Water – Background Information

Major contributors to contamination of water in the NAR include landfill, former mine sites and heavy industry. For example, an historic 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 levels of heavy metal contamination have been recorded in each of these rivers. As estuaries within the NAR are popular recreation sites and important fish and bird habitat, it is essential that water quality does not drop below relevant Water Quality Guidelines (DAWE 2020).

Public drinking water sources and their catchments are protected by a series of by-laws and protection plans (DWER 2020) that prohibit polluting activities. Land use planning and zoning decisions currently discourage the location of potentially contaminating uses in proximity to sensitive environments. Where contaminated sites exist, they should be managed following the Contaminated Sites Act 2003.

Declining rainfall, reducing surface water flows and groundwater recharge overall, along with a steadily increasing demand for potable water for settlements and industry has resulted in declining availability of water sustaining environmental processes. The Mid West Regional Water Supply Strategy launched in 2015 provides an overview of how available water resources will be matched with agriculture, mining, urban expansion and industry needs in the northern part of the region.

The Department of Water and Environmental Regulation is currently refining the legislation for water resources management. The purpose is to enable the productive use of water, protect water resources and their environments, and ensure that water resources are shared, managed and used well. The proposed Water Resources Management Bill will replace the six different Acts under which water resources in Western Australia are currently managed.

Erosion is the transport of soil, sediment and rock fragments by wind and water. River banks are more vulnerable to erosion when riparian vegetation has been cleared or disturbed by grazing, flood, or development. Sedimentation refers to the settling of eroded material transported by water as flows slow (DWER 2020). Excess sediment can block waterways, cause upstream flooding, redirect water flows and cause further erosion, fill deep permanent pools and ruin crucial refuge habitat. Increased fine sediment suspended in the water column (turbidity) reduces light penetration, increases the risk of algal blooms, and can clog the gills of fish. 

The Irwin, Chapman, Moore and Greenough rivers and estuaries all have extensive sedimentation problems due to widespread clearing in the riparian zone, broad-scale catchment erosion and a network of poorly maintained artificial drainage systems. Sedimentation, water quality decline and hydrology changes have led to a decline in freshwater fishing for recreation across most of the NAR. Management actions include fencing and revegetating foreshore areas, implementing farming practices that reduce erosion and maintain vegetation cover or intercept, slow and infiltrate surface runoff. When river banks are too unstable to support vegetation, installing engineered management options can help secure and protect waterways (DWER 2020).

Eutrophication occurs when water becomes overly enriched with nutrients and minerals and is a widespread problem in the NAR. Excess nutrients from livestock waste and fertilisers are significant contributors to eutrophication in the NAR. Excess nutrients cause algal blooms that starve aquatic flora and fauna of oxygen. This leaves wetlands and estuaries in a toxic state and reduces food resources for terrestrial species that feed on aquatic flora and fauna. For example, Gingin Brook feeds very high levels of soluble phosphorus into the Moore estuary, causing regular summer algae blooms. Inland wetland systems such as Lake Logue/Indoon are becoming increasingly eutrophic due to excess nutrients entering from the surrounding farming areas.

Information is critical to developing robust strategies to reduce eutrophication. Monitoring nutrient levels and water quality wherever possible is vital (Selman & Greenhalgh 2009).  Fertiliser should be applied according to soil type to avoid excessive leaching of nutrients. Revegetation projects along waterways can assist by taking up excess nutrients. Nutrient load caused by livestock may be prevented by fencing off rivers and streams so they are not accessed for grazing.

Flooding occurs when sufficiently heavy or prolonged rainfall produces runoff which overflows the banks of rivers and wetlands. Flooding is only a concern in the NAR when it affects infrastructure located in flood prone areas along rivers and coastlines. Impacts of flooding are particularly severe where natural waterways or flow paths have been modified or development has been inappropriately located. Severe floods are infrequent in the NAR but when flooding does take place, the resulting damage to property, infrastructure and land can be considerable.

The Department of Water and Environmental Regulation has undertaken floodplain mapping for the Greenough, Chapman, and Irwin rivers. Consistent with State Planning Policy 2.9 Water Resources, all new developments must consider impacts on water resources and potential impacts of flooding on the proposed infrastructure. Where climate change is leading to an increase in the intensity of rainfall events and the frequency and severity of coastal storms, additional assessments of flood risk and mitigation and adaptation options may be needed.

Despite widespread clearing for agriculture in the NAR, narrow bands of native vegetation remain along all of the major waterways. The extent of riparian vegetation around wetlands and estuaries has not been assessed. Most riparian zone vegetation in the NAR is degraded and in decline, driven by grazing, salinity, the presence of feral animals and exotic weeds (Richardson et al. 2014). Access to rivers and streams by livestock is largely unrestricted and has resulted in severe losses of fringing vegetation. This is turn leads to erosion and sedimentation.

River restoration projects aimed at alleviating the extent of erosion and sedimentation involve stabilising riverbanks by planting native trees and shrubs, livestock exclusion fencing along water-courses and realigning large woody debris to slow and direct water flow.  Reducing the number of feral animals, particularly pigs and goats, that have access to water courses also reduces damage. Foreshore assessments have been developed for the Chapman, Greenough and Hutt rivers. These assessments identify the most degraded foreshore areas that can be targeted for urgent rehabilitation.

Dryland salinity, where salts stored deep below ground are brought close to the surface by a rising water table, is a significant cause of land degradation in the region. Shallow-rooted annual crops and pastures transpire significantly less water than the native vegetation they replaced, leading to an increase in groundwater recharge, rising  groundwater levels and the development of shallow water tables in areas where often none existed previously (DPIRD 2019). Rising groundwater levels mobilise soluble salts stored below ground and concentrate them in the root zone. Salts are then transported by runoff into waterways and wetlands. Rising water tables and increasing groundwater discharge have also caused inundation and waterlogging of streamlines and valley floors. Salt water intrusion may occur where groundwater is extracted from freshwater aquifers in coastal areas, contaminating water supply.

Over most of the region, the impact of rainfall on groundwater trends is still less than the impact of clearing and most water tables continue to rise in areas cleared and developed for agriculture after 1960. As these areas approach a ‘new’ hydrological equilibrium, climate change will become the dominant controller of groundwater level trends and impact the extent of dryland salinity. Roughly half of the region, particularly the inland areas, is at a moderate to high risk of worsening dryland salinity (DPIRD 2019). Strategies to manage dryland salinity include revegetating cleared areas with native vegetation, planting saline tolerant species, grazing exclusions and engineering options (DPIRD 2020).