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Blackwood RegionIssuesThe Blackwood Region faces environmental challenges common to many areas of south-west Western Australia - salinity, erosion, waterlogging and the decline of biodiversity. Over 80% of the streams and wetlands in the Blackwood Catchment are severely degraded due to clearing, grazing, burning and erosion. Salinity
Western Australia's soils have accumulated massive amounts of salt brought in by rain from the sea over tens of thousands of years. Salinity problems arise when the watertable (the level at which the soil is saturated with groundwater) rises. This causes salt stored in the soil to dissolve and also rise to (or near) the surface. In nature trees play an important role drawing water from the soil into their leaves and back to the atmosphere, this keeps the water table deeper in the soil. When trees are cleared and replaced with shallow-rooted annual crops and pastures less water is drawn from the ground. The result is a rise in water table, bringing salt stored in the soil to the surface, which then affects plant growth and agricultural production, as well as increasing the salinity levels of streams and lakes. (Source: Water Fact 15) With the widespread clearing of the native vegetation of the inland areas of the south-west and replacing it with crops and pastures salinity is a major issue in the region. Salinity in the Blackwood River, with an 85% cleared catchment has gone from about 500 mg/L in the mid 1950's to approximately 4500 - 6000 mg/L in the mainstream reach at present. (Source: SWCC Technical Report 2004) The only remaining fresh water tributaries of the Blackwood River are located downstream of Bridgetown where the catchment is forested and rainfall is high, however the main Blackwood River channel in the lower catchment is expected to double is salinity over the next 60 years. Freshwater fish have left the brackish reaches of the river. What can we do about salinity?
Planting trees and other deep rooted perennials will lower the groundwater and keep the salt back in the sub-soil, where it is harmless. Saline areas can be planted with salt tolerant vegetation such as samphires or saltbush. Managing salinity however is much more than planting trees. It requires an understanding of the recharge (water soaking into the ground) and discharge (the loss of groundwater through seepage at the soil surface). To find out how effectively salinity is being managed, it is important to measure the changes in the water table and the salinity of the groundwater, on a regular basis, with observation bores and piezometers. Nutrients
Other problems in the Blackwood River include the increase in the amount of nutrients and pollutants entering the river. Algal blooms routinely occur in the Blackwood. Nutrients, mainly nitrogen and phosphorus, are washed into the river system in organic matter (manure, crop residues etc) and fertiliser from agricultural and urban areas. When the algae die they use up oxygen which can kill animal and microbial life in the river. As the process worsens algal blooms become more frequent and they can include toxic species which make the waterways unsuitable for stock watering and recreation. (Source: SWCC Technical Report 2004) Oxygen depletion in the summer river pools is probably the main cause for the decline of marron populations in the river. Sediment
The river pools are filling up with sediment and organic matter, a result of erosion across the catchment. This is altering the flow and affecting the habitats of aquatic animals. River pools provide essential habitat for many aquatic animals during the summer / autumn months when streams stop flowing. The loss of river pools would have a catastrophic effect on the river ecosystem. Large sediment accumulations can retard stream flow and cause upstream flooding. (Source: SWCC Technical Report 2004) Habitat loss
Active clearing, passive clearing by livestock, wetland filling, salinisation and erosion has lead to the loss of native fringing vegetation which is important habitat for macroinvertebrates, fish and other animals. Many parts of the river system retain only its immediate fringing vegetation and often then only in patches and with fewer of the original native species. Loss of fringing vegetation means there are less branches and trunks of trees falling in the water (important habitat for animals) and that more sunlight hits the water surface, increasing its temperature and creating conditions that favour pest species such as mosquito fish and carp. (Source: SWCC Technical Report 2004) Reduced water flow
In the past dams and weirs were built with little regard for downstream flows and the water requirements of flora and fauna. (Source: SWCC Technical Report 2004) An emerging issue for the lower Blackwood is pumping water from the Yarragadee aquifer to supplement the growing need for potable water for Perth and the south-west of WA. Taking water from underground has the potential to reduce freshwater flows to the lower tributaries of the Blackwood. (Source Brearley 2005) References and Further Information Pen, Dr Luke J., 1999, Managing Our Rivers; A guide to the nature and management of the streams of south-west Western Australia, Water and Rivers Commission Water and Rivers Commission 2004, The Importance of Western Australia's Waterways, PDF Document accessed on Department of Environment web site 19/07/2006 South West Catchments Council, 2004, Technical Report: Water Assets of the South West Region Brearley, Anne, 2005, Earnest Hodgkin's Swanland: estuaries and coastal lagoons of Southwestern Australia, University of Western Australia Press Water Fact 15, Salinity, The Water and Rivers Commission, June 2000
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