Estuaries are the places where rivers and the sea meet. They are typically semi-enclosed coastal bodies of water with a connection with the open sea and within which seawater is measurably diluted with fresh water from land drainage. There are 40 estuaries within the Corangamite region, of all shapes and sizes. Estuaries of the Corangamite region are highly valued for recreational use, particularly the Barwon River estuary and the numerous estuaries along the Great Ocean Road.
The region’s estuaries are also important environmental assets of the region’s coastline. They support a range of distinctive aquatic and terrestrial plants and animals, including rare and threatened species and communities. They are important drought refuges, provide significant breeding and feeding areas for birds, and spawning areas and nursery habitat for fish. Vegetation and saltwater marshes (including the nationally vulnerable Coastal Saltmarsh) adjacent to estuaries maintain water quality, assist with nutrient cycling, and provide a buffer to catchment-derived sediments and pollutants entering the marine environment.
The Barwon River estuary is the only estuary in the region that is permanently open. All the rest are classed as “intermittently open/closed estuaries”, which means they sometimes close to the ocean. Corangamite CMA regulates artificial openings of estuaries throughout the region. An artificial opening most commonly occurs when the estuary mouth is closed off from the ocean due to a berm (a sandbar) that has naturally built up, meaning the river water cannot empty into the ocean. The region’s estuaries are normally quite resilient to coastal processes such as tidal exchanges, shoreline recession and natural estuary openings.
Habitats in estuaries include shallow wetlands, saltmarshes, mangroves, seagrass beds, sandy and muddy sediments as well as the water column itself. These habitats support a range of waterbirds, fish and invertebrates. The survival, health and distribution of these plants and animals are dependent on various physical and chemical processes operating in each estuary.
Assessment of current condition and trends
The condition of estuaries within the Corangamite region was assessed as part of the 2010 Index of Stream Condition, with 18 estuaries included in the assessment (two in the Barwon Basin and sixteen in the Otway Coast Basin). From this assessment, 61% of estuaries were classified as being in moderate to excellent condition.
There are 40 estuaries in the Corangamite region. All of these estuaries, with the exception of the Barwon River, are intermittent estuaries. This means that they have sandbars that periodically close their connection to the ocean. The closure of an estuary entrance can result in an increase in water level and inundation of adjacent land. Inundation is a natural process and plays an important role in the life cycle of many species and the cycling of nutrients. Periodic inundation of adjacent wetlands and fringing vegetation is also necessary to ensure their ongoing health. For some estuaries, reduced freshwater inflows reduce the frequency of flushing flows that open estuary entrances. Artificial estuary openings are typically undertaken to protect human assets and infrastructure. Artificial estuary openings are not undertaken for environmental reasons.
Artificially opening the estuary entrance to allow the excess water to flow out to sea can reduce the social and economic costs associated with estuaries flooding. However, there are environmental impacts associated with this intervention. The detrimental effects of artificial estuary entrance opening events can include:
- disruption to the natural patterns of variation in water quality
- impacts on plant and animal species, including mass fish deaths
- disruption of animal migration and reproductive cycles.
The Corangamite CMA regulates artificial estuary openings in the region, issuing works on waterway approvals under the Water Act 1989.
EstuaryWatch is a successful citizen science program that supports community members to actively participate in the monitoring of estuary health. EstuaryWatch volunteers are passionate about their local environment and meet once a month to collect valuable data on the condition of their local estuary. The Corangamite EstuaryWatch program was established in 2006 in response to a groundswell of community interest, and a lack of long term data on the condition of Victoria’s estuaries. There are currently 61 active EstuaryWatch volunteers monitoring 11 Estuaries in the Corangamite region. EstuaryWatch data has been used to educate and inform better estuary management. Water quality data, estuary observations and photos collected by EstuaryWatch have been referred to as part of algal bloom, fish death and storm surge response and has been incorporated into estuary management plans, research projects and the decision support tool for artificial estuary openings in Victoria, the Estuary Entrance Management Support System (EEMSS). The monitoring undertaken by the EstuaryWatch program follows standardised methodologies set in Standard Operating Procedure manuals to ensure EstuaryWatch produces credible data. The data is stored on the State-wide EstuaryWatch Database.
The Corangamite EstuaryWatch Program monitors the following water quality parameters:
• electrical Conductivity
• dissolved Oxygen.
Estuary mouth condition monitoring, and photo-point surveys are regularly conducted. Estuary mouth condition monitoring takes place where the river meets the sea. Each month, EstuaryWatch begins with estuary mouth condition monitoring. Volunteers take a series of photos of the estuary mouth. These photos are taken from the same location each month so that the condition of the estuary mouth can be compared over time. One month the estuary might be closed to the sea, the next month it might be open to the sea.
Major threats and drivers of change
With sea levels projected to rise on average by 0.8-1.1 metres by 2100, combined with an increase in storm surge events and reduced inflows, climate change is expected to greatly impact all estuaries in the region. Climate change impact is expected to be higher in the estuaries of the Curdies, Gellibrand, Aire and Anglesea rivers. It is also expected to have a major impact on the Barwon River estuary, especially in the upper reaches of the estuary into Lake Connewarre.
Many of the estuaries in the Corangamite region are surrounded by dense coastal settlements (e.g. Lorne, Torquay, Barwon Heads, Peterborough and Apollo Bay), and can be exposed to intensive levels of recreation and use. The establishment of townships within close proximity to many of the region’s estuaries places pressure on how these function. There is a high level of expectation that estuaries will be opened when there is a threat of flooding to private property and infrastructure. The effect of run-off and pollution from these settlements can also have an impact on estuary health.
Other threats to the condition of estuaries include:
• unauthorised estuary entrance openings, e.g. use of machinery to remove sand from the estuary mouth so it flows into the ocean, under unfavourable circumstances can cause fish death events, reduced water quality and interference with life cycles
• changes in water regimes – particularly reduced freshwater inflows from rivers
• high levels of sediment and nutrients
• pollution events, e.g. oil spills
• habitat modification
• land-claim (creating new land from areas that were previously below high tide)
• invasion by weeds or pest animals
• salinisation, acidification and acid sulfate soils.