There is a strong consensus among Australian jurisdictions that:
- point sources of pollution are no longer significantly affecting the Australian environment; discharges of this type of pollution are tightly regulated, licensed and monitored
- current and future diffuse pollution to catchments often results from historical land clearing and land-use changes; these legacy issues, which affect water quality and flow regimes, are difficult and expensive to reverse.
Large-scale land clearing for agriculture has been largely curtailed in Australia, but the legacy of sedimentation, nutrient enrichment and salinisation of rivers is ongoing. Although the millennium drought slowed some of these degrading processes in places, it is likely that subsequent flooding will bring these issues back to the forefront of environmental concerns.
Bushfires are a natural and common feature of the Australian environment. When extensive and intense, and in our higher rainfall areas (such as the fires in Victorian catchments in 2003 and 2009), they can have serious short-term impacts on water quality and very long-term impacts on water supplies. Following the 2003 bushfires, which burned 1 million hectares of Victorian high country, studies showed that the maximum reduction in water yield (at about 25–30 years after the fires) was 692 gigalitres for the Murray River and 155 gigalitres for the Gippsland Lakes.49 Analyses estimated that, one year after the fires, the Ovens, Kiewa, Hume and Snowy catchments would deliver approximately 30 times more total suspended solids, 5 times more total nitrogen, and 8 times more total phosphorus than before the fires.50 Following the bushfires of January 2009 (which affected catchments of five of Melbourne’s nine major dams), Melbourne Water had to shift some 10 gigalitres of supplies out of storages made vulnerable to contamination from ash and debris. The effects of bushfires on water quality, and the measures that can be taken before, during and after bushfires to minimise these impacts have been recently and comprehensively reviewed.51
Land drainage and clearing for urban expansion place obvious pressures and, in some cases, irrevocable impacts on local wetlands and rivers. For example, the Southern River catchment is one of the fastest developing areas of Perth, but development is impeded by shallow groundwater and multiple wetlands of high conservation value. It is likely that future alterations in land use may significantly affect the current water balance. As inundation and shallow watertables are incompatible with urban infrastructure, it is expected that the urban development will include watertable control measures. Consequently, development is likely to increase the export of nutrients to the Canning River and Swan River estuary. These urbanisation effects have been closely modelled to support urban design that will minimise such impacts.52