State of the Environment Report 2007
Erosion occurs when soil becomes unstable and erodes from the land, and the banks and beds of waterways. Soil erosion is a natural process that is at its greatest during floods. Land with steep slopes and friable soils in high rainfall areas are particularly susceptible to erosion and can contribute large amounts of soil to waterways and wetlands. Erosion is exacerbated by human activities that remove vegetation cover or cause soil disturbance.
Eroded soil is gradually deposited into waterways and wetlands by wind and water erosion, or via mass collapse of soil structures (such as land slides, bank collapse and slumping). When eroded soil is deposited into waterways and wetlands it becomes sediment. Fine sediments (e.g. silt) can be suspended in water and carried quickly downstream and out to sea. Coarser sediments (e.g. sand, pebbles and rocks) slowly roll along the riverbed and deposit in pools, wetlands and river channels. During low flows, sediments begin to settle out of the water in a process termed 'sedimentation'. In contrast, during large flows and floods, sediments are scoured from the riverbed or banks of waterways and transported downstream or out to sea. Over time this dynamic process of sediment deposition and scouring contributes to the reshaping of waterways. In wetlands, sedimentation is more problematic, contributing to the gradual filling of wetlands over time.
Problems arise when erosion and sedimentation rates are too fast. Excessive erosion can damage fringing vegetation and undermine infrastructure such as bridges, roads and buildings located close to the waterline. High levels of suspended sediment can make the water muddy, affecting aquatic animals and reducing habitat. It can also exacerbate flooding of nearby land, fill water reservoirs, foul water supplies and clog irrigation and drainage pipes. Often nutrients and contaminants are bound to eroded soil and may contribute to contamination or eutrophication problems.
Turbidity and suspended solids are commonly used as indicators of levels of erosion and sedimentation in waterways and wetlands. 'Suspended solids' refers to the amount of sediment or organic matter in the water column. Turbidity is a measure of the light scattering properties of water, which may be affected by sediment, organic matter or colour of the water. Most South West waterways draining forested catchments are generally low in suspended solids and turbidity. In these areas the presence of healthy fringing vegetation protects riverbanks from erosion and intercepts soils eroded from catchments. However, waterways draining agricultural catchments (such as the Avon, Blackwood, Murray, Collie, Preston, Pallinup, Oldfield, and Young rivers) have high suspended solids or turbidity. Although many monitored waterways in WA show short-term changes in turbidity or suspended solids in recent years (Figure IW6.1), further investigation is required to determine their cause.
![Figure IW6.1: Suspended solids and turbidity in waterways. [Data source: Department of Environment [ver. 2005]; Analysis: EPA; Presentation: EPA.]](/site/files/images/Figure-IW6.1_Large.jpg)
Waterways in the Mid West, Pilbara and Kimberley (including the Chapman, Gascoyne, Murchison, Ashburton, Fortescue, Fitzroy and Ord rivers) are well known for their high sediment transport rates and braided, sediment-filled river channels. Many of the catchments of these waterways are susceptible to sedimentation due to steep terrain, dry soils and low vegetation cover. Livestock access to waterways is also believed to exacerbate erosion and sedimentation in pastoral areas. For example, since Lake Argyle was constructed on the Ord River in the 1970s, sediment has been deposited at rates of 19-28 million cubic metres per year into the reservoir (Wasson et al., 1994). By 1994, sediment deposition had reduced the useable storage by 600 million cubic metres, representing a little over 10% of the original volume of the reservoir (Doupe & Pettit, 2002). Although there are no recent modelled estimates, reduced pastoral activities and effective catchment restoration efforts (Payne et al., 2004) suggest that sediment levels in the Ord River (and into Lake Argyle) may be falling. There is no other reliable information about sedimentation trends in other WA wetlands.
There is very little information available to determine the extent of sedimentation on river pools, with most evidence being anecdotal. For example, prior to agricultural development in the Avon River catchment there were about 26 major pools along the river between Cobblers Pool and Yenyinning Lakes, varying up to 70 m wide, 2 km long and 10 m deep. Nearly all have since filled with sediment (Pen, 1999; Water and Rivers Commission & Avon Waterways Committee, 2002), much of which has been mobilised with the rapid formation of gullies and networks of eroded channels in the upper catchment. River training (straightening of river channels) and minimal retention of vegetation near waterways has also contributed (see 'Loss or degradation of fringing and instream vegetation'). Unlike in normal rivers, flood events appear unable to scour the excess sediments from these pools (Pen, 1999) and consequently they are unlikely to return to their previous state without significant management intervention (i.e. dredging). Similarly, mapping in 1841 indicated pools in the Brunswick River were once 2-5 m deep but they are now full with sediment (Department of Environment, unpublished data).
