The Sydney Catchment Authority (SCA) recognises that its dams and weirs affect the natural flow of water downstream. The SCA releases water from the storages to downstream rivers through environmental flows to help restore the ecological processes and biodiversity of water dependent ecosystems. Environmental flows represent around two to three percent of the total water we use each day.
Daily variable environmental flows
The SCA publishes a weekly report on daily environmental flows from the following dams and water suppy weirs: Avon, Cataract, Cordeaux, Nepean, Tallowa and Woronora dams, and Broughtons Pass and Pheasants Nest weirs.
Current environmental flows
The SCA releases five megalitres of water each day from Warragamba Dam to dilute discharge from the Wallacia sewage treatment plant into the Warragamba River (average 0.6 megalitres per day). Another 17 megalitres of water is released each day in winter, increasing to 25 megalitres in summer, for Sydney Water to extract at its North Richmond Water Filtration Plant. These releases are specified in the Water Sharing Plan for the Greater Metropolitan Region Unregulated River Water Sources 2011.
In the 2010 Metropolitan Water Plan, investigations into a new environmental flow release regime from Warragamba Dam were announced. The Department of Finance and Services is currently coordinating these investigations with substantial contributions from the SCA. A decision on environmental flows from Warragamba Dam is to be included in the next Metropolitan Water Plan.
The SCA releases water from Tallowa Dam and Wingecarribee and Fitzroy Falls reservoirs to help improve the environmental health of the rivers downstream and sustain riparian rights.
At Wingecarribee Reservoir, at least three million litres of water is sent downstream every day for environmental purposes.
At Tallowa Dam, daily variable flows for environmental purposes began on 15 July 2009. Environmental flows and improved movement of fish up and down the river were made possible by the installation of a new offtake at Tallowa Dam, a new spillway gate at the top of the dam which ensures the environmental flows are at a similar temperature to the downstream river, and a mechanical fish lift to allow native fish to move upstream and downstream. At times of low flows, all inflows to Tallowa Dam up to 371 million litres a day (depending on the season) are released to the downstream river. At times of higher flow, an additional 20 percent of inflows to Tallowa Dam are released to the downstream river.
At Fitzroy Falls Reservoir, environmental release levels are linked to inflow rates measured at Wildes Meadow Creek.
Upper Nepean system
The SCA introduced daily variable flows from the Upper Nepean dams and water supply weirs for environmental purposes from 1 July 2010. Improvements to weirs along the Hawkesbury-Nepean River help the new flows make it downstream, with modified or replaced fishways to allow fish to move more freely up and down the river to breed.
At times of low flow, inflows to the Upper Nepean dams and water supply weirs are released to the downstream river. Daily variable inflows of up to 20.1 million litres are released from Nepean Dam, 6.8 million litres from Avon Dam, 4.5 million litres from Cordeaux Dam and 14.5 million litres from Cataract Dam. These releases are passed through Pheasants Nest and Broughtons Pass weirs to the downstream river. Inflows from the catchments between the dams and weirs are also released from the weirs, including up to 4.4 million litres from Pheasants Nest Weir and up to 4.5 million litres from Broughtons Pass Weir. At times of higher flow, an additional 20 percent of inflows to each dam and water supply weir are released to the downstream rivers.
The SCA introduced daily variable flows from Woronora Dam for environmental purposes from 15 July 2009. At times of low flows, all inflows up to 4.1 million litres a day are released to the downstream river. At times of higher flow, an additional 20 percent of inflows to Woronora Dam are released to the downstream river.
Blue Mountains system
The Blue Mountains system is comprised of six small reservoirs located high in the catchment. Apart from overflows during periods of high rainfall, there are no current environmental releases from these dams.