Our Water Vulnerabilities
The rivers, wetlands and water are vulnerable as a result of:
Rising water tables, increased runoff and drainage of saline surface and groundwater has transported large amounts of salt into receiving waterways and wetlands in the NAR. Rising water tables and increasing groundwater discharge has also caused inundation and waterlogging of streamlines and valley floors. When the salinity levels within waterscapes start to rise, the riparian vegetation is replaced by more salt tolerant species and macroinvertebrate diversity is reduced.
Along the coastal areas adjacent to the rivers are freshwater aquifers. Where extraction of groundwater occurs, there is risk that may lead to salt water intrusion and consequently contaminating the freshwater supply (DoW, 2009).
The effects of sedimentation are widespread in the NAR. In particular, the Irwin, Chapman, Moore and Greenough rivers have extensive problems, as do the estuaries, which is a result of significant sediment inputs from the contributing rivers. Widespread clearing in the NAR has altered sediment regimes through the influx of fine silt and clay particles from soil erosion. Deposition of sediment from the channel or through broad catchment erosion has caused the filling of river pools, smothering of aquatic habitat, reduction of channel capacity and channel avulsions (a new channel breaking out adjacent to the old channel. Artificial drainage systems built throughout the region also contribute considerable quantities of sediment to waterscapes, as many of these structures are poorly maintained and erode rapidly Anecdotal reports suggest that freshwater ﬁshing used to be a signiﬁcant pastime across most of the NAR, but sedimentation, water quality decline and changes in hydrology have limited this recreation to the estuaries (NACC, 2005). Crustaceans are still collected by recreational ﬁshers in several parts of the region.
Loss of Riparian Vegetation
The ecological sustainability of rivers and wetlands are largely dependent on the presence of healthy foreshore vegetation and sound water quality. Despite widespread clearing in the NAR, narrow bands of native vegetation still remain along major waterways, yet the extent of riparian vegetation surrounding wetlands is largely unknown. In most instances, however, this vegetation has been damaged by salinisation, waterlogging, grazing and trampling by domestic and feral animals and invasion by weeds, and is continuing to deteriorate. Stock access to rivers and streams is generally unrestricted and has resulted in severe loss of fringing vegetation with subsequent erosion and sedimentation.
Major contributors to point source contamination entering waterscapes in the NAR include former mine sites, landfill and heavy industry. An old mine site at Galena is leaching heavy metals from tailings into a tributary of the Murchison River. Similar mine sites occur on tributaries of the Hutt, Greenough, Chapman and Bowes Rivers and moderate contamination of heavy metals has been recorded. Lead contamination from Nokenina Brook has also been recorded in the Bowes River. As estuaries within the NAR are popular recreation sites it is necessary that water quality does not drop below relevant Water Quality Guidelines.
Flooding occurs when sufficiently heavy or prolonged rainfall produces run off which overflows the banks of the rivers and wetlands. Flood problems in the NAR arise where settlement has taken place in flood prone areas along rivers and coastlines, for example Moora and Dongara. Floodplains continue to be under pressure from more intensive uses despite the significant flood risk and this pressure is increasing as land becomes scarce. Severe floods do not happen frequently in the NAR, however when flooding does take place the resulting damage to property, infrastructure and land can be considerable.
Nutrients enter waterscapes either attached to soil particles or dissolved in water. A significant contribution to nutrient enrichment in the NAR results from animal waste and fertilisers. Point sources of nutrient enrichment arise from animal feedlots/ abattoirs and septic tanks. Eutrophication and resultant algal blooms can have an impact on aquatic flora and fauna through oxygen starvation and on vertebrate fauna through toxicity and the reduction in food resources. Throughout the NAR are routinely affected by eutrophication. Gingin Brook feeds very high levels of soluble phosphorus into the Moore estuary, causing regular summer macroalgae blooms. The Chapman estuary also has high nutrient levels and experiences a consistent algal bloom. Intermittent river flow mobilises large amounts of animal waste which results in offensive smelling algal blooms in river pools. Wetlands such as Lake Indoon are becoming increasingly eutrophic, as a result of increased runoff in the catchment transporting extra nutrients from farming areas.