Soils are more susceptible to erosion if unprotected by vegetation, or exposed to wind or water flow (see 'Loss or degradation of fringing and instream vegetation'). Livestock can be particularly damaging if they have unrestricted access to waterways and wetlands. Fringing native vegetation may be trampled or over-grazed, thereby increasing soil disturbance and the potential for bank erosion or mass slumping. Farm machinery and herding of livestock tends to compact soil, thereby increasing the erosive potential of water runoff over land. Waterlogged and inundated areas are also susceptible to water erosion, particularly if stocked or cultivated while wet. Salinisation can also degrade soil structure making the soil more susceptible to erosion.
Clearing land for development results in large areas of soil being exposed for long periods of time (see 'Loss or degradation of native vegetation'). Nearby wetlands, waterways and drains can be affected by high sediment levels unless developers provide measures to protect the exposed soil. When the land has been developed, the relative amount of impervious surfaces (e.g. roads, houses, paths) increases, amplifying erosion potential by increasing the speed of water runoff. Forestry, cropping, pastoralism, mining activities and fires all have the potential to increase erosion and sedimentation rates once vegetation cover is removed.
Agriculture Extension Program: The Department of Agriculture promotes farming methods to minimise land erosion. Grade or contour banks and interceptor drains are used on sloping agricultural land to minimise erosion. Recent surveys indicate that 50-70% of Wheatbelt farmers use surface water management practices compared to only 30-40% of farmers in wetter parts of the South West (Department of Agriculture, 2006). Fencing wetlands and waterways from grazing animals is necessary to prevent bank erosion and degradation of fringing vegetation: 33% of pastoralists restrict grazing stock from river and creek frontages compared to 45-50% of farmers in the South West agricultural region.
Natural Heritage Trust/National Action Plan for Salinity and Water Quality (NHT/NAP) programs: All regional natural resource management groups have recognised erosion and/or sedimentation as a threat to inland waters. They are proposing projects for the management of affected waterways and wetlands, and protection of valued natural assets at risk, including fencing, rehabilitation of river pools, sediment retention devices, ponding banks and rehabilitation of fringing vegetation.
Avon River Pools Recovery Program: has been responsible for rehabilitating degraded waterways, including revegetation of fringe areas, recontouring banks, introducing riffles and snags, and dredging of river pools. Much of this activity is addressing previous river training of the Avon River in the 1950s-1970s that caused significant increases in river erosion and sedimentation rates.
Ord River Regeneration Project: commenced in 1960 in response to excessive sediment entering Lake Argyle. From the 1960s to 1980s, extensive rehabilitation occurred including intensive strip contour cultivation and seeding, fencing, donkey eradication and de-stocking. In 2002 the area was surveyed to assess changes in vegetation and soils, and confirmed profound improvements in vegetation and soil condition (Payne et al., 2004).
Modelling: The CSIRO and the Centre for Catchment Hydrology are currently developing a Sednet software model that predicts sediment loss from catchments and various land use types. The model is intended for use in regional planning and to assist with setting erosion and sedimentation targets.
Erosion and sedimentation is a widespread problem affecting many WA waterways and wetlands. Better catchment and waterways management is needed to stop further degradation and reverse damage to important areas. Increased sediment in water detrimentally affects aquatic biodiversity. Finer sediment may also carry phosphorus, contributing to eutrophication problems. Sediment build-up can exacerbate flooding where channels have become shallower and outlets blocked. This in turn can increase flood magnitude and frequency, increasing the risk of damage to buildings, roads, bridges, pipes, farmland and other infrastructure. Deposition of sediments in water supply dams can reduce volume capacity and may require costly treatment to remove sediment and fine particulates. Erosion of banks can intrude on productive land, forcing landowners to act to stem further loss of valuable land or infrastructure. Often this means fencing and rehabilitating areas much larger than would have been necessary to stop the problem in the first instance.
4.19 Enhance routine monitoring of waterways and wetlands currently impacted by, and at future risk of, erosion and sedimentation.
4.20 Implement appropriate planning processes to ensure foreshore development is controlled.
See also 'Loss or degradation of fringing and instream vegetation':
