The estuarine reach of the Barwon River contains a system of wetlands and lakes including Lake Connewarre, Reedy Lake, Hospital Swamps, Salt Swamp and Murtnaghurt Lagoon. Water for the environment can be used to manage water levels in Reedy Lake and Hospital Swamps, which connect to the Barwon River.
Environmental watering objectives in the Lower Barwon wetlands
Reedy Lake and Hospital Swamps form part of the internationally recognised Port Phillip Bay (Western Shoreline) and Bellarine Peninsula Ramsar site, which are used by many thousands of migratory birds from around the world. The wetlands support about 47 threatened flora and fauna species and communities. These include some of Victoria's rarest species (such as the brolga, orangebellied parrot, Australasian bittern, growling grass frog, Australian grayling and dwarf galaxias) and subtropical and temperate coastal saltmarsh communities.
Reedy Lake supports a range of vegetation communities including coastal saltmarsh, herbfields and reed beds. Reedy Lake was originally a partly-ephemeral system, but river regulation meant the lake was permanently inundated from the 1970s until 2016. Permanent inundation favoured the reed bed community in the lake and over time it has increased its extent and replaced much of the coastal saltmarsh and herbfield communities and openwater habitat. While reed beds form an important part of the lake's ecosystem, their continued expansion has reduced habitat diversity and the number and diversity of internationally-important migratory waterbirds that were able to use the wetland.
The Corangamite CMA has implemented the first two years of a four-year watering regime at Reedy Lake which includes three years of partial summer/autumn drying and one year of full summer inundation. This has already helped to control carp numbers and improve conditions for communities of coastal saltmarsh and herbfields. Achieving a more-natural wetting and drying regime is the single most important management activity to protect the ecology of the lower Barwon wetlands.
Hospital Swamps is made up of five unique wetland basins that support important ecological processes and significant ecological values including large areas of threatened coastal saltmarsh and diverse waterbird populations. Vegetation communities in Hospital Swamps have remained largely unchanged over time due to the maintenance of natural wetting and drying cycles.
Social and economic values
The lower Barwon wetlands are located close to Geelong, which is the second-largest city in Victoria. The wetlands form a very important part of the region's social fabric and are valued for their beauty, ecological significance and recreational uses. In particular, the wetlands are used heavily by Geelong Field and Game for hunting and by Geelong Field Naturalists for birdwatching and recreation. The system also supports a commercial eel fishery.
High streamflow in the Lower Barwon River in winter 2017 led to three flood events that filled Reedy Lake and Hospital Swamps. The Corangamite CMA closed the inlet regulators after floodwaters receded, and it left them closed over summer to allow the wetlands to dry.
The outlet to Reedy Lake was opened in summer to allow water levels to draw down naturally to between 0.1–0.3 m AHD, implementing the second year of a three-year partial summer/autumn drying regime. Before 2016–17, the lake had been kept at a constant water level for over eight years.
It is likely to take several years for the lake's ecosystem to adjust to the new water regime, but monitoring at Reedy Lake has already shown minor changes to the diversity and abundance of plants and animals since the drying regime was implemented. Also, initial investigations have confirmed the partial drying regime will not be detrimental to native fish species or increase the risk of acid sulphate soils.
A natural pattern of wetting and drying has been implemented in Hospital Swamps for longer than Reedy Lake. Inflows from the Barwon River are allowed to enter the wetland in the winter/spring period, and water is then actively drawn down over the drier summer months. High rainfall in spring 2017 increased water levels in Hospital Swamps above the normal winter-fill level, extending the subsequent period for drying and drawing down. The wetland needs to dry regularly to maintain the balance between freshwater and saltwater processes required to support the diverse mix of vegetation communities and provide feeding and breeding habitat for waterbirds and native fish.
Scope of environmental watering in 2018-19
Table 1 shows potential environmental watering actions and associated environmental objectives.
The main objective for environmental watering in the lower Barwon wetlands is to implement natural wetting and drying cycles. Reedy Lake and Hospital Swamps will be filled in winter and spring when water levels in the Barwon River are high, and they will be allowed to draw down over summer. These water regimes will be managed using regulators that control flow in either direction between the Barwon River and the wetlands.
Hospital Swamps has had an appropriate wetting and drying regime for many years and there is no plan to change its management in 2018–19. The plan for Reedy Lake will be to implement the third year of the partial drying regime. Reedy Lake will be allowed to fill in winter and spring and then draw down in summer, to reduce the extent of reed beds and allow other vegetation communities to recolonise. Unlike previous years when the outlet has been manipulated to maintain the water level at about 0.3 m AHD, water levels will be allowed to naturally fluctuate between 0.1–0.3 m AHD, as a result of tidal influences. This approach is practical from an operational perspective as it requires less-frequent adjustment of the regulators and will allow natural fluctuations in salinity.
Potential environmental watering
Autumn/winter/spring filling flows (in March/April–October) 1
The inlet to Reedy Lake will be opened in autumn in response to a sustained increase in flows in the Barwon River
Spring/early summer drawdown (October– January) and then variable low water levels (around 0.1–0.3m AHD) throughout summer/ autumn (January–March/April)
The inlet to Reedy Lake will be closed and the outlet opened to allow water levels to drop to about 0.1–0.3 m AHD; during this period, the inlet and outlet may be manipulated if required to maximise the drawdown or to introduce saltwater to the lake
Autumn/winter filling flows (in May–November) 1
Hospital Swamps will be connected to the Barwon River for at least 6 weeks by keeping the inlet and outlet open
Summer/autumn drawdown (in December–March/April)
The inlet to Hospital Swamps will be closed to allow water levels to drop through evaporation; during this period, the outlet will be opened for short periods if a summer storm increases water levels above 0.85 m AHD
1 Water can only be diverted into the lower Barwon wetlands when water levels in the Barwon River are above 0.7 m AHD at the lower Barrage Gauging Station, in line with provisions for accessing water under the environmental entitlement.
In preparing its seasonal watering proposal, Corangamite CMA considered and assessed the risks of environmental watering and identified mitigation strategies. Program partners continually reassess risks and mitigation actions throughout the water year.
Table 2 shows the partners, stakeholder organisations and individuals with which Corangamite CMA engaged when preparing the lower Barwon wetlands seasonal watering proposal.
Seasonal watering proposals are informed by longer-term regional catchment strategies, regional waterway strategies, environmental flow studies, water management plans and other studies. These incorporate a range of environmental, cultural, social and economic perspectives and longerterm integrated catchment and waterway management objectives. For further details, refer to the Corangamite Regional Catchment Strategy and the Corangamite Waterway Strategy.
Over the last six years, the Corangamite CMA has consulted extensively about the planned watering regimes for Reedy Lake and Hospital Swamps with diverse stakeholders and interest groups representing over 1,500 people. These people have been involved in developing the original environmental flows study and in subsequent scientific work about ecological risks, vegetation monitoring, alternative management approaches and infrastructure operations. The results of this work show that lowering water levels at Reedy Lake is the only feasible management practice that will mitigate threats to the ecological health of the wetland and ensure all user groups can continue to use the system in future.
Table 3.7.6 Partners and stakeholders engaged in developing the lower Barwon wetlands seasonal watering proposal
|Partner and stakeholder engagement|