region by the CRC Reef Research Centre confirm that very little sediment of land origin occurs in the mid-shelf or outer reefs, and most terrestrial-derived sediment is confined to near-shore locations. The sediments retrieved as grab samples from these areas are predominantly of ancient volcanic air-carried ash.
While these recent research findings may relieve some of the worst fears that have arisen that erosion from land clearing might eventually destroy the outer reef, there is no doubt that the Cairns region of the Great Barrier Reef, for example has been severely affected by induced erosion over the past 100 years. The shallow zone of the inshore reefs has been markedly affected, and coastal rivers have become drains bringing eroded mud to estuary mouths. Mud has accumulated rapidly in the Cairns waterfront region since the 1950s, probably at 10 to 15 times the natural rate (Wolansky and Spagnol 2000).
Figure 15: Burdekin River plumes in 1974 (the largest flood ever modelled) and 1995 (an average year) at 63 and 65 days after the event.
Source: CRC Reef Research Centre
* These case studies are provided as examples only, and the findings presented here cannot necessarily be applied to other areas.