Environmental values
The Werribee system supports a range of native fish, including Australian grayling, river blackfish, flathead gudgeon, short- finned eel, tupong, Australian smelt, several species of galaxiids and a large population of black bream in the estuary. Several species of frogs, a diverse waterbug community and platypus inhabit the upper and lower reaches. The freshwater-saltwater interface of the Wirribi Yaluk (Werribee River) estuary is a regionally significant ecosystem due to the many aquatic plants and animals it supports, and it provides nursery habitat for juvenile freshwater and estuarine fish species (such as black bream).
Recent conditions
The Werribee system experienced wetter-than-average conditions throughout most of 2021-22. The second consecutive year of above-average rainfall meant Pykes Creek Reservoir and Melton Reservoir both spilled through most of winter and spring and into early summer. Allocations against both high- and low-reliability water shares in Melton Reservoir reached 100 percent by early January 2022. Lake Merrimu had above-average inflows, resulting in the highest volume available under the Werribee River environmental entitlement since 2016-17.
Spills from Melton Reservoir between 10 June and 25 December 2021 provided noteworthy flows through reaches 8 and 9 (including bankfull flows in November 2021) and the Werribee River estuary. Despite wet conditions, Pyrites Creek did not have substantial natural inflows in 2021-22 because most flow was harvested in Lake Merrimu, where storage peaked at 82 percent capacity in December 2021, the highest recording since late 2013.
Water-quality-monitoring equipment was installed at Cobbledicks Ford in reach 8 and was upgraded in the Wirribi Yaluk (Werribee River) estuary. The data collected will be used to predict potential algal blooms and inform the delivery of water for the environment to mitigate blooms.
Water for the environment was managed in the Werribee system in accordance with a wet climate scenario in 2021-22. Most planned watering actions were fully achieved through a combination of natural flows, environmental water deliveries and operational flows. In Pyrites Creek, water for the environment was used to deliver four spring freshes and three spring/summer high flows and to maintain low flows to the end of December. In the Werribee River, water for the environment was used to deliver two spring/summer freshes. Passing flows below the Werribee Diversion Weir met the low-flow watering actions during late summer, autumn and winter.
The only planned watering action that was not fully achieved was a summer fresh in the lower Werribee River. Water for the environment was ordered and released from Melton Reservoir to deliver a fresh in late January, but the order was cut short due to operational requirements. Natural freshes before and after the planned event are likely to have met the environmental objectives on this occasion. Melbourne Water, Southern Rural Water and the VEWH are collaborating to reduce the likelihood of similar operational constraints affecting future environmental water orders.
