An environmental flow is water released from a dam or weir, or left in the river, to improve the health of the river downstream.
The dams for greater Sydney have been built specifically to capture water that can be used for human supply. Recognition of the importance of maintaining some flow down river has resulted in major works to modify dams and weirs to allow for these environmental flow releases.
Environmental flows can help native fish and other species to breed, reduce the growth of weeds, and improve the suitability of the river for swimming and boating. Healthy rivers are also valued and enjoyed for their own sake, and support ecosystems that form a part of the natural landscape.
To enable environmental flow releases from the Upper Nepean dams and ensure the water passed downstream makes it all the way to the ocean, major upgrades to a number of dams and weirs were required.
In 2006, work began on Avon Dam to install new infrastructure to allow the release of variable environmental flows. The variable flow releases mimic natural flows because the amount of water released is linked to the amount of water flowing naturally into the dams. A multi-level off-take also permits the release of water of suitable quality and temperature.
The new flows began from Avon Dam in March 2008.
Similar works at Cataract, Cordeaux and Nepean dams have been completed and new variable flows began on 1 July 2010. Modifications to 10 other weirs on the Upper Nepean River will be completed by the end of 2010. These modifications allow not only the passage of environmental flows but also free passage by native fish in the river. The total cost of these projects is $39 million.
An environmental monitoring program is in place to measure changes in the downstream river health to see if these flows are having the desired effect on the river. It is expected that over the next 10 years the new environmental flows will result in increases in the length of the river that is swimmable, navigability by boats and able to be fished. It is now critical to the success of this initiative that provisions in the water sharing plans continue to protect those environmental releases.
In mid 2009, new variable environmental flows from Tallowa Dam to the lower Shoalhaven River were introduced. The new rules reflect significant community consultation and scientific analysis undertaken to assess river health, economic, and cultural needs. An innovative new mechanical fishlift and downstream fish passage were also constructed as part of this major upgrade project.
In line with adaptive management framework recommended by the Hawkesbury-Nepean River Management Forum (the Forum), the introduction of environmental flows for the Hawkesbury-Nepean river system has been a staged process. This process builds on the knowledge and monitoring of new flows from other dams. With the progressive implementation of flows from the Upper Nepean dams, the next major investment will be in securing long-term environmental flows from Warragamba Dam.
To find out more visit the websites below or view Chapter 7 of the 2010 Metropolitan Water Plan (PDF).
Please contact us if you would like to know more about Environmental Flows.