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Reducing Bushfire Impacts on Water Quality

The impacts of bushfire on water quality are caused mainly by loss of riparian vegetation and influx of sediment, ash and burnt organic material from the catchment, so the main priority for rehabilitation work is to prevent sediment and destructive debris from reaching the streams, and to restore overhanging riparian vegetation.

For example, in Victoria, after the 2003 alpine fires, the Dept of Sustainability and Environment staff undertook critical works to minimise the potential for landslips and restore stream environments. They focused on:

  • soil conservation works;
  • rehabilitation of more than 6000 km of fire control lines constructed during firefighting operations on private and public land;
  • clearing debris to stop it being washed into waterways;
  • repairing and revegetating eroded riverbanks; and
  • upgrading water storage areas and treatment works to ensure drinkable water was available to the alpine resorts and fire-affected townships.

In the Australian Capital Territory (ACT), apart from work to manage many hectares of burnt pine trees in plantations, two key priorities have been:

  • to restore riparian vegetation in bands wide enough to trap sediment moving downslope; and
  • aerial seeding of sterile rye grass and planting of shrubs to stabilise bare areas.

The effectiveness of different widths of riparian vegetation is being studied during revegetation of the Cotter River catchment in ACT.

Work elsewhere in Australia suggests that, to trap sediment approaching a small stream, a 25m wide riparian zone is needed if the slopes are 5% or more. The wider the stream, the wider the riparian zone needed. At least 6 m width of grass buffer strip is needed if significant sediment movement is likely or if the slope is steep enough to concentrate overland flows. However, trap efficiency may also be dependent on nick points cutting through riparian zones.

Key reference

Sediment and ash deposits in channels and pools in dammed streams can be flushed by careful management of flow releases from the dams.

In-stream rehabilitation of river beds smothered with sediment has been explored at sites such as the Granite Creeks of Victoria and stretches of the Murray and Murrumbidgee rivers. The work is usually intended to directly improve habitat for fish and macroinvertebrates rather than water quality, and consists largely of installing snags. The snags, usually substantial logs or tree trunks, change the water flow patterns, allowing holes to be scoured in the channel bed; they also offer shelter or cover for and from predators, and surfaces for invertebrates to attach to.

Key reference

  • Cottingham et al., 2005
  • Cottingham et al., 2003
  • Davis and Finlayson, 2000

Snags a valuable but scarce resource. (See attached file: snag.pdf)