The Importance of Groundwater to the Blackwood and Other Iconic Rivers of the South-West
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Department of Water Department of Regional Development The importance of groundwater to the Blackwood and other iconic rivers of the south-west Yarragadee and Leederville aquifers discharge into the Blackwood River near Darradup measurement station, not far from Nannup The Blackwood, Margaret, Donnelly, Warren, Margaret River’s pools full, even during the long, and Capel rivers in the south-west of Western dry summer months. Australia play a major role in supporting local Our new findings show how the Blackwood, one of industry, agriculture, tourism and the environment. the region’s most recognised rivers, is supported Given their importance to the region, the by natural groundwater flows. The Blackwood Government of Western Australia has made a $1.6 has the region’s longest section of permanent million investment made possible by Royalties for groundwater-fed flow and it uniquely displays Regions funding to better understand the rivers the reverse of typical river behaviour by being in the face of a drying climate and increasing more saline during winter than in summer. This groundwater use. phenomenon is the result of salt in the catchment For the first time, the Department of Water’s and the fresh groundwater feeding into the river. expert scientists have studied the connection Throughout the year, freshwater from the between these five rivers and the major aquifers Yarragadee and Leederville aquifers discharges of the region. In doing so, we have discovered a into the Blackwood just downstream of Nannup. number of unique attributes about the rivers and During winter, rainfall over the salt-affected underlying aquifers. headwaters of the Blackwood washes downstream and increases salinity of the flow. In summer, The discovery with less rainfall, the fresh groundwater has more We have quantified the influence that groundwater influence on the quality of the river water. has on keeping parts of the Blackwood, Donnelly, Our studies also show how the ecology of Warren and Capel flowing all year round, and the September 2016 Securing Western Australia’s water future the Blackwood has changed in response to In studying the Blackwood in particular, we the different levels of salinity. During winter, determined the chemical tracers found in the freshwater fish species such as Balston’s pygmy rain and groundwater samples are only present perch (listed as a ‘Vulnerable’ species under the downstream of where the Yarragadee and federal Environmental Protection and Biodiversity Leederville aquifers come to the surface, or Conservation Act 1999) seek refuge in the outcrop. This tells us that the water that supports freshwater tributaries like Milleannup Brook or the rivers spends very little time underground Poison Gully. before flowing back out again at different points. During summer when the river freshens they move By measuring summer flow volumes upstream back into the main channel. The groundwater and downstream of these points, we have discharge in summer is also important for the determined how much groundwater flows into the migration of freshwater cobbler, the largest rivers and how it keeps water flowing or pools full freshwater fish species in the south west. year round. Further decreases in water quality and Why are these discoveries important? groundwater volume will be detrimental to the rivers and the aquatic species they support. Studying the rivers of the south-west provides Careful management of groundwater around the an excellent indicator of how the region’s rivers is important to looking after these systems. groundwater systems are responding to reduced rainfall and groundwater use. This helps us How did we do it? predict the likely changes in groundwater-to-river A specialised water study, using rain gauges and outflow over the coming years. targeted aquifer and river water sampling, tracked Because so many businesses, communities, rainfall using chemical tracers or ‘fingerprints’ as tourists and aquatic species rely on the rivers of it soaked into the ground and through aquifers the south-west, this sort of research gives us the and rivers. This has helped define recharge knowledge we need to understand and manage patterns and how the river’s flow may be the sustainability of the resource in the face of a impacted by water use and fluctuations in rainfall drying climate. linked to climate change. River flow salinity changes over a 12-month period Blackwood River salinity (mg/L) Margaret River salinity (mg/L) Winter flow Summer flow Further information River health assessment in the lower catchment of the Blackwood River Department of Water • Busselton office Travis Cattlin • Senior Hydrogeologist www.water.wa.gov.au [email protected] • Phone (08) 9781 0125 © Government of Western Australia WT11976 0916.