Flow Release Trials
Ecological Water Requirements often require the release of water from dams to maintain flows in downstream reaches of creeks, especially in summer when reduced flows, depth and oxygen levels may place resident fauna at risk.
With a continuing drying climate in the southwest of Western Australia, it is becoming increasingly difficult to meet the stipulated environmental flows in some systems. As a result, the volumes being released are often under review. In some instances the flows have been reduced, but only after thorough investigation of the effects of reduced flows on water quality and habitat inundation. For the lower Helena River, WRM were commissioned to determine suitable flows to sustain and/or improve the health of aquatic ecosystems downstream of Mundaring Weir and the Lower Helena Pipehead Dam (PHD).
The project was funded by the Water Corporation, and supported river restoration activities and on-going monitoring of the Helena River by the Swan River Trust, Eastern Metropolitan Regional Council and the State Department of Water. Trial environmental flow releases from the Lower Helena Pipehead Dam were used to relate coverage of in-stream habitat (high biodiversity riffle areas) to declining flows, and to define a minimum threshold flow to prevent anoxia in river pools.
The study determined that a minimum summer baseflow of 9 L/sec was required to inundate riffle habitat and maintain water quality in permanent river pools. Subsequently, summer releases from the PHD were implemented and follow up monitoring revealed increased abundance of native fish, crayfish and macroinvertebrates.