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 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 fish species and estuarine species (such as black bream).
Recent conditions
The Werribee system catchment experienced wetter-than-average conditions in autumn 2020 and throughout 2020-21. High rainfall in autumn 2020 caused Pykes Creek Reservoir and Melton Reservoir to fill and spill, and both storages spilled on several occasions throughout late spring and summer 2020-21. Allocations against high-reliability water shares in Melton Reservoir reached 100 percent by December 2020, and low-reliability water shares reached 80 percent by March 2021. In contrast, Lake Merrimu had below-average inflows and contributed only a small volume to the environmental entitlement in 2020-21.
Pyrites Creek did not have any significant natural high-flow events in 2020-21. Water for the environment was used to deliver a spring fresh, a spring/summer high flow and maintain low flows to the end of December. These flows enabled connections
between habitat pools for native fish, frogs and waterbugs. The flows supported the recruitment and growth of native vegetation within the creek and on the banks, and it flushed sediment from pools along the length of the reach. Most of the water delivered during the high flow in September and the fresh in December was re-harvested at Melton Reservoir and used to deliver watering actions in lower Wirribi Yaluk (Werribee River).
Spills from Melton Reservoir provided some large natural flows through reaches 8 and 9 and the Werribee River estuary during spring and summer. Water for the environment was used to deliver a partial spring/summer high flow in September to support the upstream migration of native fish from the estuary. Small environmental flows were delivered during summer and autumn to freshen the lower reaches and flush algal blooms that developed near the Werribee Zoo. The storage manager’s increased passing flows below the Werribee Diversion Weir met the low-flow watering actions during late summer, autumn and winter.
Water for the environment was managed in the Werribee system in accord with an average climate scenario in 2020-21. Most planned watering actions were fully met. The spring high flow in Pyrites Creek and winter/spring fresh in the lower Wirribi Yaluk (Werribee River) were only partially delivered due to capacity constraints at reservoir outlets, but the expected watering effects and environmental objectives for these flows were likely met.
Delivering a partial winter/spring fresh in 2021-22 under dry and average conditions and a full winter/spring fresh under wet conditions will remain a high priority, to support black bream recruitment following a fish death event in early 2020 linked to low oxygen and an excess influx of nutrients in stormwater.